Messages in this thread | | | Date | Tue, 28 May 2013 23:02:09 +0800 | From | Xiao Guangrong <> | Subject | Re: [PATCH v7 04/11] KVM: MMU: zap pages in batch |
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On 05/28/2013 08:18 AM, Marcelo Tosatti wrote: > On Mon, May 27, 2013 at 10:20:12AM +0800, Xiao Guangrong wrote: >> On 05/25/2013 04:34 AM, Marcelo Tosatti wrote: >>> On Thu, May 23, 2013 at 03:55:53AM +0800, Xiao Guangrong wrote: >>>> Zap at lease 10 pages before releasing mmu-lock to reduce the overload >>>> caused by requiring lock >>>> >>>> After the patch, kvm_zap_obsolete_pages can forward progress anyway, >>>> so update the comments >>>> >>>> [ It improves kernel building 0.6% ~ 1% ] >>> >>> Can you please describe the overload in more detail? Under what scenario >>> is kernel building improved? >> >> Yes. >> >> The scenario is we do kernel building, meanwhile, repeatedly read PCI rom >> every one second. >> >> [ >> echo 1 > /sys/bus/pci/devices/0000\:00\:03.0/rom >> cat /sys/bus/pci/devices/0000\:00\:03.0/rom > /dev/null >> ] > > Can't see why it reflects real world scenario (or a real world > scenario with same characteristics regarding kvm_mmu_zap_all vs faults)? > > Point is, it would be good to understand why this change > is improving performance? What are these cases where breaking out of > kvm_mmu_zap_all due to either (need_resched || spin_needbreak) on zapped > < 10 ?
When guest read ROM, qemu will set the memory to map the device's firmware, that is why kvm_mmu_zap_all can be called in the scenario.
The reasons why it heart the performance are: 1): Qemu use a global io-lock to sync all vcpu, so that the io-lock is held when we do kvm_mmu_zap_all(). If kvm_mmu_zap_all() is not efficient, all other vcpus need wait a long time to do I/O.
2): kvm_mmu_zap_all() is triggered in vcpu context. so it can block the IPI request from other vcpus.
Is it enough?
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