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Subject[patch 41/58] block: revert part of 18ce3751ccd488c78d3827e9f6bf54e6322676fb
2.6.27-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Jens Axboe <jens.axboe@oracle.com>

commit 78f707bfc723552e8309b7c38a8d0cc51012e813 upstream.

The above commit added WRITE_SYNC and switched various places to using
that for committing writes that will be waited upon immediately after
submission. However, this causes a performance regression with AS and CFQ
for ext3 at least, since sync_dirty_buffer() will submit some writes with
WRITE_SYNC while ext3 has sumitted others dependent writes without the sync
flag set. This causes excessive anticipation/idling in the IO scheduler
because sync and async writes get interleaved, causing a big performance
regression for the below test case (which is meant to simulate sqlite
like behaviour).

---- test case ----
int main(int argc, char **argv)
{
int fdes, i;
FILE *fp;
struct timeval start;
struct timeval end;
struct timeval res;

gettimeofday(&start, NULL);
for (i=0; i<ROWS; i++) {
fp = fopen("test_file", "a");
fprintf(fp, "Some Text Data\n");
fdes = fileno(fp);
fsync(fdes);
fclose(fp);
}
gettimeofday(&end, NULL);
timersub(&end, &start, &res);
fprintf(stdout, "time to write %d lines is %ld(msec)\n", ROWS,
(res.tv_sec*1000000 + res.tv_usec)/1000);
return 0;
}
-------------------
Thanks to Sean.White@APCC.com for tracking down this performance
regression and providing a test case.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
fs/buffer.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -3042,7 +3042,7 @@ int sync_dirty_buffer(struct buffer_head
if (test_clear_buffer_dirty(bh)) {
get_bh(bh);
bh->b_end_io = end_buffer_write_sync;
- ret = submit_bh(WRITE_SYNC, bh);
+ ret = submit_bh(WRITE, bh);
wait_on_buffer(bh);
if (buffer_eopnotsupp(bh)) {
clear_buffer_eopnotsupp(bh);



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