[Xen-devel] Performance issues with large grants

[Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>]

I am trying to map a large number of pages in an HVM domain and am
running into significant performance issues. Currently, mapping 1280
pages will take 11 seconds to complete, which is not very usable. I
have determined that the most costly part of the update is the IOMMU
flush which takes about 9ms per page (the domain doing the mapping
has a PCI device forwarded to it). Allowing this update to be disabled
and batched to the end of the update improves the speed to around 800ms,
but this introduces instability in the network drivers, which stop
receiving host-to-guest packets.

Is there a better method to batch the P2M updates for grant table
modifications that I should try? The improved speed is still slower
than I would expect (about 40ms), so there are other high-cost actions
being performed in addition to the IOMMU flush. I have also considered
allowing mapping 2M pages between domains as an alternate solution;
this would be a larger API change, and I'd be interested in comments on
the general usefulness of 2M grants.

-- 
Daniel De Graaf
National Security Agency

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