Re: [Qemu-devel] Re: QEMU-KVM and video performance

[Gerhard Wiesinger]

On Wed, 21 Apr 2010, Avi Kivity wrote:

> On 04/21/2010 09:50 PM, Gerhard Wiesinger wrote:
> > > > I don't think changing VGA window is a problem because there are
> > > > 500.000-1Mio changes/s possible.
> > >
> > > 1MB/s, 500k-1M changes/s.... Coincidence?  Is it taking a page fault
> > > or trap on every write?
> >
> >
> > To clarify:
> > Memory Performance writing to segmen A000 is about 1MB/st.
>
> That indicates a fault every write (assuming 8-16 bit writes).  If you're 
> using 256 color vga and not switching banks, this indicates a bug.

Yes, 256 color VGA and no bank switches involved.

> > Calling INT 10 set/get window function with different windows (e.g. 
> > toggling between window page 0 and 1) is about 500.000 to 1Mio function 
> > calls per second.
>
> That's suprisingly fast. I'd expect 100-200k/sec.

Sorry, I mixed up the numbers:
1.) QEMU-KVM: ~111k
2.) QEMU only: 500k-1Mio

> Please run kvm_stat and report output for both tests to confirm.

See below. 2nd column is per second statistic when running the test.

> > To get real good VGA performance both parameters should be:
> > About >50MB/s for writes to segment A000
> > ~500.000 bank switches per second.
>
> First should be doable easily, second is borderline.
>
> > I think this is very easy to distingish:
> > 1.) VGA Segment A000 is legacy and should be handled through QEMU and not 
> > through KVM (because it is much more faster). Also 16 color modes should be 
> > fast enough there.
> > 2.) All other flat PCI memory accesses should be handled through KVM (there 
> > is a specialized driver loaded for that PCI device in the non legacy OS).
> >
> > Is that easily possible?
>
> No.  Code can run in either qemu or kvm, not both.  You can switch between 
> them based on access statistics (early versions of qemu-kvm did that, without 
> the statistics part), but this isn't trivial.

Hmmm. Ok, 2 different opinions about the memory write performance:
Easily or not possible?

Thnx for you help so far.

Ciao,
Gerhard

--
http://www.wiesinger.com/

-------------
kvm_stat
Please mount debugfs ('mount -t debugfs debugfs /sys/kernel/debug')
and ensure the kvm modules are loaded

lsmod|grep -i kvm
kvm_amd                38276  0
kvm                   162288  1 kvm_amd

mount|grep -i debug

=>
mount -t debugfs debugfs /sys/kernel/debug

int10perf: INT10h Performance tests:
kvm statistics
 efer_reload                  0       0
 exits                 37648629  456206
 fpu_reload             8512535  455983
 halt_exits                2084       0
 halt_wakeup               2047       0
 host_state_reload      8513213  456011
 hypercalls                   0       0
 insn_emulation        29182065       0
 insn_emulation_fail          0       0
 invlpg                       0       0
 io_exits               8386082  455975
 irq_exits                51713     214
 irq_injections           21797      36
 irq_window                   0       0
 largepages                   0       0
 mmio_exits              242781       0
 mmu_cache_miss             150       0
 mmu_flooded                  0       0
 mmu_pde_zapped               0       0
 mmu_pte_updated              0       0
 mmu_pte_write             8192       0
 mmu_recycled                 0       0
 mmu_shadow_zapped          151       0
 mmu_unsync                   0       0
 mmu_unsync_global            0       0
 nmi_injections               0       0
 nmi_window                   0       0
 pf_fixed                 16935       0
 pf_guest                     0       0
 remote_tlb_flush             2       0
 request_irq                  0       0
 request_nmi                  0       0
 signal_exits                 1       0
 tlb_flush                 2251       0

Running VGA memory tests in same VGA page in Video Mode VESA 101h:
kvm statistics

 efer_reload                  0       0
 exits                 18470836  554582
 fpu_reload             2147833    3469
 halt_exits                2083       0
 halt_wakeup               2047       0
 host_state_reload      2148186    3470
 hypercalls                   0       0
 insn_emulation         7688203  554244
 insn_emulation_fail          0       0
 invlpg                       0       0
 io_exits              10701583      18
 irq_exits                50781     321
 irq_injections           25251      18
 irq_window                   0       0
 largepages                   0       0
 mmio_exits              162847    3241
 mmu_cache_miss             154       0
 mmu_flooded                  0       0
 mmu_pde_zapped               0       0
 mmu_pte_updated              0       0
 mmu_pte_write             8192       0
 mmu_recycled                 0       0
 mmu_shadow_zapped          155       0
 mmu_unsync                   0       0
 mmu_unsync_global            0       0
 nmi_injections               0       0
 nmi_window                   0       0
 pf_fixed                 16936       0
 pf_guest                     0       0
 remote_tlb_flush             5       0
 request_irq                  0       0
 request_nmi                  0       0
 signal_exits                 1       0
 tlb_flush                  112       0