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

[Jens Axboe]

On Thu, Apr 18 2013, Roger Pau Monné wrote:
> On 18/04/13 16:26, Jens Axboe wrote:
> >>>>>>> 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.
> > 
> > What kernel are you testing? The plugging is usually what will trigger a
> > run of the queue, for rq based drivers. What does your fio job look
> > like?
> > 
> 
> I'm currently testing on top of Konrad for-jens-3.9 branch, which is
> 3.8.0-rc7. This is how my fio job looks like:
> 
> [read]
> rw=read
> size=900m
> bs=4k
> directory=/root/fio
> loops=100000
> 
> I've tried several bs sizes; 4k, 16k, 128k, 1m, 10m, and as far as I can
> see requests from the queue don't have more than 64 segments.
> 
> Also tried a simple dd with several bs sizes and got the same result.

So you're running normal buffered IO. Most of the IO should be coming
out of read-ahead, you could try tweaking the size of the window.

-- 
Jens Axboe

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