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

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Message-ID: <1327831380.14602.6.camel@edumazet-laptop>
Date:	Sun, 29 Jan 2012 11:03:00 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Wu Fengguang <wfg@...ux.intel.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>
Subject: Re: Bad SSD performance with recent kernels

Le dimanche 29 janvier 2012 à 13:59 +0800, Wu Fengguang a écrit :

> What's the block size? If it's < 4k, performance might be hurt.
> 
>         blockdev --getbsz /dev/sda
> 

# blockdev --getbsz /dev/sda
4096



> > FYI, I started a bisection.
> 
> Thank you! If the bisection would take much human time, it should be
> easier to collect some blktrace data on reading /dev/sda for analyzes.
> 

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.

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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