On Tue, 19 Jun 2012 10:51:18 +0800, Asias He <asias@xxxxxxxxxx> 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@xxxxxxxxxx> wrote: > >>>> On 06/18/2012 03:46 PM, Rusty Russell wrote: > >>>>> On Mon, 18 Jun 2012 14:53:10 +0800, Asias He<asias@xxxxxxxxxx> 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. Confused. So, without merging we get 6k exits (per second?) How many do we get when we use the request-based IO path? If your device is slow, then you won't be able to make many requests per second: why worry about exit costs? If your device is fast (eg. ram), you've already shown that your patch is a win, right? Cheers, Rusty. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
