linux-kernel - Re: Bad SSD performance with recent kernels

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Date:	Mon, 30 Jan 2012 22:48:21 +0800
From:	Wu Fengguang <wfg@...ux.intel.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Herbert Poetzl <herbert@...hfloor.at>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>,
	Li Shaohua <shaohua.li@...el.com>
Subject: Re: Bad SSD performance with recent kernels

On Sun, Jan 29, 2012 at 11:03:00AM +0100, Eric Dumazet wrote:

> Very strange, my bissection ended on following commit :
> 
> commit 805f6b5e1cbfedfb9b3d354013e7f4b13a79270f
> Author: Tao Ma <boyu.mt@...bao.com>
> Date:   Fri Mar 11 20:11:59 2011 +0100
> 
>     blktrace: Use rq->cmd_flags directly in blk_add_trace_rq.
> 
> This makes no sense.

Yeah, commit 55602dd66f535 ("fs: make generic file read/write
functions plug") looks like the real root cause and should probably
be reverted as a whole. Buffered read/write syscalls are not directly
tied to IO after all (readahead/writeback have the right plug points).

Thanks,
Fengguang

> hdparm uses 2MB block reads, so read_ahead (128KB) is too small for best
> perf
> 
> # cat /sys/class/block/sda/queue/read_ahead_kb
> 128
> 
> # dd if=/dev/sda of=/dev/null bs=128k 
> ^C
> 63744+0 enregistrements lus
> 63743+0 enregistrements écrits
> 8354922496 octets (8,4 GB) copiés, 39,975 s, 209 MB/s
> 
> # hdparm -t /dev/sda
> 
> /dev/sda:
>  Timing buffered disk reads: 510 MB in  3.00 seconds = 169.75 MB/sec
> 
> # uname -a
> Linux edumazet-laptop 3.0.0-15-generic #26-Ubuntu SMP Fri Jan 20
> 17:23:00 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux
> 
> 
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