linux-kernel - Re: scsi-mq V2

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Message-ID: <20140710135034.GR12478@kvack.org>
Date:	Thu, 10 Jul 2014 09:50:34 -0400
From:	Benjamin LaHaise <bcrl@...ck.org>
To:	Jens Axboe <axboe@...nel.dk>
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 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.

		-ben

> -- 
> Jens Axboe

-- 
"Thought is the essence of where you are now."
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