buf0flu.cc

/*****************************************************************************

Copyright (C) 1995, 2010, Innobase Oy. All Rights Reserved.

This program is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License as published by the Free Software
Foundation; version 2 of the License.

This program is distributed in the hope that it will be useful, but WITHOUT
ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with
this program; if not, write to the Free Software Foundation, Inc., 51 Franklin
St, Fifth Floor, Boston, MA 02110-1301 USA

*****************************************************************************/

/**************************************************/
#include "buf0flu.h"

#ifdef UNIV_NONINL
#include "buf0flu.ic"
#endif

#include "buf0buf.h"
#include "srv0srv.h"
#include "page0zip.h"
#ifndef UNIV_HOTBACKUP
#include "ut0byte.h"
#include "ut0lst.h"
#include "page0page.h"
#include "fil0fil.h"
#include "buf0lru.h"
#include "buf0rea.h"
#include "ibuf0ibuf.h"
#include "log0log.h"
#include "os0file.h"
#include "trx0sys.h"

/**********************************************************************
These statistics are generated for heuristics used in estimating the
rate at which we should flush the dirty blocks to avoid bursty IO
activity. Note that the rate of flushing not only depends on how many
dirty pages we have in the buffer pool but it is also a fucntion of
how much redo the workload is generating and at what rate. */
/* @{ */

#define BUF_FLUSH_STAT_N_INTERVAL 20

static buf_flush_stat_t buf_flush_stat_arr[BUF_FLUSH_STAT_N_INTERVAL];

static ulint    buf_flush_stat_arr_ind;

static buf_flush_stat_t buf_flush_stat_cur;

static buf_flush_stat_t buf_flush_stat_sum;

static ulint buf_lru_flush_page_count = 0;

/* @} */

#if defined UNIV_DEBUG || defined UNIV_BUF_DEBUG
/******************************************************************/
static
ibool
buf_flush_validate_low(
/*===================*/
  buf_pool_t* buf_pool);  
#endif /* UNIV_DEBUG || UNIV_BUF_DEBUG */

/******************************************************************/
static
buf_page_t*
buf_flush_insert_in_flush_rbt(
/*==========================*/
  buf_page_t* bpage)  
{
  const ib_rbt_node_t*  c_node;
  const ib_rbt_node_t*  p_node;
  buf_page_t*   prev = NULL;
  buf_pool_t*   buf_pool = buf_pool_from_bpage(bpage);

  ut_ad(buf_flush_list_mutex_own(buf_pool));

  /* Insert this buffer into the rbt. */
  c_node = rbt_insert(buf_pool->flush_rbt, &bpage, &bpage);
  ut_a(c_node != NULL);

  /* Get the predecessor. */
  p_node = rbt_prev(buf_pool->flush_rbt, c_node);

  if (p_node != NULL) {
    buf_page_t**  value;
    value = rbt_value(buf_page_t*, p_node);
    prev = *value;
    ut_a(prev != NULL);
  }

  return(prev);
}

/*********************************************************/
static
void
buf_flush_delete_from_flush_rbt(
/*============================*/
  buf_page_t* bpage)  
{
#ifdef UNIV_DEBUG
  ibool   ret = FALSE;
#endif /* UNIV_DEBUG */
  buf_pool_t* buf_pool = buf_pool_from_bpage(bpage);

  ut_ad(buf_flush_list_mutex_own(buf_pool));

#ifdef UNIV_DEBUG
  ret =
#endif /* UNIV_DEBUG */
  rbt_delete(buf_pool->flush_rbt, &bpage);
  ut_ad(ret);
}

/*****************************************************************/
static
int
buf_flush_block_cmp(
/*================*/
  const void* p1,   
  const void* p2)   
{
  int     ret;
  const buf_page_t* b1 = *(const buf_page_t**) p1;
  const buf_page_t* b2 = *(const buf_page_t**) p2;
#ifdef UNIV_DEBUG
  buf_pool_t*   buf_pool = buf_pool_from_bpage(b1);
#endif /* UNIV_DEBUG */

  ut_ad(b1 != NULL);
  ut_ad(b2 != NULL);

  ut_ad(buf_flush_list_mutex_own(buf_pool));

  ut_ad(b1->in_flush_list);
  ut_ad(b2->in_flush_list);

  if (b2->oldest_modification > b1->oldest_modification) {
    return(1);
  } else if (b2->oldest_modification < b1->oldest_modification) {
    return(-1);
  }

  /* If oldest_modification is same then decide on the space. */
  ret = (int)(b2->space - b1->space);

  /* Or else decide ordering on the offset field. */
  return(ret ? ret : (int)(b2->offset - b1->offset));
}

/********************************************************************/
UNIV_INTERN
void
buf_flush_init_flush_rbt(void)
/*==========================*/
{
  ulint i;

  for (i = 0; i < srv_buf_pool_instances; i++) {
    buf_pool_t* buf_pool;

    buf_pool = buf_pool_from_array(i);

    buf_flush_list_mutex_enter(buf_pool);

    /* Create red black tree for speedy insertions in flush list. */
    buf_pool->flush_rbt = rbt_create(
      sizeof(buf_page_t*), buf_flush_block_cmp);

    buf_flush_list_mutex_exit(buf_pool);
  }
}

/********************************************************************/
UNIV_INTERN
void
buf_flush_free_flush_rbt(void)
/*==========================*/
{
  ulint i;

  for (i = 0; i < srv_buf_pool_instances; i++) {
    buf_pool_t* buf_pool;

    buf_pool = buf_pool_from_array(i);

    buf_flush_list_mutex_enter(buf_pool);

#ifdef UNIV_DEBUG_VALGRIND
  {
    ulint zip_size = buf_block_get_zip_size(block);

    if (UNIV_UNLIKELY(zip_size)) {
      UNIV_MEM_ASSERT_RW(block->page.zip.data, zip_size);
    } else {
      UNIV_MEM_ASSERT_RW(block->frame, UNIV_PAGE_SIZE);
    }
  }
#endif /* UNIV_DEBUG_VALGRIND */
#if defined UNIV_DEBUG || defined UNIV_BUF_DEBUG
    ut_a(buf_flush_validate_low(buf_pool));
#endif /* UNIV_DEBUG || UNIV_BUF_DEBUG */

    rbt_free(buf_pool->flush_rbt);
    buf_pool->flush_rbt = NULL;

    buf_flush_list_mutex_exit(buf_pool);
  }
}

/********************************************************************/
UNIV_INTERN
void
buf_flush_insert_into_flush_list(
/*=============================*/
  buf_pool_t* buf_pool, 
  buf_block_t*  block,    
  ib_uint64_t lsn)    
{
  ut_ad(!buf_pool_mutex_own(buf_pool));
  ut_ad(log_flush_order_mutex_own());
  ut_ad(mutex_own(&block->mutex));

  buf_flush_list_mutex_enter(buf_pool);

  ut_ad((UT_LIST_GET_FIRST(buf_pool->flush_list) == NULL)
        || (UT_LIST_GET_FIRST(buf_pool->flush_list)->oldest_modification
      <= lsn));

  /* If we are in the recovery then we need to update the flush
  red-black tree as well. */
  if (UNIV_LIKELY_NULL(buf_pool->flush_rbt)) {
    buf_flush_list_mutex_exit(buf_pool);
    buf_flush_insert_sorted_into_flush_list(buf_pool, block, lsn);
    return;
  }

  ut_ad(buf_block_get_state(block) == BUF_BLOCK_FILE_PAGE);
  ut_ad(!block->page.in_flush_list);

  ut_d(block->page.in_flush_list = TRUE);
  block->page.oldest_modification = lsn;
  UT_LIST_ADD_FIRST(list, buf_pool->flush_list, &block->page);

#ifdef UNIV_DEBUG_VALGRIND
  {
    ulint zip_size = buf_block_get_zip_size(block);

    if (UNIV_UNLIKELY(zip_size)) {
      UNIV_MEM_ASSERT_RW(block->page.zip.data, zip_size);
    } else {
      UNIV_MEM_ASSERT_RW(block->frame, UNIV_PAGE_SIZE);
    }
  }
#endif /* UNIV_DEBUG_VALGRIND */
#if defined UNIV_DEBUG || defined UNIV_BUF_DEBUG
  ut_a(buf_flush_validate_low(buf_pool));
#endif /* UNIV_DEBUG || UNIV_BUF_DEBUG */

  buf_flush_list_mutex_exit(buf_pool);
}

/********************************************************************/
UNIV_INTERN
void
buf_flush_insert_sorted_into_flush_list(
/*====================================*/
  buf_pool_t* buf_pool, 
  buf_block_t*  block,    
  ib_uint64_t lsn)    
{
  buf_page_t* prev_b;
  buf_page_t* b;

  ut_ad(!buf_pool_mutex_own(buf_pool));
  ut_ad(log_flush_order_mutex_own());
  ut_ad(mutex_own(&block->mutex));
  ut_ad(buf_block_get_state(block) == BUF_BLOCK_FILE_PAGE);

  buf_flush_list_mutex_enter(buf_pool);

  /* The field in_LRU_list is protected by buf_pool->mutex, which
  we are not holding.  However, while a block is in the flush
  list, it is dirty and cannot be discarded, not from the
  page_hash or from the LRU list.  At most, the uncompressed
  page frame of a compressed block may be discarded or created
  (copying the block->page to or from a buf_page_t that is
  dynamically allocated from buf_buddy_alloc()).  Because those
  transitions hold block->mutex and the flush list mutex (via
  buf_flush_relocate_on_flush_list()), there is no possibility
  of a race condition in the assertions below. */
  ut_ad(block->page.in_LRU_list);
  ut_ad(block->page.in_page_hash);
  /* buf_buddy_block_register() will take a block in the
  BUF_BLOCK_MEMORY state, not a file page. */
  ut_ad(!block->page.in_zip_hash);

  ut_ad(!block->page.in_flush_list);
  ut_d(block->page.in_flush_list = TRUE);
  block->page.oldest_modification = lsn;

#ifdef UNIV_DEBUG_VALGRIND
  {
    ulint zip_size = buf_block_get_zip_size(block);

    if (UNIV_UNLIKELY(zip_size)) {
      UNIV_MEM_ASSERT_RW(block->page.zip.data, zip_size);
    } else {
      UNIV_MEM_ASSERT_RW(block->frame, UNIV_PAGE_SIZE);
    }
  }
#endif /* UNIV_DEBUG_VALGRIND */

#ifdef UNIV_DEBUG_VALGRIND
  {
    ulint zip_size = buf_block_get_zip_size(block);

    if (UNIV_UNLIKELY(zip_size)) {
      UNIV_MEM_ASSERT_RW(block->page.zip.data, zip_size);
    } else {
      UNIV_MEM_ASSERT_RW(block->frame, UNIV_PAGE_SIZE);
    }
  }
#endif /* UNIV_DEBUG_VALGRIND */

  prev_b = NULL;

  /* For the most part when this function is called the flush_rbt
  should not be NULL. In a very rare boundary case it is possible
  that the flush_rbt has already been freed by the recovery thread
  before the last page was hooked up in the flush_list by the
  io-handler thread. In that case we'll  just do a simple
  linear search in the else block. */
  if (buf_pool->flush_rbt) {

    prev_b = buf_flush_insert_in_flush_rbt(&block->page);

  } else {

    b = UT_LIST_GET_FIRST(buf_pool->flush_list);

    while (b && b->oldest_modification
           > block->page.oldest_modification) {
      ut_ad(b->in_flush_list);
      prev_b = b;
      b = UT_LIST_GET_NEXT(list, b);
    }
  }

  if (prev_b == NULL) {
    UT_LIST_ADD_FIRST(list, buf_pool->flush_list, &block->page);
  } else {
    UT_LIST_INSERT_AFTER(list, buf_pool->flush_list,
             prev_b, &block->page);
  }

#if defined UNIV_DEBUG || defined UNIV_BUF_DEBUG
  ut_a(buf_flush_validate_low(buf_pool));
#endif /* UNIV_DEBUG || UNIV_BUF_DEBUG */

  buf_flush_list_mutex_exit(buf_pool);
}

/********************************************************************/
UNIV_INTERN
ibool
buf_flush_ready_for_replace(
/*========================*/
  buf_page_t* bpage)  
{
#ifdef UNIV_DEBUG
  buf_pool_t* buf_pool = buf_pool_from_bpage(bpage);
  ut_ad(buf_pool_mutex_own(buf_pool));
#endif
  ut_ad(mutex_own(buf_page_get_mutex(bpage)));
  ut_ad(bpage->in_LRU_list);

  if (UNIV_LIKELY(buf_page_in_file(bpage))) {

    return(bpage->oldest_modification == 0
           && buf_page_get_io_fix(bpage) == BUF_IO_NONE
           && bpage->buf_fix_count == 0);
  }

  ut_print_timestamp(stderr);
  fprintf(stderr,
    "  InnoDB: Error: buffer block state %lu"
    " in the LRU list!\n",
    (ulong) buf_page_get_state(bpage));
  ut_print_buf(stderr, bpage, sizeof(buf_page_t));
  putc('\n', stderr);

  return(FALSE);
}

/********************************************************************/
UNIV_INLINE
ibool
buf_flush_ready_for_flush(
/*======================*/
  buf_page_t* bpage,  
  enum buf_flush  flush_type)
{
#ifdef UNIV_DEBUG
  buf_pool_t* buf_pool = buf_pool_from_bpage(bpage);
  ut_ad(buf_pool_mutex_own(buf_pool));
#endif
  ut_a(buf_page_in_file(bpage));
  ut_ad(mutex_own(buf_page_get_mutex(bpage)));
  ut_ad(flush_type == BUF_FLUSH_LRU || BUF_FLUSH_LIST);

  if (bpage->oldest_modification != 0
      && buf_page_get_io_fix(bpage) == BUF_IO_NONE) {
    ut_ad(bpage->in_flush_list);

    if (flush_type != BUF_FLUSH_LRU) {

      return(TRUE);

    } else if (bpage->buf_fix_count == 0) {

      /* If we are flushing the LRU list, to avoid deadlocks
      we require the block not to be bufferfixed, and hence
      not latched. */

      return(TRUE);
    }
  }

  return(FALSE);
}

/********************************************************************/
UNIV_INTERN
void
buf_flush_remove(
/*=============*/
  buf_page_t* bpage)  
{
  buf_pool_t* buf_pool = buf_pool_from_bpage(bpage);

  ut_ad(buf_pool_mutex_own(buf_pool));
  ut_ad(mutex_own(buf_page_get_mutex(bpage)));
  ut_ad(bpage->in_flush_list);

  buf_flush_list_mutex_enter(buf_pool);

  switch (buf_page_get_state(bpage)) {
  case BUF_BLOCK_ZIP_PAGE:
    /* Clean compressed pages should not be on the flush list */
  case BUF_BLOCK_ZIP_FREE:
  case BUF_BLOCK_NOT_USED:
  case BUF_BLOCK_READY_FOR_USE:
  case BUF_BLOCK_MEMORY:
  case BUF_BLOCK_REMOVE_HASH:
    ut_error;
    return;
  case BUF_BLOCK_ZIP_DIRTY:
    buf_page_set_state(bpage, BUF_BLOCK_ZIP_PAGE);
    UT_LIST_REMOVE(list, buf_pool->flush_list, bpage);
    buf_LRU_insert_zip_clean(bpage);
    break;
  case BUF_BLOCK_FILE_PAGE:
    UT_LIST_REMOVE(list, buf_pool->flush_list, bpage);
    break;
  }

  /* If the flush_rbt is active then delete from there as well. */
  if (UNIV_LIKELY_NULL(buf_pool->flush_rbt)) {
    buf_flush_delete_from_flush_rbt(bpage);
  }

  /* Must be done after we have removed it from the flush_rbt
  because we assert on in_flush_list in comparison function. */
  ut_d(bpage->in_flush_list = FALSE);

  bpage->oldest_modification = 0;

#if defined UNIV_DEBUG || defined UNIV_BUF_DEBUG
  ut_a(buf_flush_validate_low(buf_pool));
#endif /* UNIV_DEBUG || UNIV_BUF_DEBUG */

  buf_flush_list_mutex_exit(buf_pool);
}

/*******************************************************************/
UNIV_INTERN
void
buf_flush_relocate_on_flush_list(
/*=============================*/
  buf_page_t* bpage,  
  buf_page_t* dpage)  
{
  buf_page_t* prev;
  buf_page_t*   prev_b = NULL;
  buf_pool_t* buf_pool = buf_pool_from_bpage(bpage);

  ut_ad(buf_pool_mutex_own(buf_pool));
  /* Must reside in the same buffer pool. */
  ut_ad(buf_pool == buf_pool_from_bpage(dpage));

  ut_ad(mutex_own(buf_page_get_mutex(bpage)));

  buf_flush_list_mutex_enter(buf_pool);

  /* FIXME: At this point we have both buf_pool and flush_list
  mutexes. Theoretically removal of a block from flush list is
  only covered by flush_list mutex but currently we do
  have buf_pool mutex in buf_flush_remove() therefore this block
  is guaranteed to be in the flush list. We need to check if
  this will work without the assumption of block removing code
  having the buf_pool mutex. */
  ut_ad(bpage->in_flush_list);
  ut_ad(dpage->in_flush_list);

  /* If recovery is active we must swap the control blocks in
  the flush_rbt as well. */
  if (UNIV_LIKELY_NULL(buf_pool->flush_rbt)) {
    buf_flush_delete_from_flush_rbt(bpage);
    prev_b = buf_flush_insert_in_flush_rbt(dpage);
  }

  /* Must be done after we have removed it from the flush_rbt
  because we assert on in_flush_list in comparison function. */
  ut_d(bpage->in_flush_list = FALSE);

  prev = UT_LIST_GET_PREV(list, bpage);
  UT_LIST_REMOVE(list, buf_pool->flush_list, bpage);

  if (prev) {
    ut_ad(prev->in_flush_list);
    UT_LIST_INSERT_AFTER(
      list,
      buf_pool->flush_list,
      prev, dpage);
  } else {
    UT_LIST_ADD_FIRST(
      list,
      buf_pool->flush_list,
      dpage);
  }

  /* Just an extra check. Previous in flush_list
  should be the same control block as in flush_rbt. */
  ut_a(!buf_pool->flush_rbt || prev_b == prev);

#if defined UNIV_DEBUG || defined UNIV_BUF_DEBUG
  ut_a(buf_flush_validate_low(buf_pool));
#endif /* UNIV_DEBUG || UNIV_BUF_DEBUG */

  buf_flush_list_mutex_exit(buf_pool);
}

/********************************************************************/
UNIV_INTERN
void
buf_flush_write_complete(
/*=====================*/
  buf_page_t* bpage)  
{
  enum buf_flush  flush_type;
  buf_pool_t* buf_pool = buf_pool_from_bpage(bpage);

  ut_ad(bpage);

  buf_flush_remove(bpage);

  flush_type = buf_page_get_flush_type(bpage);
  buf_pool->n_flush[flush_type]--;

  if (flush_type == BUF_FLUSH_LRU) {
    /* Put the block to the end of the LRU list to wait to be
    moved to the free list */

    buf_LRU_make_block_old(bpage);

    buf_pool->LRU_flush_ended++;
  }

  /* fprintf(stderr, "n pending flush %lu\n",
  buf_pool->n_flush[flush_type]); */

  if (buf_pool->n_flush[flush_type] == 0
      && buf_pool->init_flush[flush_type] == FALSE) {

    /* The running flush batch has ended */

    os_event_set(buf_pool->no_flush[flush_type]);
  }
}

/********************************************************************/
static
void
buf_flush_sync_datafiles(void)
/*==========================*/
{
  /* Wake possible simulated aio thread to actually post the
  writes to the operating system */
  os_aio_simulated_wake_handler_threads();

  /* Wait that all async writes to tablespaces have been posted to
  the OS */
  os_aio_wait_until_no_pending_writes();

  /* Now we flush the data to disk (for example, with fsync) */
  fil_flush_file_spaces(FIL_TABLESPACE);

  return;
}

/********************************************************************/
static
void
buf_flush_buffered_writes(void)
/*===========================*/
{
  byte*   write_buf;
  ulint   len;
  ulint   len2;
  ulint   i;

  if (!srv_use_doublewrite_buf || trx_doublewrite == NULL) {
    /* Sync the writes to the disk. */
    buf_flush_sync_datafiles();
    return;
  }

  mutex_enter(&(trx_doublewrite->mutex));

  /* Write first to doublewrite buffer blocks. We use synchronous
  aio and thus know that file write has been completed when the
  control returns. */

  if (trx_doublewrite->first_free == 0) {

    mutex_exit(&(trx_doublewrite->mutex));

    return;
  }

  for (i = 0; i < trx_doublewrite->first_free; i++) {

    const buf_block_t*  block;

    block = (buf_block_t*) trx_doublewrite->buf_block_arr[i];

    if (buf_block_get_state(block) != BUF_BLOCK_FILE_PAGE
        || block->page.zip.data) {
      /* No simple validate for compressed pages exists. */
      continue;
    }

    if (UNIV_UNLIKELY
        (memcmp(block->frame + (FIL_PAGE_LSN + 4),
          block->frame + (UNIV_PAGE_SIZE
              - FIL_PAGE_END_LSN_OLD_CHKSUM + 4),
          4))) {
      ut_print_timestamp(stderr);
      fprintf(stderr,
        "  InnoDB: ERROR: The page to be written"
        " seems corrupt!\n"
        "InnoDB: The lsn fields do not match!"
        " Noticed in the buffer pool\n"
        "InnoDB: before posting to the"
        " doublewrite buffer.\n");
    }

    if (!block->check_index_page_at_flush) {
    } else if (page_is_comp(block->frame)) {
      if (UNIV_UNLIKELY
          (!page_simple_validate_new(block->frame))) {
corrupted_page:
        buf_page_print(block->frame, 0);

        ut_print_timestamp(stderr);
        fprintf(stderr,
          "  InnoDB: Apparent corruption of an"
          " index page n:o %lu in space %lu\n"
          "InnoDB: to be written to data file."
          " We intentionally crash server\n"
          "InnoDB: to prevent corrupt data"
          " from ending up in data\n"
          "InnoDB: files.\n",
          (ulong) buf_block_get_page_no(block),
          (ulong) buf_block_get_space(block));

        ut_error;
      }
    } else if (UNIV_UNLIKELY
         (!page_simple_validate_old(block->frame))) {

      goto corrupted_page;
    }
  }

  /* increment the doublewrite flushed pages counter */
  srv_dblwr_pages_written+= trx_doublewrite->first_free;
  srv_dblwr_writes++;

  len = ut_min(TRX_SYS_DOUBLEWRITE_BLOCK_SIZE,
         trx_doublewrite->first_free) * UNIV_PAGE_SIZE;

  write_buf = trx_doublewrite->write_buf;
  i = 0;

  fil_io(OS_FILE_WRITE, TRUE, TRX_SYS_SPACE, 0,
         trx_doublewrite->block1, 0, len,
         (void*) write_buf, NULL);

  for (len2 = 0; len2 + UNIV_PAGE_SIZE <= len;
       len2 += UNIV_PAGE_SIZE, i++) {
    const buf_block_t* block = (buf_block_t*)
      trx_doublewrite->buf_block_arr[i];

    if (UNIV_LIKELY(!block->page.zip.data)
        && UNIV_LIKELY(buf_block_get_state(block)
           == BUF_BLOCK_FILE_PAGE)
        && UNIV_UNLIKELY
        (memcmp(write_buf + len2 + (FIL_PAGE_LSN + 4),
          write_buf + len2
          + (UNIV_PAGE_SIZE
             - FIL_PAGE_END_LSN_OLD_CHKSUM + 4), 4))) {
      ut_print_timestamp(stderr);
      fprintf(stderr,
        "  InnoDB: ERROR: The page to be written"
        " seems corrupt!\n"
        "InnoDB: The lsn fields do not match!"
        " Noticed in the doublewrite block1.\n");
    }
  }

  if (trx_doublewrite->first_free <= TRX_SYS_DOUBLEWRITE_BLOCK_SIZE) {
    goto flush;
  }

  len = (trx_doublewrite->first_free - TRX_SYS_DOUBLEWRITE_BLOCK_SIZE)
    * UNIV_PAGE_SIZE;

  write_buf = trx_doublewrite->write_buf
    + TRX_SYS_DOUBLEWRITE_BLOCK_SIZE * UNIV_PAGE_SIZE;
  ut_ad(i == TRX_SYS_DOUBLEWRITE_BLOCK_SIZE);

  fil_io(OS_FILE_WRITE, TRUE, TRX_SYS_SPACE, 0,
         trx_doublewrite->block2, 0, len,
         (void*) write_buf, NULL);

  for (len2 = 0; len2 + UNIV_PAGE_SIZE <= len;
       len2 += UNIV_PAGE_SIZE, i++) {
    const buf_block_t* block = (buf_block_t*)
      trx_doublewrite->buf_block_arr[i];

    if (UNIV_LIKELY(!block->page.zip.data)
        && UNIV_LIKELY(buf_block_get_state(block)
           == BUF_BLOCK_FILE_PAGE)
        && UNIV_UNLIKELY
        (memcmp(write_buf + len2 + (FIL_PAGE_LSN + 4),
          write_buf + len2
          + (UNIV_PAGE_SIZE
             - FIL_PAGE_END_LSN_OLD_CHKSUM + 4), 4))) {
      ut_print_timestamp(stderr);
      fprintf(stderr,
        "  InnoDB: ERROR: The page to be"
        " written seems corrupt!\n"
        "InnoDB: The lsn fields do not match!"
        " Noticed in"
        " the doublewrite block2.\n");
    }
  }

flush:
  /* Now flush the doublewrite buffer data to disk */

  fil_flush(TRX_SYS_SPACE);

  /* We know that the writes have been flushed to disk now
  and in recovery we will find them in the doublewrite buffer
  blocks. Next do the writes to the intended positions. */

  for (i = 0; i < trx_doublewrite->first_free; i++) {
    const buf_block_t* block = (buf_block_t*)
      trx_doublewrite->buf_block_arr[i];

    ut_a(buf_page_in_file(&block->page));
    if (UNIV_LIKELY_NULL(block->page.zip.data)) {
      fil_io(OS_FILE_WRITE | OS_AIO_SIMULATED_WAKE_LATER,
             FALSE, buf_page_get_space(&block->page),
             buf_page_get_zip_size(&block->page),
             buf_page_get_page_no(&block->page), 0,
             buf_page_get_zip_size(&block->page),
             (void*)block->page.zip.data,
             (void*)block);

      /* Increment the counter of I/O operations used
      for selecting LRU policy. */
      buf_LRU_stat_inc_io();

      continue;
    }

    ut_a(buf_block_get_state(block) == BUF_BLOCK_FILE_PAGE);

    if (UNIV_UNLIKELY(memcmp(block->frame + (FIL_PAGE_LSN + 4),
           block->frame
           + (UNIV_PAGE_SIZE
              - FIL_PAGE_END_LSN_OLD_CHKSUM + 4),
           4))) {
      ut_print_timestamp(stderr);
      fprintf(stderr,
        "  InnoDB: ERROR: The page to be written"
        " seems corrupt!\n"
        "InnoDB: The lsn fields do not match!"
        " Noticed in the buffer pool\n"
        "InnoDB: after posting and flushing"
        " the doublewrite buffer.\n"
        "InnoDB: Page buf fix count %lu,"
        " io fix %lu, state %lu\n",
        (ulong)block->page.buf_fix_count,
        (ulong)buf_block_get_io_fix(block),
        (ulong)buf_block_get_state(block));
    }

    fil_io(OS_FILE_WRITE | OS_AIO_SIMULATED_WAKE_LATER,
           FALSE, buf_block_get_space(block), 0,
           buf_block_get_page_no(block), 0, UNIV_PAGE_SIZE,
           (void*)block->frame, (void*)block);

    /* Increment the counter of I/O operations used
    for selecting LRU policy. */
    buf_LRU_stat_inc_io();
  }

  /* Sync the writes to the disk. */
  buf_flush_sync_datafiles();

  /* We can now reuse the doublewrite memory buffer: */
  trx_doublewrite->first_free = 0;

  mutex_exit(&(trx_doublewrite->mutex));
}

/********************************************************************/
static
void
buf_flush_post_to_doublewrite_buf(
/*==============================*/
  buf_page_t* bpage)  
{
  ulint zip_size;
try_again:
  mutex_enter(&(trx_doublewrite->mutex));

  ut_a(buf_page_in_file(bpage));

  if (trx_doublewrite->first_free
      >= 2 * TRX_SYS_DOUBLEWRITE_BLOCK_SIZE) {
    mutex_exit(&(trx_doublewrite->mutex));

    buf_flush_buffered_writes();

    goto try_again;
  }

  zip_size = buf_page_get_zip_size(bpage);

  if (UNIV_UNLIKELY(zip_size)) {
    UNIV_MEM_ASSERT_RW(bpage->zip.data, zip_size);
    /* Copy the compressed page and clear the rest. */
    memcpy(trx_doublewrite->write_buf
           + UNIV_PAGE_SIZE * trx_doublewrite->first_free,
           bpage->zip.data, zip_size);
    memset(trx_doublewrite->write_buf
           + UNIV_PAGE_SIZE * trx_doublewrite->first_free
           + zip_size, 0, UNIV_PAGE_SIZE - zip_size);
  } else {
    ut_a(buf_page_get_state(bpage) == BUF_BLOCK_FILE_PAGE);
    UNIV_MEM_ASSERT_RW(((buf_block_t*) bpage)->frame,
           UNIV_PAGE_SIZE);

    memcpy(trx_doublewrite->write_buf
           + UNIV_PAGE_SIZE * trx_doublewrite->first_free,
           ((buf_block_t*) bpage)->frame, UNIV_PAGE_SIZE);
  }

  trx_doublewrite->buf_block_arr[trx_doublewrite->first_free] = bpage;

  trx_doublewrite->first_free++;

  if (trx_doublewrite->first_free
      >= 2 * TRX_SYS_DOUBLEWRITE_BLOCK_SIZE) {
    mutex_exit(&(trx_doublewrite->mutex));

    buf_flush_buffered_writes();

    return;
  }

  mutex_exit(&(trx_doublewrite->mutex));
}
#endif /* !UNIV_HOTBACKUP */

/********************************************************************/
UNIV_INTERN
void
buf_flush_init_for_writing(
/*=======================*/
  byte*   page,   
  void*   page_zip_,  
  ib_uint64_t newest_lsn) 
{
  ut_ad(page);

  if (page_zip_) {
    page_zip_des_t* page_zip = static_cast<page_zip_des_t *>(page_zip_);
    ulint   zip_size = page_zip_get_size(page_zip);
    ut_ad(zip_size);
    ut_ad(ut_is_2pow(zip_size));
    ut_ad(zip_size <= UNIV_PAGE_SIZE);

    switch (UNIV_EXPECT(fil_page_get_type(page), FIL_PAGE_INDEX)) {
    case FIL_PAGE_TYPE_ALLOCATED:
    case FIL_PAGE_INODE:
    case FIL_PAGE_IBUF_BITMAP:
    case FIL_PAGE_TYPE_FSP_HDR:
    case FIL_PAGE_TYPE_XDES:
      /* These are essentially uncompressed pages. */
      memcpy(page_zip->data, page, zip_size);
      /* fall through */
    case FIL_PAGE_TYPE_ZBLOB:
    case FIL_PAGE_TYPE_ZBLOB2:
    case FIL_PAGE_INDEX:
      mach_write_to_8(page_zip->data
          + FIL_PAGE_LSN, newest_lsn);
      memset(page_zip->data + FIL_PAGE_FILE_FLUSH_LSN, 0, 8);
      mach_write_to_4(page_zip->data
          + FIL_PAGE_SPACE_OR_CHKSUM,
          srv_use_checksums
          ? page_zip_calc_checksum(
            page_zip->data, zip_size)
          : BUF_NO_CHECKSUM_MAGIC);
      return;
    }

    ut_print_timestamp(stderr);
    fputs("  InnoDB: ERROR: The compressed page to be written"
          " seems corrupt:", stderr);
    ut_print_buf(stderr, page, zip_size);
    fputs("\nInnoDB: Possibly older version of the page:", stderr);
    ut_print_buf(stderr, page_zip->data, zip_size);
    putc('\n', stderr);
    ut_error;
  }

  /* Write the newest modification lsn to the page header and trailer */
  mach_write_to_8(page + FIL_PAGE_LSN, newest_lsn);

  mach_write_to_8(page + UNIV_PAGE_SIZE - FIL_PAGE_END_LSN_OLD_CHKSUM,
      newest_lsn);

  /* Store the new formula checksum */

  mach_write_to_4(page + FIL_PAGE_SPACE_OR_CHKSUM,
      srv_use_checksums
      ? buf_calc_page_new_checksum(page)
      : BUF_NO_CHECKSUM_MAGIC);

  /* We overwrite the first 4 bytes of the end lsn field to store
  the old formula checksum. Since it depends also on the field
  FIL_PAGE_SPACE_OR_CHKSUM, it has to be calculated after storing the
  new formula checksum. */

  mach_write_to_4(page + UNIV_PAGE_SIZE - FIL_PAGE_END_LSN_OLD_CHKSUM,
      srv_use_checksums
      ? buf_calc_page_old_checksum(page)
      : BUF_NO_CHECKSUM_MAGIC);
}

#ifndef UNIV_HOTBACKUP
/********************************************************************/
static
void
buf_flush_write_block_low(
/*======================*/
  buf_page_t* bpage)  
{
  ulint zip_size  = buf_page_get_zip_size(bpage);
  page_t* frame   = NULL;

#ifdef UNIV_DEBUG
  buf_pool_t* buf_pool = buf_pool_from_bpage(bpage);
  ut_ad(!buf_pool_mutex_own(buf_pool));
#endif

#ifdef UNIV_LOG_DEBUG
  static ibool univ_log_debug_warned;
#endif /* UNIV_LOG_DEBUG */

  ut_ad(buf_page_in_file(bpage));

  /* We are not holding buf_pool->mutex or block_mutex here.
  Nevertheless, it is safe to access bpage, because it is
  io_fixed and oldest_modification != 0.  Thus, it cannot be
  relocated in the buffer pool or removed from flush_list or
  LRU_list. */
  ut_ad(!buf_pool_mutex_own(buf_pool));
  ut_ad(!buf_flush_list_mutex_own(buf_pool));
  ut_ad(!mutex_own(buf_page_get_mutex(bpage)));
  ut_ad(buf_page_get_io_fix(bpage) == BUF_IO_WRITE);
  ut_ad(bpage->oldest_modification != 0);

#ifdef UNIV_IBUF_COUNT_DEBUG
  ut_a(ibuf_count_get(bpage->space, bpage->offset) == 0);
#endif
  ut_ad(bpage->newest_modification != 0);

#ifdef UNIV_LOG_DEBUG
  if (!univ_log_debug_warned) {
    univ_log_debug_warned = TRUE;
    fputs("Warning: cannot force log to disk if"
          " UNIV_LOG_DEBUG is defined!\n"
          "Crash recovery will not work!\n",
          stderr);
  }
#else
  /* Force the log to the disk before writing the modified block */
  log_write_up_to(bpage->newest_modification, LOG_WAIT_ALL_GROUPS, TRUE);
#endif
  switch (buf_page_get_state(bpage)) {
  case BUF_BLOCK_ZIP_FREE:
  case BUF_BLOCK_ZIP_PAGE: /* The page should be dirty. */
  case BUF_BLOCK_NOT_USED:
  case BUF_BLOCK_READY_FOR_USE:
  case BUF_BLOCK_MEMORY:
  case BUF_BLOCK_REMOVE_HASH:
    ut_error;
    break;
  case BUF_BLOCK_ZIP_DIRTY:
    frame = bpage->zip.data;
    if (UNIV_LIKELY(srv_use_checksums)) {
      ut_a(mach_read_from_4(frame + FIL_PAGE_SPACE_OR_CHKSUM)
           == page_zip_calc_checksum(frame, zip_size));
    }
    mach_write_to_8(frame + FIL_PAGE_LSN,
        bpage->newest_modification);
    memset(frame + FIL_PAGE_FILE_FLUSH_LSN, 0, 8);
    break;
  case BUF_BLOCK_FILE_PAGE:
    frame = bpage->zip.data;
    if (!frame) {
      frame = ((buf_block_t*) bpage)->frame;
    }

    buf_flush_init_for_writing(((buf_block_t*) bpage)->frame,
             bpage->zip.data
             ? &bpage->zip : NULL,
             bpage->newest_modification);
    break;
  }

  if (!srv_use_doublewrite_buf || !trx_doublewrite) {
    fil_io(OS_FILE_WRITE | OS_AIO_SIMULATED_WAKE_LATER,
           FALSE, buf_page_get_space(bpage), zip_size,
           buf_page_get_page_no(bpage), 0,
           zip_size ? zip_size : UNIV_PAGE_SIZE,
           frame, bpage);
  } else {
    buf_flush_post_to_doublewrite_buf(bpage);
  }
}

# if defined UNIV_DEBUG || defined UNIV_IBUF_DEBUG
/********************************************************************/
UNIV_INTERN
ibool
buf_flush_page_try(
/*===============*/
  buf_pool_t* buf_pool, 
  buf_block_t*  block)    
{
  ut_ad(buf_pool_mutex_own(buf_pool));
  ut_ad(buf_block_get_state(block) == BUF_BLOCK_FILE_PAGE);
  ut_ad(mutex_own(&block->mutex));

  if (!buf_flush_ready_for_flush(&block->page, BUF_FLUSH_LRU)) {
    return(FALSE);
  }

  if (buf_pool->n_flush[BUF_FLUSH_LRU] > 0
      || buf_pool->init_flush[BUF_FLUSH_LRU]) {
    /* There is already a flush batch of the same type running */
    return(FALSE);
  }

  buf_pool->init_flush[BUF_FLUSH_LRU] = TRUE;

  buf_page_set_io_fix(&block->page, BUF_IO_WRITE);

  buf_page_set_flush_type(&block->page, BUF_FLUSH_LRU);

  if (buf_pool->n_flush[BUF_FLUSH_LRU]++ == 0) {

    os_event_reset(buf_pool->no_flush[BUF_FLUSH_LRU]);
  }

  /* VERY IMPORTANT:
  Because any thread may call the LRU flush, even when owning
  locks on pages, to avoid deadlocks, we must make sure that the
  s-lock is acquired on the page without waiting: this is
  accomplished because buf_flush_ready_for_flush() must hold,
  and that requires the page not to be bufferfixed. */

  rw_lock_s_lock_gen(&block->lock, BUF_IO_WRITE);

  /* Note that the s-latch is acquired before releasing the
  buf_pool mutex: this ensures that the latch is acquired
  immediately. */

  mutex_exit(&block->mutex);
  buf_pool_mutex_exit(buf_pool);

  /* Even though block is not protected by any mutex at this
  point, it is safe to access block, because it is io_fixed and
  oldest_modification != 0.  Thus, it cannot be relocated in the
  buffer pool or removed from flush_list or LRU_list. */

  buf_flush_write_block_low(&block->page);

  buf_pool_mutex_enter(buf_pool);
  buf_pool->init_flush[BUF_FLUSH_LRU] = FALSE;

  if (buf_pool->n_flush[BUF_FLUSH_LRU] == 0) {
    /* The running flush batch has ended */
    os_event_set(buf_pool->no_flush[BUF_FLUSH_LRU]);
  }

  buf_pool_mutex_exit(buf_pool);
  buf_flush_buffered_writes();

  return(TRUE);
}
# endif /* UNIV_DEBUG || UNIV_IBUF_DEBUG */

/********************************************************************/
static
void
buf_flush_page(
/*===========*/
  buf_pool_t* buf_pool, 
  buf_page_t* bpage,    
  enum buf_flush  flush_type) 
{
  mutex_t*  block_mutex;
  ibool   is_uncompressed;

  ut_ad(flush_type == BUF_FLUSH_LRU || flush_type == BUF_FLUSH_LIST);
  ut_ad(buf_pool_mutex_own(buf_pool));
  ut_ad(buf_page_in_file(bpage));

  block_mutex = buf_page_get_mutex(bpage);
  ut_ad(mutex_own(block_mutex));

  ut_ad(buf_flush_ready_for_flush(bpage, flush_type));

  buf_page_set_io_fix(bpage, BUF_IO_WRITE);

  buf_page_set_flush_type(bpage, flush_type);

  if (buf_pool->n_flush[flush_type] == 0) {

    os_event_reset(buf_pool->no_flush[flush_type]);
  }

  buf_pool->n_flush[flush_type]++;

  is_uncompressed = (buf_page_get_state(bpage) == BUF_BLOCK_FILE_PAGE);
  ut_ad(is_uncompressed == (block_mutex != &buf_pool->zip_mutex));

  switch (flush_type) {
    ibool is_s_latched;
  case BUF_FLUSH_LIST:
    /* If the simulated aio thread is not running, we must
    not wait for any latch, as we may end up in a deadlock:
    if buf_fix_count == 0, then we know we need not wait */

    is_s_latched = (bpage->buf_fix_count == 0);
    if (is_s_latched && is_uncompressed) {
      rw_lock_s_lock_gen(&((buf_block_t*) bpage)->lock,
             BUF_IO_WRITE);
    }

    mutex_exit(block_mutex);
    buf_pool_mutex_exit(buf_pool);

    /* Even though bpage is not protected by any mutex at
    this point, it is safe to access bpage, because it is
    io_fixed and oldest_modification != 0.  Thus, it
    cannot be relocated in the buffer pool or removed from
    flush_list or LRU_list. */

    if (!is_s_latched) {
      buf_flush_buffered_writes();

      if (is_uncompressed) {
        rw_lock_s_lock_gen(&((buf_block_t*) bpage)
               ->lock, BUF_IO_WRITE);
      }
    }

    break;

  case BUF_FLUSH_LRU:
    /* VERY IMPORTANT:
    Because any thread may call the LRU flush, even when owning
    locks on pages, to avoid deadlocks, we must make sure that the
    s-lock is acquired on the page without waiting: this is
    accomplished because buf_flush_ready_for_flush() must hold,
    and that requires the page not to be bufferfixed. */

    if (is_uncompressed) {
      rw_lock_s_lock_gen(&((buf_block_t*) bpage)->lock,
             BUF_IO_WRITE);
    }

    /* Note that the s-latch is acquired before releasing the
    buf_pool mutex: this ensures that the latch is acquired
    immediately. */

    mutex_exit(block_mutex);
    buf_pool_mutex_exit(buf_pool);
    break;

  default:
    ut_error;
  }

  /* Even though bpage is not protected by any mutex at this
  point, it is safe to access bpage, because it is io_fixed and
  oldest_modification != 0.  Thus, it cannot be relocated in the
  buffer pool or removed from flush_list or LRU_list. */

#ifdef UNIV_DEBUG
  if (buf_debug_prints) {
    fprintf(stderr,
      "Flushing %u space %u page %u\n",
      flush_type, bpage->space, bpage->offset);
  }
#endif /* UNIV_DEBUG */
  buf_flush_write_block_low(bpage);
}

/***********************************************************/
static
ulint
buf_flush_try_neighbors(
/*====================*/
  ulint   space,    
  ulint   offset,   
  enum buf_flush  flush_type, 
  ulint   n_flushed,  
  ulint   n_to_flush) 
{
  ulint   i;
  ulint   low;
  ulint   high;
  ulint   count = 0;
  buf_pool_t* buf_pool = buf_pool_get(space, offset);

  ut_ad(flush_type == BUF_FLUSH_LRU || flush_type == BUF_FLUSH_LIST);

  if (UT_LIST_GET_LEN(buf_pool->LRU) < BUF_LRU_OLD_MIN_LEN
      || !srv_flush_neighbor_pages) {
    /* If there is little space, it is better not to flush
    any block except from the end of the LRU list */

    low = offset;
    high = offset + 1;
  } else {
    /* When flushed, dirty blocks are searched in
    neighborhoods of this size, and flushed along with the
    original page. */

    ulint buf_flush_area;
  
    buf_flush_area  = ut_min(
      BUF_READ_AHEAD_AREA(buf_pool),
      buf_pool->curr_size / 16);

    low = (offset / buf_flush_area) * buf_flush_area;
    high = (offset / buf_flush_area + 1) * buf_flush_area;
  }

  /* fprintf(stderr, "Flush area: low %lu high %lu\n", low, high); */

  if (high > fil_space_get_size(space)) {
    high = fil_space_get_size(space);
  }

  for (i = low; i < high; i++) {

    buf_page_t* bpage;

    if ((count + n_flushed) >= n_to_flush) {

      /* We have already flushed enough pages and
      should call it a day. There is, however, one
      exception. If the page whose neighbors we
      are flushing has not been flushed yet then
      we'll try to flush the victim that we
      selected originally. */
      if (i <= offset) {
        i = offset;
      } else {
        break;
      }
    }

    buf_pool = buf_pool_get(space, i);

    buf_pool_mutex_enter(buf_pool);

    /* We only want to flush pages from this buffer pool. */
    bpage = buf_page_hash_get(buf_pool, space, i);

    if (!bpage) {

      buf_pool_mutex_exit(buf_pool);
      continue;
    }

    ut_a(buf_page_in_file(bpage));

    /* We avoid flushing 'non-old' blocks in an LRU flush,
    because the flushed blocks are soon freed */

    if (flush_type != BUF_FLUSH_LRU
        || i == offset
        || buf_page_is_old(bpage)) {
      mutex_t* block_mutex = buf_page_get_mutex(bpage);

      mutex_enter(block_mutex);

      if (buf_flush_ready_for_flush(bpage, flush_type)
          && (i == offset || !bpage->buf_fix_count)) {
        /* We only try to flush those
        neighbors != offset where the buf fix
        count is zero, as we then know that we
        probably can latch the page without a
        semaphore wait. Semaphore waits are
        expensive because we must flush the
        doublewrite buffer before we start
        waiting. */

        buf_flush_page(buf_pool, bpage, flush_type);
        ut_ad(!mutex_own(block_mutex));
        ut_ad(!buf_pool_mutex_own(buf_pool));
        count++;
        continue;
      } else {
        mutex_exit(block_mutex);
      }
    }
    buf_pool_mutex_exit(buf_pool);
  }

  return(count);
}

/********************************************************************/
static
ibool
buf_flush_page_and_try_neighbors(
/*=============================*/
  buf_page_t* bpage,    
  enum buf_flush  flush_type, 
  ulint   n_to_flush, 
  ulint*    count)    
{
  mutex_t*  block_mutex;
  ibool   flushed = FALSE;
#ifdef UNIV_DEBUG
  buf_pool_t* buf_pool = buf_pool_from_bpage(bpage);
#endif /* UNIV_DEBUG */

  ut_ad(buf_pool_mutex_own(buf_pool));

  block_mutex = buf_page_get_mutex(bpage);
  mutex_enter(block_mutex);

  ut_a(buf_page_in_file(bpage));

  if (buf_flush_ready_for_flush(bpage, flush_type)) {
    ulint   space;
    ulint   offset;
    buf_pool_t* buf_pool;

    buf_pool = buf_pool_from_bpage(bpage);

    buf_pool_mutex_exit(buf_pool);

    /* These fields are protected by both the
    buffer pool mutex and block mutex. */
    space = buf_page_get_space(bpage);
    offset = buf_page_get_page_no(bpage);

    mutex_exit(block_mutex);

    /* Try to flush also all the neighbors */
    *count += buf_flush_try_neighbors(space,
              offset,
              flush_type,
              *count,
              n_to_flush);

    buf_pool_mutex_enter(buf_pool);
    flushed = TRUE;
  } else {
    mutex_exit(block_mutex);
  }

  ut_ad(buf_pool_mutex_own(buf_pool));

  return(flushed);
}

/*******************************************************************/
static
ulint
buf_flush_LRU_list_batch(
/*=====================*/
  buf_pool_t* buf_pool, 
  ulint   max)    
{
  buf_page_t* bpage;
  ulint   count = 0;

  ut_ad(buf_pool_mutex_own(buf_pool));

  do {
    /* Start from the end of the list looking for a
    suitable block to be flushed. */
    bpage = UT_LIST_GET_LAST(buf_pool->LRU);

    /* Iterate backwards over the flush list till we find
    a page that isn't ready for flushing. */
    while (bpage != NULL
           && !buf_flush_page_and_try_neighbors(
        bpage, BUF_FLUSH_LRU, max, &count)) {

      bpage = UT_LIST_GET_PREV(LRU, bpage);
    }
  } while (bpage != NULL && count < max);

  /* We keep track of all flushes happening as part of LRU
  flush. When estimating the desired rate at which flush_list
  should be flushed, we factor in this value. */
  buf_lru_flush_page_count += count;

  ut_ad(buf_pool_mutex_own(buf_pool));

  return(count);
}

/*******************************************************************/
static
ulint
buf_flush_flush_list_batch(
/*=======================*/
  buf_pool_t* buf_pool, 
  ulint   min_n,    
  ib_uint64_t lsn_limit)  
{
  ulint   len;
  buf_page_t* bpage;
  ulint   count = 0;

  ut_ad(buf_pool_mutex_own(buf_pool));

  /* If we have flushed enough, leave the loop */
  do {
    /* Start from the end of the list looking for a suitable
    block to be flushed. */

    buf_flush_list_mutex_enter(buf_pool);

    /* We use len here because theoretically insertions can
    happen in the flush_list below while we are traversing
    it for a suitable candidate for flushing. We'd like to
    set a limit on how farther we are willing to traverse
    the list. */
    len = UT_LIST_GET_LEN(buf_pool->flush_list);
    bpage = UT_LIST_GET_LAST(buf_pool->flush_list);

    if (bpage) {
      ut_a(bpage->oldest_modification > 0);
    }

    if (!bpage || bpage->oldest_modification >= lsn_limit) {

      /* We have flushed enough */
      buf_flush_list_mutex_exit(buf_pool);
      break;
    }

    ut_a(bpage->oldest_modification > 0);

    ut_ad(bpage->in_flush_list);

    buf_flush_list_mutex_exit(buf_pool);

    /* The list may change during the flushing and we cannot
    safely preserve within this function a pointer to a
    block in the list! */
    while (bpage != NULL
           && len > 0
           && !buf_flush_page_and_try_neighbors(
        bpage, BUF_FLUSH_LIST, min_n, &count)) {

      buf_flush_list_mutex_enter(buf_pool);

      /* If we are here that means that buf_pool->mutex
       was not released in buf_flush_page_and_try_neighbors()
      above and this guarantees that bpage didn't get
      relocated since we released the flush_list
      mutex above. There is a chance, however, that
      the bpage got removed from flush_list (not
      currently possible because flush_list_remove()
      also obtains buf_pool mutex but that may change
      in future). To avoid this scenario we check
      the oldest_modification and if it is zero
      we start all over again. */
      if (bpage->oldest_modification == 0) {
        buf_flush_list_mutex_exit(buf_pool);
        break;
      }

      bpage = UT_LIST_GET_PREV(list, bpage);

      ut_ad(!bpage || bpage->in_flush_list);

      buf_flush_list_mutex_exit(buf_pool);

      --len;
    }

  } while (count < min_n && bpage != NULL && len > 0);

  ut_ad(buf_pool_mutex_own(buf_pool));

  return(count);
}

/*******************************************************************/
static
ulint
buf_flush_batch(
/*============*/
  buf_pool_t* buf_pool, 
  enum buf_flush  flush_type, 
  ulint   min_n,    
  ib_uint64_t lsn_limit)  
{
  ulint   count = 0;

  ut_ad(flush_type == BUF_FLUSH_LRU || flush_type == BUF_FLUSH_LIST);
#ifdef UNIV_SYNC_DEBUG
  ut_ad((flush_type != BUF_FLUSH_LIST)
        || sync_thread_levels_empty_gen(TRUE));
#endif /* UNIV_SYNC_DEBUG */

  buf_pool_mutex_enter(buf_pool);

  /* Note: The buffer pool mutex is released and reacquired within
  the flush functions. */
  switch(flush_type) {
  case BUF_FLUSH_LRU:
    count = buf_flush_LRU_list_batch(buf_pool, min_n);
    break;
  case BUF_FLUSH_LIST:
    count = buf_flush_flush_list_batch(buf_pool, min_n, lsn_limit);
    break;
  default:
    ut_error;
  }

  buf_pool_mutex_exit(buf_pool);

  buf_flush_buffered_writes();

#ifdef UNIV_DEBUG
  if (buf_debug_prints && count > 0) {
    fprintf(stderr, flush_type == BUF_FLUSH_LRU
      ? "Flushed %lu pages in LRU flush\n"
      : "Flushed %lu pages in flush list flush\n",
      (ulong) count);
  }
#endif /* UNIV_DEBUG */

  srv_buf_pool_flushed += count;

  return(count);
}

/******************************************************************/
static
void
buf_flush_common(
/*=============*/
  enum buf_flush  flush_type, 
  ulint   page_count) 
{
  buf_flush_buffered_writes();

  ut_a(flush_type == BUF_FLUSH_LRU || flush_type == BUF_FLUSH_LIST);

#ifdef UNIV_DEBUG
  if (buf_debug_prints && page_count > 0) {
    fprintf(stderr, flush_type == BUF_FLUSH_LRU
      ? "Flushed %lu pages in LRU flush\n"
      : "Flushed %lu pages in flush list flush\n",
      (ulong) page_count);
  }
#endif /* UNIV_DEBUG */

  srv_buf_pool_flushed += page_count;

  if (flush_type == BUF_FLUSH_LRU) {
    /* We keep track of all flushes happening as part of LRU
    flush. When estimating the desired rate at which flush_list
    should be flushed we factor in this value. */
    buf_lru_flush_page_count += page_count;
  }
}

/******************************************************************/
static
ibool
buf_flush_start(
/*============*/
  buf_pool_t* buf_pool, 
  enum buf_flush  flush_type) 
{
  buf_pool_mutex_enter(buf_pool);

  if (buf_pool->n_flush[flush_type] > 0
     || buf_pool->init_flush[flush_type] == TRUE) {

    /* There is already a flush batch of the same type running */

    buf_pool_mutex_exit(buf_pool);

    return(FALSE);
  }

  buf_pool->init_flush[flush_type] = TRUE;

  buf_pool_mutex_exit(buf_pool);

  return(TRUE);
}

/******************************************************************/
static
void
buf_flush_end(
/*==========*/
  buf_pool_t* buf_pool, 
  enum buf_flush  flush_type) 
{
  buf_pool_mutex_enter(buf_pool);

  buf_pool->init_flush[flush_type] = FALSE;

  if (buf_pool->n_flush[flush_type] == 0) {

    /* The running flush batch has ended */

    os_event_set(buf_pool->no_flush[flush_type]);
  }

  buf_pool_mutex_exit(buf_pool);
}

/******************************************************************/
UNIV_INTERN
void
buf_flush_wait_batch_end(
/*=====================*/
  buf_pool_t* buf_pool, 
  enum buf_flush  type)   
{
  ut_ad(type == BUF_FLUSH_LRU || type == BUF_FLUSH_LIST);

  if (buf_pool == NULL) {
    ulint i;

    for (i = 0; i < srv_buf_pool_instances; ++i) {
      buf_pool_t* i_buf_pool = buf_pool_from_array(i);

      os_event_wait(i_buf_pool->no_flush[type]);
    }
  } else {
    os_event_wait(buf_pool->no_flush[type]);
  }
}

/*******************************************************************/
UNIV_INTERN
ulint
buf_flush_LRU(
/*==========*/
  buf_pool_t* buf_pool, 
  ulint   min_n)    
{
  ulint   page_count;

  if (!buf_flush_start(buf_pool, BUF_FLUSH_LRU)) {
    return(ULINT_UNDEFINED);
  }

  page_count = buf_flush_batch(buf_pool, BUF_FLUSH_LRU, min_n, 0);

  buf_flush_end(buf_pool, BUF_FLUSH_LRU);

  buf_flush_common(BUF_FLUSH_LRU, page_count);

  return(page_count);
}

/*******************************************************************/
UNIV_INTERN
ulint
buf_flush_list(
/*===========*/
  ulint   min_n,    
  ib_uint64_t lsn_limit)  
{
  ulint   i;
  ulint   total_page_count = 0;
  ibool   skipped = FALSE;

  if (min_n != ULINT_MAX) {
    /* Ensure that flushing is spread evenly amongst the
    buffer pool instances. When min_n is ULINT_MAX
    we need to flush everything up to the lsn limit
    so no limit here. */
    min_n = (min_n + srv_buf_pool_instances - 1)
       / srv_buf_pool_instances;
  }

  /* Flush to lsn_limit in all buffer pool instances */
  for (i = 0; i < srv_buf_pool_instances; i++) {
    buf_pool_t* buf_pool;
    ulint   page_count = 0;

    buf_pool = buf_pool_from_array(i);

    if (!buf_flush_start(buf_pool, BUF_FLUSH_LIST)) {
      /* We have two choices here. If lsn_limit was
      specified then skipping an instance of buffer
      pool means we cannot guarantee that all pages
      up to lsn_limit has been flushed. We can
      return right now with failure or we can try
      to flush remaining buffer pools up to the
      lsn_limit. We attempt to flush other buffer
      pools based on the assumption that it will
      help in the retry which will follow the
      failure. */
      skipped = TRUE;

      continue;
    }

    page_count = buf_flush_batch(
      buf_pool, BUF_FLUSH_LIST, min_n, lsn_limit);

    buf_flush_end(buf_pool, BUF_FLUSH_LIST);

    buf_flush_common(BUF_FLUSH_LIST, page_count);

    total_page_count += page_count;
  }

  return(lsn_limit != IB_ULONGLONG_MAX && skipped
         ? ULINT_UNDEFINED : total_page_count);
}
 
/******************************************************************/
static
ulint
buf_flush_LRU_recommendation(
/*=========================*/
  buf_pool_t* buf_pool)   
{
  buf_page_t* bpage;
  ulint   n_replaceable;
  ulint   distance  = 0;

  buf_pool_mutex_enter(buf_pool);

  n_replaceable = UT_LIST_GET_LEN(buf_pool->free);

  bpage = UT_LIST_GET_LAST(buf_pool->LRU);

  while ((bpage != NULL)
         && (n_replaceable < BUF_FLUSH_FREE_BLOCK_MARGIN(buf_pool)
       + BUF_FLUSH_EXTRA_MARGIN(buf_pool))
         && (distance < BUF_LRU_FREE_SEARCH_LEN(buf_pool))) {

    mutex_t* block_mutex = buf_page_get_mutex(bpage);

    mutex_enter(block_mutex);

    if (buf_flush_ready_for_replace(bpage)) {
      n_replaceable++;
    }

    mutex_exit(block_mutex);

    distance++;

    bpage = UT_LIST_GET_PREV(LRU, bpage);
  }

  buf_pool_mutex_exit(buf_pool);

  if (n_replaceable >= BUF_FLUSH_FREE_BLOCK_MARGIN(buf_pool)) {

    return(0);
  }

  return(BUF_FLUSH_FREE_BLOCK_MARGIN(buf_pool)
         + BUF_FLUSH_EXTRA_MARGIN(buf_pool)
         - n_replaceable);
}

/*********************************************************************/
UNIV_INTERN
void
buf_flush_free_margin(
/*==================*/
  buf_pool_t* buf_pool)   
{
  ulint n_to_flush;

  n_to_flush = buf_flush_LRU_recommendation(buf_pool);

  if (n_to_flush > 0) {
    ulint n_flushed;

    n_flushed = buf_flush_LRU(buf_pool, n_to_flush);

    if (n_flushed == ULINT_UNDEFINED) {
      /* There was an LRU type flush batch already running;
      let us wait for it to end */

      buf_flush_wait_batch_end(buf_pool, BUF_FLUSH_LRU);
    }
  }
}

/*********************************************************************/
UNIV_INTERN
void
buf_flush_free_margins(void)
/*========================*/
{
  ulint i;

  for (i = 0; i < srv_buf_pool_instances; i++) {
    buf_pool_t* buf_pool;

    buf_pool = buf_pool_from_array(i);

    buf_flush_free_margin(buf_pool);
  }
}

/*********************************************************************
Update the historical stats that we are collecting for flush rate
heuristics at the end of each interval.
Flush rate heuristic depends on (a) rate of redo log generation and
(b) the rate at which LRU flush is happening. */
UNIV_INTERN
void
buf_flush_stat_update(void)
/*=======================*/
{
  buf_flush_stat_t* item;
  ib_uint64_t   lsn_diff;
  ib_uint64_t   lsn;
  ulint     n_flushed;

  lsn = log_get_lsn();
  if (buf_flush_stat_cur.redo == 0) {
    /* First time around. Just update the current LSN
    and return. */
    buf_flush_stat_cur.redo = lsn;
    return;
  }

  item = &buf_flush_stat_arr[buf_flush_stat_arr_ind];

  /* values for this interval */
  lsn_diff = lsn - buf_flush_stat_cur.redo;
  n_flushed = buf_lru_flush_page_count
        - buf_flush_stat_cur.n_flushed;

  /* add the current value and subtract the obsolete entry. */
  buf_flush_stat_sum.redo += lsn_diff - item->redo;
  buf_flush_stat_sum.n_flushed += n_flushed - item->n_flushed;

  /* put current entry in the array. */
  item->redo = lsn_diff;
  item->n_flushed = n_flushed;

  /* update the index */
  buf_flush_stat_arr_ind++;
  buf_flush_stat_arr_ind %= BUF_FLUSH_STAT_N_INTERVAL;

  /* reset the current entry. */
  buf_flush_stat_cur.redo = lsn;
  buf_flush_stat_cur.n_flushed = buf_lru_flush_page_count;
}

/*********************************************************************
Determines the fraction of dirty pages that need to be flushed based
on the speed at which we generate redo log. Note that if redo log
is generated at a significant rate without corresponding increase
in the number of dirty pages (for example, an in-memory workload)
it can cause IO bursts of flushing. This function implements heuristics
to avoid this burstiness.
@return number of dirty pages to be flushed / second */
UNIV_INTERN
ulint
buf_flush_get_desired_flush_rate(void)
/*==================================*/
{
  ulint   i;
  lint    rate;
  ulint   redo_avg;
  ulint   n_dirty = 0;
  ulint   n_flush_req;
  ulint   lru_flush_avg;
  ib_uint64_t lsn = log_get_lsn();
  ulint   log_capacity = log_get_capacity();

  /* log_capacity should never be zero after the initialization
  of log subsystem. */
  ut_ad(log_capacity != 0);

  /* Get total number of dirty pages. It is OK to access
  flush_list without holding any mutex as we are using this
  only for heuristics. */
  for (i = 0; i < srv_buf_pool_instances; i++) {
    buf_pool_t* buf_pool;

    buf_pool = buf_pool_from_array(i);
    n_dirty += UT_LIST_GET_LEN(buf_pool->flush_list);
  }

  /* An overflow can happen if we generate more than 2^32 bytes
  of redo in this interval i.e.: 4G of redo in 1 second. We can
  safely consider this as infinity because if we ever come close
  to 4G we'll start a synchronous flush of dirty pages. */
  /* redo_avg below is average at which redo is generated in
  past BUF_FLUSH_STAT_N_INTERVAL + redo generated in the current
  interval. */
  redo_avg = (ulint) (buf_flush_stat_sum.redo
          / BUF_FLUSH_STAT_N_INTERVAL
          + (lsn - buf_flush_stat_cur.redo));

  /* An overflow can happen possibly if we flush more than 2^32
  pages in BUF_FLUSH_STAT_N_INTERVAL. This is a very very
  unlikely scenario. Even when this happens it means that our
  flush rate will be off the mark. It won't affect correctness
  of any subsystem. */
  /* lru_flush_avg below is rate at which pages are flushed as
  part of LRU flush in past BUF_FLUSH_STAT_N_INTERVAL + the
  number of pages flushed in the current interval. */
  lru_flush_avg = buf_flush_stat_sum.n_flushed
      / BUF_FLUSH_STAT_N_INTERVAL
      + (buf_lru_flush_page_count
         - buf_flush_stat_cur.n_flushed);

  n_flush_req = (n_dirty * redo_avg) / log_capacity;

  /* The number of pages that we want to flush from the flush
  list is the difference between the required rate and the
  number of pages that we are historically flushing from the
  LRU list */
  rate = n_flush_req - lru_flush_avg;
  return(rate > 0 ? (ulint) rate : 0);
}

#if defined UNIV_DEBUG || defined UNIV_BUF_DEBUG
/******************************************************************/
static
ibool
buf_flush_validate_low(
/*===================*/
  buf_pool_t* buf_pool)   
{
  buf_page_t*   bpage;
  const ib_rbt_node_t*  rnode = NULL;

  ut_ad(buf_flush_list_mutex_own(buf_pool));

  UT_LIST_VALIDATE(list, buf_page_t, buf_pool->flush_list,
       ut_ad(ut_list_node_313->in_flush_list));

  bpage = UT_LIST_GET_FIRST(buf_pool->flush_list);

  /* If we are in recovery mode i.e.: flush_rbt != NULL
  then each block in the flush_list must also be present
  in the flush_rbt. */
  if (UNIV_LIKELY_NULL(buf_pool->flush_rbt)) {
    rnode = rbt_first(buf_pool->flush_rbt);
  }

  while (bpage != NULL) {
    const ib_uint64_t om = bpage->oldest_modification;

    ut_ad(buf_pool_from_bpage(bpage) == buf_pool);

    ut_ad(bpage->in_flush_list);

    /* A page in buf_pool->flush_list can be in
    BUF_BLOCK_REMOVE_HASH state. This happens when a page
    is in the middle of being relocated. In that case the
    original descriptor can have this state and still be
    in the flush list waiting to acquire the
    buf_pool->flush_list_mutex to complete the relocation. */
    ut_a(buf_page_in_file(bpage)
         || buf_page_get_state(bpage) == BUF_BLOCK_REMOVE_HASH);
    ut_a(om > 0);

    if (UNIV_LIKELY_NULL(buf_pool->flush_rbt)) {
      buf_page_t** prpage;

      ut_a(rnode);
      prpage = rbt_value(buf_page_t*, rnode);

      ut_a(*prpage);
      ut_a(*prpage == bpage);
      rnode = rbt_next(buf_pool->flush_rbt, rnode);
    }

    bpage = UT_LIST_GET_NEXT(list, bpage);

    ut_a(!bpage || om >= bpage->oldest_modification);
  }

  /* By this time we must have exhausted the traversal of
  flush_rbt (if active) as well. */
  ut_a(rnode == NULL);

  return(TRUE);
}

/******************************************************************/
UNIV_INTERN
ibool
buf_flush_validate(
/*===============*/
  buf_pool_t* buf_pool) 
{
  ibool ret;

  buf_flush_list_mutex_enter(buf_pool);

  ret = buf_flush_validate_low(buf_pool);

  buf_flush_list_mutex_exit(buf_pool);

  return(ret);
}
#endif /* UNIV_DEBUG || UNIV_BUF_DEBUG */
#endif /* !UNIV_HOTBACKUP */