Messages in this thread | | | Subject | Re: hackbench regression with kernel 2.6.32-rc1 | From | Mike Galbraith <> | Date | Thu, 29 Oct 2009 06:46:30 +0100 |
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On Thu, 2009-10-29 at 08:50 +0800, Zhang, Yanmin wrote: > On Wed, 2009-10-28 at 15:22 +0100, Mike Galbraith wrote: > > On Wed, 2009-10-28 at 17:29 +0800, Zhang, Yanmin wrote: > > > -Mike > > > I'm investigating 5% tbench regression on Nehalem machine. perf_counter shows > > > select_task_rq_fair consumes about 5% cpu time with 2.6.32-rc1 while it consumes > > > less than 0.5% with 2.6.31. > > > > > > Patch c88d5910890 has comments to explain it, but I still can't understand why > > > to add complicated balance logic when selecting task rq. > > > > > > I will check which section in function select_task_rq_fair consumes so much time. > > > > Turn off SD_WAKE_BALANCE as it was called in rc1. See commit 182a85f. > I run testing against 2.6.32-rc1 which already includes the patch.
Duh, I checked the wrong tree.
SD_PREFER_LOCAL is still on in rc1 though (double checks;), so you'll go through the power saving code until you reach a domain containing both waker's cpu and wakee's previous cpu even if that code already found that a higher domain wasn't overloaded. Looks to me like that block wants a want_sd && qualifier.
Even it you turn SD_PREFER_LOCAL off, you can still hit the overhead if SD_POWERSAVINGS_BALANCE is set, so I'd make sure both are off and see if that's the source (likely, since the rest is already off).
-Mike
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