Messages in this thread | | | Date | Thu, 21 Jun 2012 12:49:12 +0300 | From | Dor Laor <> | Subject | Re: [PATCH 3/3] virtio-blk: Add bio-based IO path for virtio-blk |
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On 06/20/2012 07:46 AM, Asias He wrote: > 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? >
Does it merge 20 request _on top_ of what event idx does? Of course if that's the case, we should keep that.
Dor
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