Messages in this thread |  | | | Date | Wed, 15 Nov 2006 09:28:15 -0800 | | From | Jeremy Fitzhardinge <> | | Subject | Re: [PATCH] i386-pda UP optimization |
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Ingo Molnar wrote: > Eric's test shows a 5% slowdown. That's far from cheap. >
It seems like an absurdly large difference. PDA references aren't all that common in the kernel; for the %gs prefix on PDA accesses to be causing a 5% overall difference in a test like this means that the prefixes would have to be costing hundreds or thousands of cycles, which seems absurd. Particularly since Eric's patch doesn't touch head.S, so the %gs save/restore is still being executed.
Are we sure this isn't a cache layout issue? Eric, did you try evicting your executable from pagecache between runs to see if you get variation depending on what physical pages it gets put into? (Making several copies of the executable should have the same effect.)
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