Messages in this thread | | | Date | Tue, 20 Jan 2009 13:34:18 +0100 | From | Nick Piggin <> | Subject | Re: lmbench lat_mmap slowdown with CONFIG_PARAVIRT |
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On Tue, Jan 20, 2009 at 12:26:34PM +0100, Ingo Molnar wrote: > > * Nick Piggin <npiggin@suse.de> wrote: > > > Hi, > > > > I'm looking at regressions since 2.6.16, and one is lat_mmap has slowed > > down. On further investigation, a large part of this is not due to a > > _regression_ as such, but the introduction of CONFIG_PARAVIRT=y. > > > > Now, it is true that lat_mmap is basically a microbenchmark, however it > > is exercising the memory mapping and page fault handler paths, so we're > > talking about pretty important paths here. So I think it should be of > > interest. > > > > I've run the tests on a 2s8c AMD Barcelona system, binding the test to > > CPU0, and running 100 times (stddev is a bit hard to bring down, and my > > scripts needed 100 runs in order to pick up much smaller changes in the > > results -- for CONFIG_PARAVIRT, just a couple of runs should show up the > > problem). > > > > Times I believe are in nanoseconds for lmbench, anyway lower is better. > > > > non pv AVG=464.22 STD=5.56 > > paravirt AVG=502.87 STD=7.36 > > > > Nearly 10% performance drop here, which is quite a bit... hopefully > > people are testing the speed of their PV implementations against non-PV > > bare metal :) > > Ouch, that looks unacceptably expensive. All the major distros turn > CONFIG_PARAVIRT on. paravirt_ops was introduced in x86 with the express > promise to have no measurable runtime overhead. > > ( And i suspect the real life mmap cost is probably even more expensive, > as on a Barcelona all of lmbench fits into the cache hence we dont see > any real $cache overhead. )
The PV kernel has over 100K larger text size, nearly 40K alone in mm/ and kernel/. Definitely we don't see the worst of the icache or branch buffer overhead on this microbenchmark. (wow, that's a nasty amount of bloat :( )
> Jeremy, any ideas where this slowdown comes from and how it could be > fixed?
I had a bit of a poke around the profiles, but nothing stood out. However oprofile counted 50% more cycles in the kernel with PV than with non-PV. I'll have to take a look at the user/system times, because 50% seems ludicrous.... hopefully it's just oprofile noise.
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