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

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Date:	Tue, 31 Jan 2012 03:17:51 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Wu Fengguang <wfg@...ux.intel.com>,
	Shaohua Li <shaohua.li@...el.com>,
	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 30 janvier 2012 23:26, Vivek Goyal <vgoyal@...hat.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 !
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