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    SubjectRe: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself
    On 05/23/2010 06:51 PM, Michael S. Tsirkin wrote:
    >>
    >>> So locked version seems to be faster than unlocked,
    >>> and share/unshare not to matter?
    >>>
    >>>
    >> May be due to the processor using the LOCK operation as a hint to
    >> reserve the cacheline for a bit.
    >>
    > Maybe we should use atomics on index then?
    >

    This should only be helpful if you access the cacheline several times in
    a row. That's not the case in virtio (or here).

    I think the problem is that LOCKSHARE and SHARE are not symmetric, so
    they can't be directly compared.

    > OK, after adding mb in code patch will be sent separately,
    > the test works for my workstation. locked is still fastest,
    > unshared sometimes shows wins and sometimes loses over shared.
    >
    > [root@qus19 ~]# ./cachebounce share 0 1
    > CPU 0: share cacheline: 6638521 usec
    > CPU 1: share cacheline: 6638478 usec
    >

    66 ns? nice.

    > [root@qus19 ~]# ./cachebounce share 0 2
    > CPU 0: share cacheline: 14529198 usec
    > CPU 2: share cacheline: 14529156 usec
    >

    140 ns, not too bad. I hope I'm not misinterpreting the results.

    --
    error compiling committee.c: too many arguments to function



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