linux-kernel - Re: [RESEND] [PATCH] readahead:add blk_run_backing

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Date:	Thu, 16 Jul 2009 14:36:03 +0400
From:	Vladislav Bolkhovitin <vst@...b.net>
To:	Ronald Moesbergen <intercommit@...il.com>
CC:	fengguang.wu@...el.com, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, kosaki.motohiro@...fujitsu.com,
	Alan.Brunelle@...com, linux-fsdevel@...r.kernel.org,
	jens.axboe@...cle.com, randy.dunlap@...cle.com,
	Bart Van Assche <bart.vanassche@...il.com>
Subject: Re: [RESEND] [PATCH] readahead:add blk_run_backing_dev


Ronald Moesbergen, on 07/16/2009 11:32 AM wrote:
> 2009/7/15 Vladislav Bolkhovitin <vst@...b.net>:
>>> The drop with 64 max_sectors_kb on the client is a consequence of how CFQ
>>> is working. I can't find the exact code responsible for this, but from all
>>> signs, CFQ stops delaying requests if amount of outstanding requests exceeds
>>> some threshold, which is 2 or 3. With 64 max_sectors_kb and 5 SCST I/O
>>> threads this threshold is exceeded, so CFQ doesn't recover order of
>>> requests, hence the performance drop. With default 512 max_sectors_kb and
>>> 128K RA the server sees at max 2 requests at time.
>>>
>>> Ronald, can you perform the same tests with 1 and 2 SCST I/O threads,
>>> please?
> 
> Ok. Should I still use the file-on-xfs testcase for this, or should I
> go back to using a regular block device?

Yes, please

> The file-over-iscsi is quite
> uncommon I suppose, most people will export a block device over iscsi,
> not a file.

No, files are common. The main reason why people use direct block 
devices is a not supported by anything believe that comparing with files 
they "have less overhead", so "should be faster". But it isn't true and 
can be easily checked.

>> With context-RA patch, please, in those and future tests, since it should
>> make RA for cooperative threads much better.
>>
>>> You can limit amount of SCST I/O threads by num_threads parameter of
>>> scst_vdisk module.
> 
> Ok, I'll try that and include the blk_run_backing_dev,
> readahead-context and io_context patches.
> 
> Ronald.
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