linux-kernel - Re: [PATCH v2] NFS: introduce writeback wait queue

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Date:	Mon, 5 Oct 2009 07:01:10 -0400
From:	"Myklebust, Trond" <Trond.Myklebust@...app.com>
To:	"Wu Fengguang" <fengguang.wu@...el.com>
Cc:	<jens.axboe@...cle.com>, "Chris Mason" <chris.mason@...cle.com>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	<linux-fsdevel@...r.kernel.org>,
	"LKML" <linux-kernel@...r.kernel.org>, <linux-nfs@...r.kernel.org>
Subject: Re: [PATCH v2] NFS: introduce writeback wait queue

On Oct 5, 2009, at 3:11, "Wu Fengguang" <fengguang.wu@...el.com> wrote:

> Hi all,
>
> This version makes two standalone functions for easier reuse.
>
> Before patch, nr_writeback is near 1G on my 2GB laptop:
>
>     nr_writeback         nr_dirty      nr_unstable
>           203994                2           154469
>           203994                2           154469
>
> After patch, nr_writeback is limited to nfs_congestion_kb=42MB.
>
>     nr_writeback         nr_dirty      nr_unstable
>            11180            34195            11754
>             9865            36821             8234
>            10137            36695             9338
>
> One minor problem I noticed is, NFS writeback is not very smooth.
> This per 0.1s sampled trace shows that it can sometimes stuck for
> up to 0.5s:
>
>     nr_writeback         nr_dirty      nr_unstable
>            11055            37408             9599
>            10311            37315            10529
>            10869            35920            11459
>            10869            35920            11459
>            10869            35920            11459
>            10869            35920            11459
>            10869            35920            11459
>            10838            35891            10042
>            10466            35891            10414
>            10900            34744            11437
>            10249            34744            12088
>            10249            34744            12088
>            10249            34744            12088
>            10249            34744            12088
>            10249            34744            12088
>            10249            34744            12088
>            10133            34743            10663
>            10505            34743            11035
>            10970            34991            11345
>            10691            34991            11593
>            10691            34991            11593
>            10691            34991            11593
>            10691            34991            11593
>            10691            34991            11593
>
> Trond, I guess nr_writeback/nr_unstable are decreased in async RPC
> "complete" events. It is understandable that nr_dirty can sometimes
> stuck on local waits, but the "local determined" nr_dirty and "remote
> determined" nr_writeback/nr_unstable tend to stuck at the same time?
> Did I miss something (that could be obvious to you)?

It looks (at 7am in the morning after getting up at 4:30am) as though  
the number of unstable pages is remaining constant, which would mean  
that we're sending a lot of COMMIT requests (see nfsstat). Since  
COMMIT is essentially an fsync call, it means that the server is going  
to be slower.

Cheers
     Trond
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