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Re: [Xen-devel] Re: Poor performance on HVM (kernbench)

  • To: "Gianluca Guida" <gianluca.guida@xxxxxxxxxxxxx>
  • From: "George Dunlap" <dunlapg@xxxxxxxxx>
  • Date: Fri, 12 Sep 2008 12:04:38 +0100
  • Cc: Muli Ben-Yehuda <MULI@xxxxxxxxxx>, deshantm@xxxxxxxxx, Anthony Liguori <aliguori@xxxxxxxxxx>, xen-devel mailing list <xen-devel@xxxxxxxxxxxxxxxxxxx>
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On Thu, Sep 11, 2008 at 7:26 PM, Gianluca Guida
<gianluca.guida@xxxxxxxxxxxxx> wrote:
> Or, it could be a fixup table bug, but I doubt it.
> George, did you saw excessive fixup faults in the trace?

No, nothing excessive; 273,480 over 30 seconds isn't that bad.  The
main thing was that out of 15024 attempts to remove writable mappings,
13775 had to fall back to a brute-force search.

Looking at the trace, I can't really tell why there should be a
problem... I'm seeing tons of circumstances where there should only be
one writable mapping, but it falls back to brute-force search anyway.
Here's an example:

 24.999159660 -x  vmexit exit_reason EXCEPTION_NMI eip 2b105dcee330
 24.999159660 -x  wrmap-bf gfn 7453c
 24.999159660 -x fixup va 2b105f000000 gl1e 800000005caf0067 flags
 24.999748980 -x  vmentry
 24.999759577 -x  vmexit exit_reason EXCEPTION_NMI eip ffffffff8022a3b0
 24.999759577 -x fixup:unsync va ffff88007453c008 gl1e 7453c067 flags
 24.999762562 -x  vmentry
 25.002946338 -x  vmexit exit_reason CR_ACCESS eip ffffffff80491a63
 25.002946338 -x  wrmap-bf gfn 7e18c
 25.002946338 -x  oos resync full gfn 7e18c
 25.002946338 -x  wrmap-bf gfn 7453c
 25.002946338 -x  oos resync full gfn 7453c
 25.003526640 -x  vmentry

Here we see gfn 7453c:
 * promoted to be a shadow (the big 'P' in the flag string); at the
vmentry, there should be no writable mappings.
 * marked out-of-sync (one writable mapping, with fixup table)
 * re-sync'ed because of a CR write, and a brute-force search.

Note that the times behind the "wrmap-bf" and "oos resync full" are
not valid; but the whole vmexit->vmentry arc takes over 1.5


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