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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] Re: [PATCH] Allow removing writable mappings from splintered page tables.
Hmm, no really obvious low-hanging fruit. The Xen-HVM was about 9% slower than your reported numbers for Xen-PV, and the trace shows that the guest spent about that much inside the hypervisor. The breakdown: * 3.6% propagating page faults to guest * 3.0% pulling through entries from out-of-sync guest pt's to shadow pagetables * 1.4% Marking pages out of sync (of which 75% was in unsyncs that had to re-sync another page) * 0.9% cr3 switches * 0.9% handling I/O (Rounding may cause the numbers not to add up exactly.) So one of the biggest things, really, is that Linux seems to insist on mapping pages one-at-a-time as they're demand-faulted, rather than doing a batch of them. Unfortunately, having pages out-of-sync means that we must use the slow propagate path rather than the fast-propagate path, which is at least 25% slower. The only avenues for optimization I can see are: * See if there's a way to reduce the number of unsyncs that cause resyncs. Allowing more pages to go out-of-sync *might* do this; or it might just shift the same overhead into cr3 switch. * Reduce the time of "hot paths" through the hypervisor by profiling, &c. -George On Mon, Sep 15, 2008 at 6:03 PM, George Dunlap <George.Dunlap@xxxxxxxxxxxxx> wrote: > Heh... the blatant copying is flattering and annoying at the same > time. :-) Ah, the beauty of open-source... > > I've got your trace, and I'll take a look at it tomorrow. Thanks! > > -George > > On Mon, Sep 15, 2008 at 5:30 PM, Todd Deshane <deshantm@xxxxxxxxx> wrote: >> On Mon, Sep 15, 2008 at 6:38 AM, George Dunlap >> <George.Dunlap@xxxxxxxxxxxxx> wrote: >>> And your original numbers showed elapsed time to be 527s for KVM, so >>> now Xen is 8 seconds in the lead for HVM Linux. :-) Thanks for the >>> help tracking this down! >>> >> >> KVM is also working on improved page table algorithms >> http://www.mail-archive.com/kvm@xxxxxxxxxxxxxxx/msg03562.html >> >> I think the competition is a good thing. >> >>> If you have time, could you take another 30-second trace with the new >>> changes in, just for fun? I'll take a quick look and see if there's >>> any other low-hanging fruit to grab. >>> >> >> Sent the trace to you with another service called sendspace, since, for >> some reason, the trace file was much bigger. >> >> Todd >> >> -- >> Todd Deshane >> http://todddeshane.net >> check out our book: http://runningxen.com >> >> _______________________________________________ >> Xen-devel mailing list >> Xen-devel@xxxxxxxxxxxxxxxxxxx >> http://lists.xensource.com/xen-devel >> > _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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