Re: [PATCH 3/3] virtio-blk: Add bio-based IO path for virtio-blk

[Paolo Bonzini]

Il 03/07/2012 16:28, Dor Laor ha scritto:
>>>> Users using a spinning disk still get IO scheduling in the host though.
>>>> What benefit is there in doing it in the guest as well?
>>>
>>> The io scheduler waits for requests to merge and thus batch IOs
>>> together. It's not important w.r.t spinning disks since the host can
>>> do it but it causes much less vmexits which is the key issue for VMs.
>>
>> Does it make sense to use the guest's I/O scheduler at all?
> 
> That's the reason we have a noop io scheduler.

But is performance really better with noop?  We had to revert usage of
QUEUE_FLAG_NONROT in the guests because it caused performance
degradation (commit f8b12e513b953aebf30f8ff7d2de9be7e024dbbe).

The bio-based path is really QUEUE_FLAG_NONROT++, so it should really be
a special case for people who know what they're doing.  (Better would be
to improve QEMU, there's definitely room for 20% improvement...).

Paolo
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