Re: [uml-devel] UBD performance

[Chris L. <ch...@ex...>]

On Sun, Oct 09, 2005 at 09:35:15PM -0400, Jeff Dike wrote:
> On Sun, Oct 09, 2005 at 11:51:28PM +0100, Chris Lightfoot wrote:
> > (TBH I'm surprised that the AIO code shows so little
> > improvement. The kernel does report that it's using 2.6
> > host AIO, so it is using the new version. Obviously
> > issuing the writes segment-by-segment is non-ideal but I'm
> > surprised it's this bad. Could I be missing some
> > configuration step?)
> 
> Make sure you've got the o_direct patch, and are using a 4K block filesystem.
> 
> If you're not doing O_DIRECT, then io_getevents waits - i.e. it's synchronous.

OK. The host is indeed using a 4K block filesystem; I
couldn't find your O_DIRECT patch, but turning on O_DIRECT
with fcntl just after opening the backing file gives these
results:
    http://ex-parrot.com/~chris/tmp/20051010/host-vs-uml-io-results-3.png
i.e., the AIO implementation is now slower than the stock
implementation for writes of size up to about 8KB, but
faster for larger writes; it's still quite a bit slower
than the host.

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