From: Christoph Hellwig <hch@infradead.org>
To: Shaohua Li <shaohua.li@intel.com>
Cc: lkml <linux-kernel@vger.kernel.org>,
linux-mm <linux-mm@kvack.org>,
Andrew Morton <akpm@linux-foundation.org>,
Jens Axboe <axboe@kernel.dk>,
Herbert Poetzl <herbert@13thfloor.at>,
Eric Dumazet <eric.dumazet@gmail.com>,
Vivek Goyal <vgoyal@redhat.com>,
Wu Fengguang <wfg@linux.intel.com>
Subject: Re: [PATCH] fix readahead pipeline break caused by block plug
Date: Tue, 31 Jan 2012 03:36:40 -0500 [thread overview]
Message-ID: <20120131083640.GA13556@infradead.org> (raw)
In-Reply-To: <1327996780.21268.42.camel@sli10-conroe>
On Tue, Jan 31, 2012 at 03:59:40PM +0800, Shaohua Li wrote:
> Herbert Poetzl reported a performance regression since 2.6.39. The test
> is a simple dd read, but with big block size. The reason is:
>
> T1: ra (A, A+128k), (A+128k, A+256k)
> T2: lock_page for page A, submit the 256k
> T3: hit page A+128K, ra (A+256k, A+384). the range isn't submitted
> because of plug and there isn't any lock_page till we hit page A+256k
> because all pages from A to A+256k is in memory
> T4: hit page A+256k, ra (A+384, A+ 512). Because of plug, the range isn't
> submitted again.
> T5: lock_page A+256k, so (A+256k, A+512k) will be submitted. The task is
> waitting for (A+256k, A+512k) finish.
>
> There is no request to disk in T3 and T4, so readahead pipeline breaks.
>
> We really don't need block plug for generic_file_aio_read() for buffered
> I/O. The readahead already has plug and has fine grained control when I/O
> should be submitted. Deleting plug for buffered I/O fixes the regression.
>
> One side effect is plug makes the request size 256k, the size is 128k
> without it. This is because default ra size is 128k and not a reason we
> need plug here.
>
> Signed-off-by: Shaohua Li <shaohua.li@intel.com>
> Tested-by: Herbert Poetzl <herbert@13thfloor.at>
> Tested-by: Eric Dumazet <eric.dumazet@gmail.com>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Please also CC -stable on this.
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next prev parent reply other threads:[~2012-01-31 8:36 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-01-31 7:59 [PATCH] fix readahead pipeline break caused by block plug Shaohua Li
2012-01-31 8:36 ` Christoph Hellwig [this message]
2012-01-31 8:48 ` Eric Dumazet
2012-01-31 8:50 ` Herbert Poetzl
2012-01-31 8:53 ` Shaohua Li
2012-01-31 9:17 ` Eric Dumazet
2012-01-31 10:20 ` Wu Fengguang
2012-01-31 10:34 ` Wu Fengguang
2012-01-31 10:46 ` Christoph Hellwig
2012-01-31 10:57 ` Wu Fengguang
2012-01-31 11:34 ` Christoph Hellwig
2012-01-31 11:42 ` Wu Fengguang
2012-01-31 11:57 ` Christoph Hellwig
2012-01-31 12:20 ` Wu Fengguang
2012-02-01 2:25 ` Shaohua Li
2012-01-31 14:47 ` Vivek Goyal
2012-01-31 20:23 ` Vivek Goyal
2012-01-31 22:03 ` Vivek Goyal
2012-01-31 22:13 ` Andrew Morton
2012-01-31 22:22 ` Vivek Goyal
2012-02-01 3:36 ` Vivek Goyal
2012-02-01 7:10 ` Wu Fengguang
2012-02-01 16:01 ` Vivek Goyal
2012-02-01 9:18 ` Christoph Hellwig
2012-02-01 20:10 ` Vivek Goyal
2012-02-01 20:13 ` Jeff Moyer
2012-02-01 20:22 ` Andrew Morton
2012-02-01 7:02 ` Wu Fengguang
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