On Mon, Dec 01, 2014 at 12:33:15PM +0100, Cornelia Huck wrote:> On Mon, 1 Dec 2014 11:26:58 +0200 > "Michael S. Tsirkin" <mst at redhat.com> wrote: > > > For some places on data path, it might be worth it > > to cache the correct value e.g. as part of device > > structure. This replaces a branch with a memory load, > > so the gain would have to be measured, best done > > separately? > > I think we'll want to do some measuring once the basic structure is > in place anyway.What's meant by in place here?> We should make sure that e.g. s390 only takes minor > hit due to all that swapping that is needed for standard-compliant > devices. Caching the value might certainly help in some paths.Well, this is queued in linux-next for 3.19, so now's the time to do it :)
On Mon, 1 Dec 2014 13:46:45 +0200 "Michael S. Tsirkin" <mst at redhat.com> wrote:> On Mon, Dec 01, 2014 at 12:33:15PM +0100, Cornelia Huck wrote: > > On Mon, 1 Dec 2014 11:26:58 +0200 > > "Michael S. Tsirkin" <mst at redhat.com> wrote: > > > > > For some places on data path, it might be worth it > > > to cache the correct value e.g. as part of device > > > structure. This replaces a branch with a memory load, > > > so the gain would have to be measured, best done > > > separately? > > > > I think we'll want to do some measuring once the basic structure is > > in place anyway. > > What's meant by in place here?That this patchset is ready :)> > > We should make sure that e.g. s390 only takes minor > > hit due to all that swapping that is needed for standard-compliant > > devices. Caching the value might certainly help in some paths. > > Well, this is queued in linux-next for 3.19, so > now's the time to do it :)So much to do, so little time... I'm still feeling a bit uncomfortable with some of the changes (virtio-scsi etc.) as I have not been able to test them yet (as there's no converted qemu for these yet). The virtio-net and virtio-blk changes seem sane, though, and virtio-ccw should be fine as well. OTOH, it's not like we're introducing new external interfaces, so later rework should be fine.
On Mon, Dec 01, 2014 at 01:02:58PM +0100, Cornelia Huck wrote:> On Mon, 1 Dec 2014 13:46:45 +0200 > "Michael S. Tsirkin" <mst at redhat.com> wrote: > > > On Mon, Dec 01, 2014 at 12:33:15PM +0100, Cornelia Huck wrote: > > > On Mon, 1 Dec 2014 11:26:58 +0200 > > > "Michael S. Tsirkin" <mst at redhat.com> wrote: > > > > > > > For some places on data path, it might be worth it > > > > to cache the correct value e.g. as part of device > > > > structure. This replaces a branch with a memory load, > > > > so the gain would have to be measured, best done > > > > separately? > > > > > > I think we'll want to do some measuring once the basic structure is > > > in place anyway. > > > > What's meant by in place here? > > That this patchset is ready :) > > > > > > We should make sure that e.g. s390 only takes minor > > > hit due to all that swapping that is needed for standard-compliant > > > devices. Caching the value might certainly help in some paths. > > > > Well, this is queued in linux-next for 3.19, so > > now's the time to do it :) > > So much to do, so little time... > > I'm still feeling a bit uncomfortable with some of the changes > (virtio-scsi etc.) as I have not been able to test them yet (as there's > no converted qemu for these yet). The virtio-net and virtio-blk changes > seem sane, though, and virtio-ccw should be fine as well. > > OTOH, it's not like we're introducing new external interfaces, so later > rework should be fine.Right. I'll send a revision with virtio console and the rest of devices shortly.
On Mon, Dec 01, 2014 at 01:02:58PM +0100, Cornelia Huck wrote:> On Mon, 1 Dec 2014 13:46:45 +0200 > "Michael S. Tsirkin" <mst at redhat.com> wrote: > > > On Mon, Dec 01, 2014 at 12:33:15PM +0100, Cornelia Huck wrote: > > > On Mon, 1 Dec 2014 11:26:58 +0200 > > > "Michael S. Tsirkin" <mst at redhat.com> wrote: > > > > > > > For some places on data path, it might be worth it > > > > to cache the correct value e.g. as part of device > > > > structure. This replaces a branch with a memory load, > > > > so the gain would have to be measured, best done > > > > separately? > > > > > > I think we'll want to do some measuring once the basic structure is > > > in place anyway. > > > > What's meant by in place here? > > That this patchset is ready :)Also it's ready to the level where benchmarking is possible, right? I don't think you should wait until we finish polishing up commit messages.> > > > > We should make sure that e.g. s390 only takes minor > > > hit due to all that swapping that is needed for standard-compliant > > > devices. Caching the value might certainly help in some paths. > > > > Well, this is queued in linux-next for 3.19, so > > now's the time to do it :) > > So much to do, so little time... > > I'm still feeling a bit uncomfortable with some of the changes > (virtio-scsi etc.) as I have not been able to test them yet (as there's > no converted qemu for these yet). The virtio-net and virtio-blk changes > seem sane, though, and virtio-ccw should be fine as well. > > OTOH, it's not like we're introducing new external interfaces, so later > rework should be fine.