Re: lmbench lat_mmap slowdown with CONFIG_PARAVIRT

[Nick Piggin]

On Tue, Jan 20, 2009 at 01:45:00PM +0100, Ingo Molnar wrote:
> 
> * Nick Piggin <npiggin@xxxxxxx> wrote:
> 
> > On Tue, Jan 20, 2009 at 12:26:34PM +0100, Ingo Molnar wrote:
> > > 
> > > * Nick Piggin <npiggin@xxxxxxx> 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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