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

George Dunlap wrote:
So, the problem appears to be with a ton of brute-force searches to
remove writable mappings, both during resync and promotion.  My
analysis tool is reporting that of the 30 seconds or so in the trace
from xen-unstable, the guest spent a whopping 67% in the hypervisor:
 * 26% doing resyncs as a result of marking another page out-of-sync
 * 9% promoting pages
 * 27% resyncing as a result of cr3 switches
And almost the entirety of all of those can be attributed to
brute-force searches to remove writable mappings.

Fantastic (well, sort of)!

If I understand it correctly, Todd is using PV drivers in Linux HVM guests, so the reason for brute-force search is due to former L1 page-tables being used as I/O pages, not being unshadowed because they can get writable mappings out of it. It is, shortly, an unshadowing problem. Should be `easy` to fix. I wasn't using PV drivers, so I was not experiencing this behaviour.

Or, it could be a fixup table bug, but I doubt it.

George, did you saw excessive fixup faults in the trace?

Todd, could you try without PV drivers (plain qemu emulation) and see if the results get better?


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