Re: [Patch 3/3] prepopulate/cache cleared pages

[Nick Piggin <nickpiggin@yahoo.com.au>]

Arjan van de Ven wrote:
> > Arjan, just to get an idea of your workload: obviously it is a mix of
> > read and write on the mmap_sem (read only will not really benefit from
> > reducing lock width because cacheline transfers will still be there).
>
>
> yeah it's threads that each allocate, use and then free memory with
> mmap()
>
>
> > Is it coming from brk() from the allocator? Someone told me a while ago
> > that glibc doesn't have a decent amount of hysteresis in its allocator
> > and tends to enter the kernel quite a lot... that might be something
> > to look into.
>
>
> we already are working on that angle; I just posted the kernel stuff as
> a side effect basically 

OK.

[aside]
Actually I have a scalability improvement for rwsems, that moves the
actual task wakeups out from underneath the rwsem spinlock in the up()
paths. This was useful exactly on a mixed read+write workload on mmap_sem.

The difference was quite large for the "generic rwsem" algorithm because
it uses the spinlock in fastpaths a lot more than the xadd algorithm. I
think x86-64 uses the former, which is what I presume you're testing with?

Obviously this is a slightly different issue from the one you're trying to
address here, but I'll dig out patch when I get some time and you can see
if it helps.

--
SUSE Labs, Novell Inc.
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