Re: lmbench lat_mmap slowdown with CONFIG_PARAVIRT

[Ingo Molnar <mingo@elte.hu>]

* Nick Piggin <npiggin@suse.de> wrote:

> On Tue, Jan 20, 2009 at 03:03:24PM +0100, Ingo Molnar wrote:
> > 
> > * Ingo Molnar <mingo@elte.hu> wrote:
> > 
> > > > 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.
> > 
> > Here are some more precise stats done via hw counters on a perfcounters 
> > kernel using 'timec', running a modified version of the 'mmap performance 
> > stress-test' app i made years ago.
> > 
> > The MM benchmark app can be downloaded from:
> > 
> >    http://redhat.com/~mingo/misc/mmap-perf.c
> 
> BTW. the lmbench test I run directly (it's called lat_mmap.c, and gets 
> compiled into a standalone lat_mmap exec by the standard lmbench build).

doesnt that include an indeterminate number of gettimeofday() based 
calibration calls? That would make it harder to measure its total costs in 
a comparative way.

	Ingo