From: From: Eric Sandeen <sandeen@xxxxxxxxxx>
There was a bug reported on RHEL5 that a 10G dd on a 12G box
had a very, very slow sync after that.
At issue was the loop in write_cache_pages scanning all the way
to the end of the 10G file, even though the subsequent call
to mpage_da_submit_io would only actually write a smallish amt; then
we went back to the write_cache_pages loop ... wasting tons of time
in calling __mpage_da_writepage for thousands of pages we would
just revisit (many times) later.
Upstream it's not such a big issue for sys_sync because we get
to the loop with a much smaller nr_to_write, which limits the loop.
However, talking with Aneesh he realized that fsync upstream still
gets here with a very large nr_to_write and we face the same problem.
This patch makes mpage_add_bh_to_extent stop the loop after we've
accumulated 2048 pages, by setting mpd->io_done = 1; which ultimately
causes the write_cache_pages loop to break.
Repeating the test with a dirty_ratio of 80 (to leave something for
fsync to do), I don't see huge IO performance gains, but the reduction
in cpu usage is striking: 80% usage with stock, and 2% with the
below patch. Instrumenting the loop in write_cache_pages clearly
shows that we are wasting time here.
Eventually we need to change mpage_da_map_pages() also submit its I/O
to the block layer, subsuming mpage_da_submit_io(), and then change it
call ext4_get_blocks() multiple times.
Signed-off-by: Eric Sandeen <sandeen@xxxxxxxxxx>
Signed-off-by: "Theodore Ts'o" <tytso@xxxxxxx>
---
This is the slightly revised version of Eric's patch that I've added to
the ext4 patch queue. -- Ted
fs/ext4/inode.c | 9 +++++++++
1 files changed, 9 insertions(+), 0 deletions(-)
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 5c6ca10..2c12926 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2349,6 +2349,15 @@ static void mpage_add_bh_to_extent(struct mpage_da_data *mpd,
sector_t next;
int nrblocks = mpd->b_size >> mpd->inode->i_blkbits;
+ /*
+ * XXX Don't go larger than mballoc is willing to allocate
+ * This is a stopgap solution. We eventually need to fold
+ * mpage_da_submit_io() into this function and then call
+ * ext4_get_blocks() multiple times in a loop
+ */
+ if (nrblocks >= 8*1024*1024/mpd->inode->i_sb->s_blocksize)
+ goto flush_it;
+
/* check if thereserved journal credits might overflow */
if (!(EXT4_I(mpd->inode)->i_flags & EXT4_EXTENTS_FL)) {
if (nrblocks >= EXT4_MAX_TRANS_DATA) {
--
1.6.6.1.1.g974db.dirty
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