From: Vivek Goyal <vgoyal@redhat.com>
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>,
Wu Fengguang <wfg@linux.intel.com>
Subject: Re: [PATCH] fix readahead pipeline break caused by block plug
Date: Tue, 31 Jan 2012 15:23:11 -0500 [thread overview]
Message-ID: <20120131202311.GB4378@redhat.com> (raw)
In-Reply-To: <20120131144734.GA4378@redhat.com>
On Tue, Jan 31, 2012 at 09:47:34AM -0500, Vivek Goyal wrote:
> 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.
>
> Why IO is not submitted because of plug? Doesn't task now get scheduled
> out causing an unplug? IOW, are we now busy waiting somewhere preventing
> unplug?
Ok, after putting some trace points I think now I understand what is
happening.
We submit some readahead IO to device request queue but because of nested
plug, queue never gets unplugged. When read logic reaches a page which is
not in page cache, it waits for page to be read from the disk
(lock_page_killable()) and that time we flush the plug list.
So effectively read ahead logic is kind of broken in parts because of
nested plugging. Removing top level plug (generic_file_aio_read()) for
buffered reads, will allow unplugging queue earlier for readahead.
Thanks
Vivek
next prev parent reply other threads:[~2012-01-31 20:23 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
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 [this message]
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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