Messages in this thread | | | Date | Tue, 20 Jan 2009 14:41:23 +0100 | From | Nick Piggin <> | Subject | Re: lmbench lat_mmap slowdown with CONFIG_PARAVIRT |
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On Tue, Jan 20, 2009 at 01:45:00PM +0100, Ingo Molnar wrote: > > * Nick Piggin <npiggin@suse.de> wrote: > > > 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.
kbuild costs go up a bit (average of 30 builds) elapsed non-pv: AVG=53.31s STD=0.99 pv: AVG=53.54s STD=0.94
user non-pv: AVG=318.63s STD=0.19 pv: AVG=319.33s STD=0.23
system non-pv: AVG=30.56s STD=0.15 pv: AVG=31.80s STD=0.15
kernel side of the kbuild workload slows down by 4.1%. User time also increases a bit (probably more cache and branch misses).
> If you have a Core2 test-system could you please try tip/master, which > also has your do_page_fault-de-bloating patch applied?
Will try to get one to do some runs on.
Thanks, Nick
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