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Mon, 29 Sep 2008

First fix for the tbench over loopback regression.

It brought me back about 5% in Xen domain with 256 MB of ram in 4-clients tbench test:

current: 187 MB/s
patched: 194 MB/s
Patch is rather trivial: it disables TSO and GSO in loopback and generically on devices which are capable of scatter-gather (where it was automatically enabled by e5a4a72d4f8 commit, which I biseced to be guilty). Actually TSO disablement part provided more gain than GSO on SG devices.
Idea behind patches is clear: we create bigger packet, so we should have smaller overhead of its processing, but apparently GSO/TSO packet creation overhead dominates in loopback at least.

My all three (big) test machines died in various (apparently unbootable) bisections, so I tested it in small and very slow Xen domain. Because of that I did not run 2.6.22 kernel, since git operations and compilation take ages on this 'machine'. For example I was only able to perform about dozen or so git checkous/resets/bisections and compilations for the whole day.

I've posted patch to the netdev@, let's see the result.

Forgot to mention, that I wanted to sell this patch for the DST, POHMELFS or netchannels patch review next time I will post them :)

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