On 12/05/2014 04:50 AM, Peter Lieven wrote: > this patch finally introduce multiread support to virtio-blk while
s/introduce/introduces/ s/virtio-blk while/virtio-blk. While/ > multiwrite support was there for a long time read support was missing. s/time/time,/ > > To achieve this the patch does serveral things which might need futher s/serveral/several/ s/futher/further/ > explaination: s/explaination/explanation/ > > - the whole merge and multireq logic is moved from block.c into > virtio-blk. This is move is a preparation for directly creating a > coroutine out of virtio-blk. Can this move be done as a separate prerequisite patch? Mixing code motion and new features in the same patch is harder to review. > > - requests are only merged if they are strictly sequential and no s/sequential/sequential,/ > longer sorted. This simplification decreases overhead and reduces > latency. It will also merge some requests which were unmergable before. > > The old algorithm took up to 32 requests sorted them and tried to merge s/requests/requests,/ > them. The outcome was anything between 1 and 32 requests. In case of > 32 requests there were 31 requests unnecessarily delayed. > > On the other hand lets imagine e.g. 16 unmergeable requests followed s/lets/let's/ > by 32 mergable requests. The latter 32 requests would have been split > into two 16 byte requests. > > Last the simplified logic allows for a fast path if we have only a > single request in the multirequest. In this case the request is sent as > ordinary request without mulltireq callbacks. s/mulltireq/multireq/ > > As a first benchmark I installed Ubuntu 14.04.1 on a ramdisk. The number of > merged requests is in the same order while the latency is slightly decreased. > One should not stick too much to the numbers because the number of wr_requests > are highly fluctuant. I hope the numbers show that this patch is at least > not causing too big harm: > I'll leave the actual patch review to developers more knowledgeable about block behavior. -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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