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

Gerhard Wiesinger
2010-04-22T06:05:03Z

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