Re: scsi-mq V2

From: Jens Axboe
Date: Thu Jul 10 2014 - 09:52:27 EST

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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