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

[Gerhard Wiesinger <lists@wiesinger.com>]

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