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Commit dddbd6ac authored by Jan Kara's avatar Jan Kara Committed by Theodore Ts'o
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ext4: avoid unnecessary transaction stalls during writeback 



Currently ext4_writepages() submits all pages with transaction started.
When no page needs block allocation or extent conversion we can submit
all dirty pages in the inode while holding a single transaction handle
and when device is congested this can take significant amount of time.
Thus ext4_writepages() can block transaction commits for extended
periods of time.

Take for example a simple benchmark simulating PostgreSQL database
(pgioperf in mmtest). The benchmark runs 16 processes doing random reads
from a huge file, one process doing random writes to the huge file, and
one process doing sequential writes to a small files and frequently
running fsync. With unpatched kernel transaction commits take on average
~18s with standard deviation of ~41s, top 5 commit times are:

274.466639s, 126.467347s, 86.992429s, 34.351563s, 31.517653s.

After this patch transaction commits take on average 0.1s with standard
deviation of 0.15s, top 5 commit times are:

0.563792s, 0.519980s, 0.509841s, 0.471700s, 0.469899s

[ Modified so we use an explicit do_map flag instead of relying on
  io_end not being allocated, the since io_end->inode is needed for I/O
  error handling. -- tytso ]

Signed-off-by: default avatarJan Kara <jack@suse.cz>
Signed-off-by: default avatarTheodore Ts'o <tytso@mit.edu>
parent 85c8f176

fs/ext4/inode.c

+35 −0
Original line number Diff line number Diff line
@@ -1643,6 +1643,7 @@ struct mpage_da_data { 
	 */

	struct ext4_map_blocks map;

	struct ext4_io_submit io_submit;	/* IO submission data */

	unsigned int do_map:1;

};



static void mpage_release_unused_pages(struct mpage_da_data *mpd,
@@ -2179,6 +2180,9 @@ static bool mpage_add_bh_to_extent(struct mpage_da_data *mpd, ext4_lblk_t lblk, 


	/* First block in the extent? */

	if (map->m_len == 0) {

		/* We cannot map unless handle is started... */

		if (!mpd->do_map)

			return false;

		map->m_lblk = lblk;

		map->m_len = 1;

		map->m_flags = bh->b_state & BH_FLAGS;
@@ -2231,6 +2235,9 @@ static int mpage_process_page_bufs(struct mpage_da_data *mpd, 
			/* Found extent to map? */

			if (mpd->map.m_len)

				return 0;

			/* Buffer needs mapping and handle is not started? */

			if (!mpd->do_map)

				return 0;

			/* Everything mapped so far and we hit EOF */

			break;

		}
@@ -2747,6 +2754,29 @@ static int ext4_writepages(struct address_space *mapping, 
		tag_pages_for_writeback(mapping, mpd.first_page, mpd.last_page);

	done = false;

	blk_start_plug(&plug);



	/*

	 * First writeback pages that don't need mapping - we can avoid

	 * starting a transaction unnecessarily and also avoid being blocked

	 * in the block layer on device congestion while having transaction

	 * started.

	 */

	mpd.do_map = 0;

	mpd.io_submit.io_end = ext4_init_io_end(inode, GFP_KERNEL);

	if (!mpd.io_submit.io_end) {

		ret = -ENOMEM;

		goto unplug;

	}

	ret = mpage_prepare_extent_to_map(&mpd);

	/* Submit prepared bio */

	ext4_io_submit(&mpd.io_submit);

	ext4_put_io_end_defer(mpd.io_submit.io_end);

	mpd.io_submit.io_end = NULL;

	/* Unlock pages we didn't use */

	mpage_release_unused_pages(&mpd, false);

	if (ret < 0)

		goto unplug;



	while (!done && mpd.first_page <= mpd.last_page) {

		/* For each extent of pages we use new io_end */

		mpd.io_submit.io_end = ext4_init_io_end(inode, GFP_KERNEL);
@@ -2775,8 +2805,10 @@ static int ext4_writepages(struct address_space *mapping, 
				wbc->nr_to_write, inode->i_ino, ret);

			/* Release allocated io_end */

			ext4_put_io_end(mpd.io_submit.io_end);

			mpd.io_submit.io_end = NULL;

			break;

		}

		mpd.do_map = 1;



		trace_ext4_da_write_pages(inode, mpd.first_page, mpd.wbc);

		ret = mpage_prepare_extent_to_map(&mpd);
@@ -2807,6 +2839,7 @@ static int ext4_writepages(struct address_space *mapping, 
		if (!ext4_handle_valid(handle) || handle->h_sync == 0) {

			ext4_journal_stop(handle);

			handle = NULL;

			mpd.do_map = 0;

		}

		/* Submit prepared bio */

		ext4_io_submit(&mpd.io_submit);
@@ -2824,6 +2857,7 @@ static int ext4_writepages(struct address_space *mapping, 
			ext4_journal_stop(handle);

		} else

			ext4_put_io_end(mpd.io_submit.io_end);

		mpd.io_submit.io_end = NULL;



		if (ret == -ENOSPC && sbi->s_journal) {

			/*
@@ -2839,6 +2873,7 @@ static int ext4_writepages(struct address_space *mapping, 
		if (ret)

			break;

	}

unplug:

	blk_finish_plug(&plug);

	if (!ret && !cycled && wbc->nr_to_write > 0) {

		cycled = 1;