On Wed, Mar 30, 2011 at 01:22:43PM +0200, Gleb Natapov wrote:
> On Wed, Mar 30, 2011 at 12:50:53PM +0200, Avi Kivity wrote:
> > On 03/30/2011 12:47 PM, Gleb Natapov wrote:
> > >On Wed, Mar 30, 2011 at 12:38:53PM +0200, Avi Kivity wrote:
> > >> On 03/29/2011 02:08 PM, Gleb Natapov wrote:
> > >> >Currently we sync registers back and forth before/after exiting
> > >> >to userspace for IO, but during IO device model shouldn't need to
> > >> >read/write the registers, so we can as well skip those sync points. The
> > >> >only exaception is broken vmware backdor interface. The new code sync
> > >> >registers content during IO only if registers are read from/written to
> > >> >by userspace in the middle of the IO operation and this almost never
> > >> >happens in practise.
> > >>
> > >> While this is a nice idea, how much does it save in practice? It
> > >> does introduce more complexity.
> > >>
> > >
> > >I haven't measured, but can try to do so. It eliminates two copies of
> > >all registers on each MMIO/PIO read. I expect this to be measurable in
> > >workloads that do many such reads.
> > >
> >
> > I don't, especially if these are mmios to userspace. Perhaps it's
> > better to remove the copy on kernel mmio, since it's much faster, if
> > the result is simpler (there can be no KVM_SET_REGS in that
> > context).
> >
>
> The patch saves copying of 256 bytes on each MMIO/PIO read. It may not
> save a lot comparing to time it takes to do one MMIO to userspace, but
> do 1 million of those and you saved a lot of CPU cycles. I do not think
> we should abandon the optimization so easily. Unfortunately I can't run
> perf on 2.6.38 kernel right now. It gives me strange errors and when it
> doesn't it makes kernel OOPS.
>
After reboot perf started to work. I ran modified emulator.flat unit
test. It was modified to run test_cmps() in an endless loop.
Without patch:
1.71% qemu-system-x86 [kvm] [k] x86_emulate_instruction
1.51% qemu-system-x86 [kvm] [k] x86_emulate_instruction
1.68% qemu-system-x86 [kvm] [k] x86_emulate_instruction
With patch:
0.84% qemu-system-x86 [kvm] [k] x86_emulate_instruction
0.96% qemu-system-x86 [kvm] [k] x86_emulate_instruction
0.89% qemu-system-x86 [kvm] [k] x86_emulate_instruction
--
Gleb.
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