Messages in this thread | | | Date | Wed, 20 Jun 2012 12:46:43 +0800 | From | Asias He <> | Subject | Re: [PATCH 3/3] virtio-blk: Add bio-based IO path for virtio-blk |
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On 06/19/2012 02:21 PM, Dor Laor wrote: > On 06/19/2012 05:51 AM, Asias He wrote: >> On 06/18/2012 07:39 PM, Sasha Levin wrote: >>> On Mon, 2012-06-18 at 14:14 +0300, Dor Laor wrote: >>>> On 06/18/2012 01:05 PM, Rusty Russell wrote: >>>>> On Mon, 18 Jun 2012 16:03:23 +0800, Asias He<asias@redhat.com> wrote: >>>>>> On 06/18/2012 03:46 PM, Rusty Russell wrote: >>>>>>> On Mon, 18 Jun 2012 14:53:10 +0800, Asias He<asias@redhat.com> >>>>>>> wrote: >>>>>>>> This patch introduces bio-based IO path for virtio-blk. >>>>>>> >>>>>>> Why make it optional? >>>>>> >>>>>> request-based IO path is useful for users who do not want to bypass >>>>>> the >>>>>> IO scheduler in guest kernel, e.g. users using spinning disk. For >>>>>> users >>>>>> using fast disk device, e.g. SSD device, they can use bio-based IO >>>>>> path. >>>>> >>>>> 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. >>> >>> Is the amount of exits caused by virtio-blk significant at all with >>> EVENT_IDX? >> >> Yes. EVENT_IDX saves the number of notify and interrupt. Let's take the >> interrupt as an example, The guest fires 200K request to host, the >> number of interrupt is about 6K thanks to EVENT_IDX. The ratio is 200K / >> 6K = 33. The ratio of merging is 40000K / 200K = 20. >> > > In this case, why don't you always recommend bio over request based?
This case shows that IO scheduler's merging in guest saves a lot of requests to host side. Why should I recommend bio over request based here?
-- Asias
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