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From: Eric Dumazet <eric.dumazet@gmail.com>
To: Vivek Goyal <vgoyal@redhat.com>
Cc: Wu Fengguang <wfg@linux.intel.com>,
	Shaohua Li <shaohua.li@intel.com>,
	Herbert Poetzl <herbert@13thfloor.at>,
	Andrew Morton <akpm@linux-foundation.org>,
	LKML <linux-kernel@vger.kernel.org>, Jens Axboe <axboe@kernel.dk>,
	Tejun Heo <tj@kernel.org>
Subject: Re: Bad SSD performance with recent kernels
Date: Tue, 31 Jan 2012 03:17:51 +0100	[thread overview]
Message-ID: <CAL4WiiohOtcxkd9RLnJM0guN93SdPJy8Nzi6cV-fZ0-gVnTrQA@mail.gmail.com> (raw)
In-Reply-To: <20120130222643.GH30245@redhat.com>

Le 30 janvier 2012 23:26, Vivek Goyal <vgoyal@redhat.com> a écrit :
>
> Can you please run the blktrace again with this patch applied. I am curious
> to see how does traffic pattern look like now.
>
> In your previous trace, there were so many small 8 sector requests which
> were merged into 512 sector requests before dispatching to disk. (I am
> not sure why those requests are not bigger. Shouldn't readahead logic
> submit a bigger request?) Now with plug/unplug logic removed, I am assuming
> we should be doing less merging and dispatching more smaller requests. May be
> that is helping and cutting down on disk idling time.
>
> In previous logs, 512 sector request seems to be taking around 1ms to
> complete after dispatch. In between requests disk seems to be idle
> for around .5 to .6 ms. Out of this .3 ms seems to be gone in just
> coming up with new request after completion of previous one and another
> .3ms seems to be consumed in merging the smaller IOs. So if we don't wait
> for merging, it should keep disk busier for .3ms more which is 30% of time
> it takes to complete 512 sector request. So theoritically it can give
> 30% boost for this workload. (Assuming request size will not impact the
> disk throughput very severely).
>
> Anyway, some blktrace data will shed some light..
>
> Thanks
> Vivek


I'll do that when I have access to the machine again, in 6 hours or so :)

Thanks !

  parent reply	other threads:[~2012-01-31  2:17 UTC|newest]

Thread overview: 36+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-01-27  6:00 Bad SSD performance with recent kernels Herbert Poetzl
2012-01-27  6:44 ` Eric Dumazet
2012-01-28 12:51 ` Wu Fengguang
2012-01-28 13:33   ` Eric Dumazet
2012-01-29  5:59     ` Wu Fengguang
2012-01-29  8:42       ` Herbert Poetzl
2012-01-29  9:28         ` Wu Fengguang
2012-01-29 10:03       ` Eric Dumazet
2012-01-29 11:16         ` Wu Fengguang
2012-01-29 13:13           ` Eric Dumazet
2012-01-29 15:52             ` Pádraig Brady
2012-01-29 16:10             ` Wu Fengguang
2012-01-29 20:15               ` Herbert Poetzl
2012-01-30 11:18                 ` Wu Fengguang
2012-01-30 12:34                   ` Eric Dumazet
2012-01-30 14:01                     ` Wu Fengguang
2012-01-30 14:05                       ` Wu Fengguang
2012-01-30  3:17               ` Shaohua Li
2012-01-30  5:31                 ` Eric Dumazet
2012-01-30  5:45                   ` Shaohua Li
2012-01-30  7:13                 ` Herbert Poetzl
2012-01-30  7:22                   ` Shaohua Li
2012-01-30  7:36                     ` Herbert Poetzl
2012-01-30  8:12                       ` Shaohua Li
2012-01-30 10:31                         ` Shaohua Li
2012-01-30 14:28                           ` Wu Fengguang
2012-01-30 14:51                             ` Eric Dumazet
2012-01-30 22:26                               ` Vivek Goyal
2012-01-31  0:14                                 ` Shaohua Li
2012-01-31  1:07                                   ` Wu Fengguang
2012-01-31  3:00                                     ` Shaohua Li
2012-01-31  2:17                                 ` Eric Dumazet [this message]
2012-01-31  8:46                                 ` Eric Dumazet
2012-01-31  6:36                             ` Herbert Poetzl
2012-01-30 14:48         ` Wu Fengguang
2012-01-28 17:01   ` Herbert Poetzl

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