linux-kernel - Re: [PATCH v1 7/7] xen-block: implement indirect descriptors

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Date:	Thu, 18 Apr 2013 16:16:59 +0200
From:	Roger Pau Monné <roger.pau@...rix.com>
To:	Jens Axboe <axboe@...nel.dk>
CC:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"martin.peterson@...cle.com" <martin.peterson@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>
Subject: Re: [PATCH v1 7/7] xen-block: implement indirect descriptors

On 18/04/13 14:43, Jens Axboe wrote:
> On Wed, Apr 17 2013, Konrad Rzeszutek Wilk wrote:
>> On Wed, Apr 17, 2013 at 07:04:51PM +0200, Roger Pau Monné wrote:
>>> On 17/04/13 16:25, Konrad Rzeszutek Wilk wrote:
>>>>>> Perhaps the xen-blkfront part of the patch should be just split out to make
>>>>>> this easier?
>>>>>>
>>>>>> Perhaps what we really should have is just the 'max' value of megabytes
>>>>>> we want to handle on the ring.
>>>>>>
>>>>>> As right now 32 ring requests * 32 segments = 4MB.  But if the user wants
>>>>>> to se the max: 32 * 4096 = so 512MB (right? each request would handle now 16MB
>>>>>> and since we have 32 of them = 512MB).
>>>>>
>>>>> I've just set that to something that brings a performance benefit
>>>>> without having to map an insane number of persistent grants in blkback.
>>>>>
>>>>> Yes, the values are correct, but the device request queue (rq) is only
>>>>> able to provide read requests with 64 segments or write requests with
>>>>> 128 segments. I haven't been able to get larger requests, even when
>>>>> setting this to 512 or higer.
>>>>
>>>> What are you using to drive the requests? 'fio'?
>>>
>>> Yes, I've tried fio with several "bs=" values, but it doesn't seem to
>>> change the size of the underlying requests. Have you been able to get
>>> bigger requests?
>>
>> Martin, Jens,
>> Any way to drive more than 128 segments?
> 
> If the driver is bio based, then there's a natural size constraint on
> the number of vecs in the bio. So to get truly large requests, the
> driver would need to merge incoming sequential IOs (similar to how it's
> done for rq based drivers).

When you say rq based drivers, you mean drivers with a request queue?

We are already using a request queue in blkfront, and I'm setting the
maximum number of segments per request using:

blk_queue_max_segments(<rq>, <segments>);

But even when setting <segments> to 256 or 512, I only get read requests
with 64 segments and write requests with 128 segments from the queue.

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