Re: [patch 00/14] remap_file_pages protection support

[Nick Piggin]

Valerie Henson wrote:
> On Sun, May 14, 2006 at 12:13:21AM +1000, Nick Piggin wrote:
>
> > OK, I got interested again, but can't get Val's ebizzy to give me
> > a find_vma constrained workload yet (though the numbers back up
> > my assertion that the vma cache is crap for threaded apps).
>
>
> Hey Nick,
>
> Glad to see you're using it!  There are (at least) two ways to do what
> you want:
>
> 1. Increase the number of threads - this gives you two vma's per
>    thread, one for stack, one for guard page:
>
>    $ ./ebizzy -t 100
>
> 2. Apply the patch at the end of this email and use -p "prevent
>    coalescing", -m "always mmap" and appropriate number of chunks,
>    size, and records to search - this works for me:
>
>    $ ./ebizzy -p -m -n 10000 -s 4096 -r 100000
>
> The original program mmapped everything with the same permissions and
> no alignment restrictions, so all the mmaps were coalesced into one.
> This version alternates PROT_WRITE permissions on the mmap'd areas
> after they are written, so you get lots of vma's:
>
> val@goober:~/ebizzy$ ./ebizzy -p -m -n 10000 -s 4096 -r 100000
>
> [2]+  Stopped                 ./ebizzy -p -m -n 10000 -s 4096 -r 100000
> val@goober:~/ebizzy$ wc -l /proc/`pgrep ebizzy`/maps
> 10019 /proc/10917/maps
>
> I haven't profiled to see if this brings find_vma to the top, though.

Hi Val,

Thanks, I've tried with your most recent ebizzy and with 256 threads and
50,000 vmas (which gives really poor mmap_cache hits), I'm still unable
to get find_vma above a few % of kernel time.

With 50,000 vmas, my per-thread vma cache is much less effective, I guess
because access is pretty random (hopefully more realistic patterns would
get a bigger improvement).

I also tried running kbuild under UML, and could not make find_vma take
much time either [in this case, the per-thread vma cache patch roughly
doubles the number of hits, from about 15%->30% (in the host)].

So I guess it's time to go back into my hole. If anyone does come across
a find_vma constrained workload (especially with threads), I'd be very
interested.

Thanks,
Nick

--
SUSE Labs, Novell Inc.
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