linux-kernel - Re: [PATCH 0/2] convert mmap_sem to a scalable rw

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Date:	Mon, 14 May 2007 14:57:28 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Eric Dumazet <dada1@...mosbay.com>, Ingo Molnar <mingo@...e.hu>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Oleg Nesterov <oleg@...sign.ru>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/2] convert mmap_sem to a scalable rw_mutex

On Mon, 2007-05-14 at 14:07 +0200, Nick Piggin wrote:
> On Fri, May 11, 2007 at 07:18:33PM +0200, Peter Zijlstra wrote:
> > On Fri, 2007-05-11 at 18:52 +0200, Eric Dumazet wrote:
> > > 
> > > But I personally find this new rw_mutex not scalable at all if you have some 
> > > writers around.
> > > 
> > > percpu_counter_sum is just a L1 cache eater, and O(NR_CPUS)
> > 
> > Yeah, that is true; there are two occurences, the one in
> > rw_mutex_read_unlock() is not strictly needed for correctness.
> > 
> > Write locks are indeed quite expensive. But given the ratio of
> > reader:writer locks on mmap_sem (I'm not all that familiar with other
> > rwsem users) this trade-off seems workable.
> 
> I guess the problem with that logic is assuming the mmap_sem read side
> always needs to be scalable. Given the ratio of threaded:unthreaded
> apps, maybe the trade-off swings away from favour?

Could be; I've been bashing my head against the wall trying to find a
scalable write side solution. But so far only got a massive dent in my
brain from the effort.

Perhaps I can do a similar optimistic locking for my rcu-btree as I did
for the radix tree. That way most of the trouble would be endowed upon
the vmas instead of the mm itself. And then it would be up to user-space
to ensure it has in the order of nr_cpu_ids arenas to work in.

Also, as Hugh pointed out in an earlier thread; mmap_sem's write side
also protects the page tables, so we'd need to fix that up too;
assumedly the write side equivalent of the vma lock would then protect
all underlying page tables....

/me drifting away, rambling incoherently,..

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