linux-kernel - Re: scsi-mq V2

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Date:	Thu, 10 Jul 2014 15:52:16 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	Benjamin LaHaise <bcrl@...ck.org>
CC:	Christoph Hellwig <hch@...radead.org>,
	"Elliott, Robert (Server Storage)" <Elliott@...com>,
	"dgilbert@...erlog.com" <dgilbert@...erlog.com>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Bart Van Assche <bvanassche@...ionio.com>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: scsi-mq V2

On 2014-07-10 15:50, Benjamin LaHaise wrote:
> On Thu, Jul 10, 2014 at 03:48:10PM +0200, Jens Axboe wrote:
>> On 2014-07-10 15:44, Benjamin LaHaise wrote:
>>> On Thu, Jul 10, 2014 at 03:39:57PM +0200, Jens Axboe wrote:
>>>> That's how fio always runs, it sets up the context with the exact queue
>>>> depth that it needs. Do we have a good enough understanding of other aio
>>>> use cases to say that this isn't the norm? I would expect it to be, it's
>>>> the way that the API would most obviously be used.
>>>
>>> The problem with this approach is that it works very poorly with per cpu
>>> reference counting's batching of references, which is pretty much a
>>> requirement now that many core systems are the norm.  Allocating the bare
>>> minimum is not the right thing to do today.  That said, the default limits
>>> on the number of requests probably needs to be raised.
>>
>> Sorry, that's a complete cop-out. Then you handle this internally,
>> allocate a bigger pool and cap the limit if you need to. Look at the
>> API. You pass in the number of requests you will use. Do you expect
>> anyone to double up, just in case? Will never happen.
>>
>> But all of this is side stepping the point that there's a real bug
>> reported here. The above could potentially explain the "it's using X
>> more CPU, or it's Y slower". The above is a softlock, it never completes.
>
> I'm not trying to cop out on this -- I'm asking for a data point to see
> if changing the request limits has any effect.

Fair enough, if the question is "does it solve the regression", then 
it's a valid data point. Rob/Doug, for fio, you can just double the 
iodepth passed in in engines/libaio:fio_libaio_init() and test with that 
and see if it makes a difference.

-- 
Jens Axboe

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