On 11/14/2012 10:44 PM, Marcelo Tosatti wrote: > On Wed, Nov 14, 2012 at 12:33:50AM +0900, Takuya Yoshikawa wrote: >> Ccing live migration developers who should be interested in this work, >> >> On Mon, 12 Nov 2012 21:10:32 -0200 >> Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote: >> >>> On Mon, Nov 05, 2012 at 05:59:26PM +0800, Xiao Guangrong wrote: >>>> Do not drop large spte until it can be insteaded by small pages so that >>>> the guest can happliy read memory through it >>>> >>>> The idea is from Avi: >>>> | As I mentioned before, write-protecting a large spte is a good idea, >>>> | since it moves some work from protect-time to fault-time, so it reduces >>>> | jitter. This removes the need for the return value. >>>> >>>> Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx> >>>> --- >>>> arch/x86/kvm/mmu.c | 34 +++++++++------------------------- >>>> 1 files changed, 9 insertions(+), 25 deletions(-) >>> >>> Its likely that other 4k pages are mapped read-write in the 2mb range >>> covered by a read-only 2mb map. Therefore its not entirely useful to >>> map read-only. >>> >>> Can you measure an improvement with this change? >> >> What we discussed at KVM Forum last week was about the jitter we could >> measure right after starting live migration: both Isaku and Chegu reported >> such jitter. >> >> So if this patch reduces such jitter for some real workloads, by lazily >> dropping largepage mappings and saving read faults until that point, that >> would be very nice! >> >> But sadly, what they measured included interactions with the outside of the >> guest, and the main cause was due to the big QEMU lock problem, they guessed. >> The order is so different that an improvement by a kernel side effort may not >> be seen easily. >> >> FWIW: I am now changing the initial write protection by >> kvm_mmu_slot_remove_write_access() to rmap based as I proposed at KVM Forum. >> ftrace said that 1ms was improved to 250-350us by the change for 10GB guest. >> My code still drops largepage mappings, so the initial write protection time >> itself may not be a such big issue here, I think. >> >> Again, if we can eliminate read faults to such an extent that guests can see >> measurable improvement, that should be very nice! >> >> Any thoughts? >> >> Thanks, >> Takuya > > OK, makes sense. I'm worried about shadow / oos interactions > with large read-only mappings (trying to remember what was the > case exactly, it might be non-existant now). Marcelo, i guess commit 38187c830cab84daecb41169948467f1f19317e3 is what you mentioned, but i do not know how it can "Simplifies out of sync shadow." :( -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
