* Sasha Levin <levinsasha928@xxxxxxxxx> wrote: > On Sat, 2011-06-04 at 11:38 +0200, Ingo Molnar wrote: > > * Sasha Levin <levinsasha928@xxxxxxxxx> wrote: > > > > > Coalescing MMIO allows us to avoid an exit every time we have a > > > MMIO write, instead - MMIO writes are coalesced in a ring which > > > can be flushed once an exit for a different reason is needed. > > > A MMIO exit is also trigged once the ring is full. > > > > > > Coalesce all MMIO regions registered in the MMIO mapper. > > > Add a coalescing handler under kvm_cpu. > > > > Does this have any effect on latency? I.e. does the guest side > > guarantee that the pending queue will be flushed after a group of > > updates have been done? > > Theres nothing that detects groups of MMIO writes, but the ring size is > a bit less than PAGE_SIZE (half of it is overhead - rest is data) and > we'll exit once the ring is full. But if the page is only filled partially and if mmio is not submitted by the guest indefinitely (say it runs a lot of user-space code) then the mmio remains pending in the partial-page buffer? If that's how it works then i *really* don't like this, this looks like a seriously mis-designed batching feature which might have improved a few server benchmarks but which will introduce random, hard to debug delays all around the place! Thanks, Ingo -- 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
