Re: [patch 00/14] remap_file_pages protection support

From: Nick Piggin
Date: Tue May 16 2006 - 09:30:01 EST

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


SUSE Labs, Novell Inc.
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