linux-kernel - Re: [PATCH] fix readahead pipeline break caused by block plug

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Message-Id: <20120131141301.ba35ffe0.akpm@linux-foundation.org>
Date:	Tue, 31 Jan 2012 14:13:01 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Shaohua Li <shaohua.li@...el.com>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>, Jens Axboe <axboe@...nel.dk>,
	Herbert Poetzl <herbert@...hfloor.at>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Wu Fengguang <wfg@...ux.intel.com>
Subject: Re: [PATCH] fix readahead pipeline break caused by block plug

On Tue, 31 Jan 2012 17:03:33 -0500
Vivek Goyal <vgoyal@...hat.com> 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.
> > 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.
> 
> For me, this patch helps only so much and does not get back all the
> performance lost in case of raw disk read. It does improve the throughput
> from around 85-90 MB/s to 110-120 MB/s but running the same dd with
> iflag=direct, gets me more than 250MB/s.
> 
> # echo 3 > /proc/sys/vm/drop_caches 
> # dd if=/dev/sdb of=/dev/null bs=1M count=1K
> 1024+0 records in
> 1024+0 records out
> 1073741824 bytes (1.1 GB) copied, 9.03305 s, 119 MB/s
> 
> echo 3 > /proc/sys/vm/drop_caches 
> # dd if=/dev/sdb of=/dev/null bs=1M count=1K iflag=direct
> 1024+0 records in
> 1024+0 records out
> 1073741824 bytes (1.1 GB) copied, 4.07426 s, 264 MB/s

Buffered I/O against the block device has a tradition of doing Weird
Things.  Do you see the same behavior when reading from a regular file?

> I think it is happening because in case of raw read we are submitting
> one page at a time to request queue

(That's not a raw read - it's using pagecache.  Please get the terms right!)

We've never really bothered making the /dev/sda[X] I/O very efficient
for large I/O's under the (probably wrong) assumption that it isn't a
very interesting case.  Regular files will (or should) use the mpage
functions, via address_space_operations.readpages().  fs/blockdev.c
doesn't even implement it.

> and by the time all the pages
> are submitted and one big merged request is formed it wates lot of time.

But that was the case in eariler kernels too.  Why did it change?


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