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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

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.

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