Re: lmbench lat_mmap slowdown with CONFIG_PARAVIRT

[Ingo Molnar <mingo@elte.hu>]

* 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. )

Jeremy, any ideas where this slowdown comes from and how it could be 
fixed?

	Ingo