Re: [PATCH 11/20] Inline get_page_from_freelist() in the fast-path

Mel Gorman
2009-02-24T15:03:41Z

On Wed, Feb 25, 2009 at 01:08:10AM +1100, Nick Piggin wrote:
> On Wednesday 25 February 2009 00:32:53 Mel Gorman wrote:
> > On Tue, Feb 24, 2009 at 02:32:37AM +1100, Nick Piggin wrote:
> > > On Monday 23 February 2009 10:17:20 Mel Gorman wrote:
> > > > In the best-case scenario, use an inlined version of
> > > > get_page_from_freelist(). This increases the size of the text but
> > > > avoids time spent pushing arguments onto the stack.
> > >
> > > I'm quite fond of inlining ;) But it can increase register pressure as
> > > well as icache footprint as well. x86-64 isn't spilling a lot more
> > > registers to stack after these changes, is it?
> >
> > I didn't actually check that closely so I don't know for sure. Is there a
> > handier way of figuring it out than eyeballing the assembly? In the end
>
> I guess the 5 second check is to look at how much stack the function
> uses. OTOH I think gcc does do a reasonable job at register allocation.
>

FWIW, 6 registers get pushed onto the stack from the calling function from
a glance of the assembly. According to the profile, about 7% of the cost of
the get_page_from_freelist() function is incurred by setting up and making
the function call. This is 2755 samples out of 35266. To compare, the cost
of zeroing was 192574 samples.

So, it's a good chunk of time, but in the grand scheme of things, time is
better spent optimising elsewhere for now.

>
> > I dropped the inline of this function anyway. It means the patches
> > reduce rather than increase text size which is a bit more clear-cut.
>
> Cool, clear cut patches for round 1 should help to get things moving.
>

Indeed

>
> > > In which case you will get extra icache footprint. What speedup does
> > > it give in the cache-hot microbenchmark case?
> >
> > I wasn't measuring with a microbenchmark at the time of writing so I don't
> > know. I was going entirely by profile counts running kernbench and the
> > time spent running the benchmark.
>
> OK. Well seeing as you have dropped this for the moment, let's not
> dwell on it ;)
>

Agreed.

--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab