linux-kernel - ffsb create_4k 16% regression

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Date:	Mon, 29 Jun 2009 16:06:58 +0800
From:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
To:	Jan Kara <jack@...e.cz>, Al Viro <viro@...iv.linux.org.uk>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: ffsb create_4k 16% regression

I run many ffsb test cases on JBODs (typically 13/12 disks). Comparing
with kernel 2.6.30, 2.6.31-rc1 has about 16% regression with
ffsb_create_4k. The sub test case creates files continuously for 10
minitues and every file is 1MB.

Bisect located below patch. 

5cee5815d1564bbbd505fea86f4550f1efdb5cd0 is first bad commit
commit 5cee5815d1564bbbd505fea86f4550f1efdb5cd0
Author: Jan Kara <jack@...e.cz>
Date:   Mon Apr 27 16:43:51 2009 +0200

    vfs: Make sys_sync() use fsync_super() (version 4)

    It is unnecessarily fragile to have two places (fsync_super() and do_sync())
    doing data integrity sync of the filesystem. Alter __fsync_super() to
    accommodate needs of both callers and use it. So after this patch
    __fsync_super() is the only place where we gather all the calls needed to
    properly send all data on a filesystem to disk.

As a matter of fact, ffsb calls sys_sync in the end to make sure all data is
flushed to disks and the flushing is counted into the result. vmstat shows
ffsb is blocked when syncing for a long time. With 2.6.30, ffsb is blocked for
a short time.

I checked the patch and did experiments to recover the original methods. Eventually,
the root cause is the patch deletes the calling to wakeup_pdflush when syncing, so
only ffsb is blocked on disk I/O. wakeup_pdflush could ask pdflush to write back pages
with ffsb at the same time.

Below patch against 2.6.31-rc1 fixes it.

Signed-off-by: Zhang Yanmin <yanmin_zhang@...ux.intel.com>

---

--- linux-2.6.31-rc1/fs/sync.c	2009-06-29 12:18:03.000000000 +0800
+++ linux-2.6.31-rc1_sync/fs/sync.c	2009-06-29 14:56:49.000000000 +0800
@@ -114,6 +114,7 @@ restart:

 SYSCALL_DEFINE0(sync)
 {
+	wakeup_pdflush(0);
 	sync_filesystems(0);
 	sync_filesystems(1);
 	if (unlikely(laptop_mode))

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