Re: performance with guests running 2.4 kernels (specifically RHEL3)

David S. Ahern
2008-04-22T20:23:07Z

I added tracers to kvm_mmu_page_fault() that include collecting tsc cycles:

1. before vcpu->arch.mmu.page_fault()
2. after vcpu->arch.mmu.page_fault()
3. after mmu_topup_memory_caches()
4. after emulate_instruction()

So the delta in the trace reports show:
- cycles required for arch.mmu.page_fault (tracer 2)
- cycles required for mmu_topup_memory_caches(tracer 3)
- cycles required for emulate_instruction() (tracer 4)

I captured trace data for ~5-seconds during one of the usual events (again this
time it was due to kscand in the guest). I ran the formatted trace data through
an awk script to summarize:

TSC cycles tracer2 tracer3 tracer4
0 - 10,000: 295067 213251 115873
10,001 - 25,000: 7682 1004 98336
25,001 - 50,000: 201 15 36
50,001 - 100,000: 100655 0 10
> 100,000: 117 0 15

This means vcpu->arch.mmu.page_fault() was called 403,722 times in the roughyl
5-second interval: 295,067 times it took < 10,000 cycles, but 100,772 times it
took longer than 50,000 cycles. The page_fault function getting run is
paging64_page_fault.

mmu_topup_memory_caches() and emulate_instruction() were both run 214,270 times,
most of them relatively quickly.

Note: I bumped the scheduling priority of the qemu threads to RR 1 so that few
host processes could interrupt it.

david

Avi Kivity wrote:
> David S. Ahern wrote:
>> I added the traces and captured data over another apparent lockup of
>> the guest.
>> This seems to be representative of the sequence (pid/vcpu removed).
>>
>> (+4776) VMEXIT [ exitcode = 0x00000000, rip = 0x00000000
>> c016127c ]
>> (+ 0) PAGE_FAULT [ errorcode = 0x00000003, virt = 0x00000000
>> c0009db4 ]
>> (+3632) VMENTRY
>> (+4552) VMEXIT [ exitcode = 0x00000000, rip = 0x00000000
>> c016104a ]
>> (+ 0) PAGE_FAULT [ errorcode = 0x0000000b, virt = 0x00000000
>> fffb61c8 ]
>> (+ 54928) VMENTRY
>>
>
> Can you oprofile the host to see where the 54K cycles are spent?
>
>> (+4568) VMEXIT [ exitcode = 0x00000000, rip = 0x00000000
>> c01610e7 ]
>> (+ 0) PAGE_FAULT [ errorcode = 0x00000003, virt = 0x00000000
>> c0009db4 ]
>> (+ 0) PTE_WRITE [ gpa = 0x00000000 00009db4 gpte = 0x00000000
>> 41c5d363 ]
>> (+8432) VMENTRY
>> (+3936) VMEXIT [ exitcode = 0x00000000, rip = 0x00000000
>> c01610ee ]
>> (+ 0) PAGE_FAULT [ errorcode = 0x00000003, virt = 0x00000000
>> c0009db0 ]
>> (+ 0) PTE_WRITE [ gpa = 0x00000000 00009db0 gpte = 0x00000000
>> 00000000 ]
>> (+ 13832) VMENTRY
>>
>>
>> (+5768) VMEXIT [ exitcode = 0x00000000, rip = 0x00000000
>> c016127c ]
>> (+ 0) PAGE_FAULT [ errorcode = 0x00000003, virt = 0x00000000
>> c0009db4 ]
>> (+3712) VMENTRY
>> (+4576) VMEXIT [ exitcode = 0x00000000, rip = 0x00000000
>> c016104a ]
>> (+ 0) PAGE_FAULT [ errorcode = 0x0000000b, virt = 0x00000000
>> fffb61d0 ]
>> (+ 0) PTE_WRITE [ gpa = 0x00000000 3d5981d0 gpte = 0x00000000
>> 3d55d047 ]
>>
>
> This indeed has the accessed bit clear.
>
>> (+ 65216) VMENTRY
>> (+4232) VMEXIT [ exitcode = 0x00000000, rip = 0x00000000
>> c01610e7 ]
>> (+ 0) PAGE_FAULT [ errorcode = 0x00000003, virt = 0x00000000
>> c0009db4 ]
>> (+ 0) PTE_WRITE [ gpa = 0x00000000 00009db4 gpte = 0x00000000
>> 3d598363 ]
>>
>
> This has the accessed bit set and the user bit clear, and the pte
> pointing at the previous pte_write gpa. Looks like a kmap_atomic().
>
>> (+8640) VMENTRY
>> (+3936) VMEXIT [ exitcode = 0x00000000, rip = 0x00000000
>> c01610ee ]
>> (+ 0) PAGE_FAULT [ errorcode = 0x00000003, virt = 0x00000000
>> c0009db0 ]
>> (+ 0) PTE_WRITE [ gpa = 0x00000000 00009db0 gpte = 0x00000000
>> 00000000 ]
>> (+ 14160) VMENTRY
>>
>> I can forward a more complete time snippet if you'd like. vcpu0 +
>> corresponding
>> vcpu1 files have 85000 total lines and compressed the files total ~500k.
>>
>> I did not see the FLOODED trace come out during this sample though I
>> did bump
>> the count from 3 to 4 as you suggested.
>>
>>
>>
>
> Bumping the count was supposed to remove the flooding...
>
>> Correlating rip addresses to the 2.4 kernel:
>>
>> c0160d00-c0161290 = page_referenced
>>
>> It looks like the event is kscand running through the pages. I
>> suspected this
>> some time ago, and tried tweaking the kscand_work_percent sysctl
>> variable. It
>> appeared to lower the peak of the spikes, but maybe I imagined it. I
>> believe
>> lowering that value makes kscand wake up more often but do less work
>> (page
>> scanning) each time it is awakened.
>>
>>
>
> What does 'top' in the guest show (perhaps sorted by total cpu time
> rather than instantaneous usage)?
>
> What host kernel are you running? How many host cpus?
>

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