Messages in this thread | | | Date | Tue, 19 Jun 2012 09:21:19 +0300 | From | Dor Laor <> | Subject | Re: [PATCH 3/3] virtio-blk: Add bio-based IO path for virtio-blk |
| |
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?
| |