From: Jens Axboe <jens.axboe@oracle.com>
Date: Tue, 17 Feb 2009 12:59:08 +0000 (+0100)
Subject: block: revert part of 18ce3751ccd488c78d3827e9f6bf54e6322676fb
X-Git-Tag: v2.6.29-rc6~40^2~2
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=78f707bfc723552e8309b7c38a8d0cc51012e813;p=pandora-kernel.git

block: revert part of 18ce3751ccd488c78d3827e9f6bf54e6322676fb

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>
---

Reading git-diff-tree failed