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 writes 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
Signed-off-by: Jan Kara <jack@xxxxxxx>
---
fs/ext4/inode.c | 24 ++++++++++++++++++++++++
1 file changed, 24 insertions(+)
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index baa87e7d1426..ff55d430938b 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2171,6 +2171,9 @@ 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->io_submit.io_end)
+ return false;
map->m_lblk = lblk;
map->m_len = 1;
map->m_flags = bh->b_state & BH_FLAGS;
@@ -2223,6 +2226,9 @@ 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->io_submit.io_end)
+ return 0;
/* Everything mapped so far and we hit EOF */
break;
}
@@ -2739,6 +2745,21 @@ 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.
+ */
+ ret = mpage_prepare_extent_to_map(&mpd);
+ /* Submit prepared bio */
+ ext4_io_submit(&mpd.io_submit);
+ /* 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);
@@ -2767,6 +2788,7 @@ 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;
}
@@ -2816,6 +2838,7 @@ 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) {
/*
@@ -2831,6 +2854,7 @@ 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;
--
2.12.0
