linux-kernel - Re: ffsb create

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Date:	Wed, 01 Jul 2009 13:33:00 +0800
From:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	LKML <linux-kernel@...r.kernel.org>, Jan Kara <jack@...e.cz>
Subject: Re: ffsb create_4k 16% regression

On Mon, 2009-06-29 at 16:06 +0800, Zhang, Yanmin wrote:
> 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))
Andrew,

Would you like to consider the patch for mm tree?

Thanks,
Yanmin


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