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    SubjectRe: [PATCH] Enhance perf to collect KVM guest os statistics from host side
    Soeren Sandmann <> writes:
    > Examples:
    > - What is going on inside QEMU?

    That's something the JIT interface could answer.

    > - Which client is the X server servicing?
    > - What parts of a python/shell/scheme/javascript program is
    > taking the most CPU time?

    I suspect for those you rather need event based tracers of some sort,
    similar to kernel trace points. Otherwise you would need own
    separate stacks and other complications.

    systemtap has some effort to use the dtrace instrumentation
    that crops up in more and more user programs for this. It wouldn't
    surprise me if that was already in python and other programs
    you're interested in.

    I presume right now it only works if you apply the utrace monstrosity
    though, but perhaps the new uprobes patches floating around
    will come to rescue.

    There also was some effort to have a pure user space
    daemon based approach for LTT, but I believe that currently
    needs own trace points.

    Again I fully expect someone to reinvent the wheel here
    and afterwards complain about "community inefficiences" :-)

    > I don't think the oprofile JIT interface solves any of these
    > problems. (In fact, I don't see why the JIT problem is even hard. The
    > JIT compiler can just generate a little ELF file with symbols in it,
    > and the profiler can pick it up through the mmap events that you get
    > through the perf interface).

    That would require keeping those temporary ELF files for
    potentially unlimited time around (profilers today look at the ELF
    files at the final analysis phase, which might be weeks away)

    Also that would be a lot of overhead for the JIT and most likely
    be a larger scale rewrite for a given JIT code base.

    -- -- Speaking for myself only.

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