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    SubjectRe: [PATCH RFC 0/4] Paravirtual spinlocks
    On Tuesday 08 July 2008 10:37:54 Jeremy Fitzhardinge wrote:
    > Rusty Russell wrote:
    > > On Tuesday 08 July 2008 05:07:49 Jeremy Fitzhardinge wrote:
    > >> At the most recent Xen Summit, Thomas Friebel presented a paper
    > >> ("Preventing Guests from Spinning Around",
    > >> investigating the
    > >> interactions between spinlocks and virtual machines. Specifically, he
    > >> looked at what happens when a lock-holding VCPU gets involuntarily
    > >> preempted.
    > >
    > > I find it interesting that gang scheduling the guest was not suggested as
    > > an obvious solution.
    > It's an obvious answer, but not an obvious solution. You trade off
    > wasting time spinning vs wasting time waiting for N vcpus to be free for
    > scheduling.

    Perhaps, but with huge numbers of cores (as The Future seems to promise) and
    significant overcommit not sure how bad this would be.

    > Or something; seems much more complex, particularly if you
    > can do a small guest tweak to solve the problem.

    But AFAICT it's one of a related set of problems where all VCPUs are required
    for a task. Hackbench comes to mind. There's going to be a lot of
    ping-ponging and you'll approach gang scheduling to get decent performance.

    > > A little disappointing that you can't patch your version inline.
    > Spinlock code isn't inlined currently, so I hadn't considered it. The
    > fast path code for both lock and unlock is nearly small enough to
    > consider it, but it seems a bit fiddly.

    Yeah, OK.


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