linux-kernel - Re: [patch 02/20] make the inode i_mmap

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Message-ID: <Pine.LNX.4.64.0712192301120.13118@schroedinger.engr.sgi.com>
Date:	Wed, 19 Dec 2007 23:04:33 -0800 (PST)
From:	Christoph Lameter <clameter@....com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Rik van Riel <riel@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 02/20] make the inode i_mmap_lock a reader/writer lock



> The only reason the x86 ticket locks have the 256 CPu limit is that
> if they go any bigger, we can't use the partial registers so would
> have to have a few more instructions.

x86_64 is going up to 4k or 16k cpus soon for our new hardware.

> A 32 bit spinlock would allow 64K cpus (ticket lock has 2 counters,
> each would be 16 bits). And it would actually shrink the spinlock in
> the case of preempt kernels too (because it would no longer have the
> lockbreak field).
> 
> And yes, I'll go out on a limb and say that 64k CPUs ought to be
> enough for anyone ;)

I think those things need a timeframe applied to it. Thats likely 
going to be true for the next 3 years (optimistic assessment ;-)).

Could you go to 32bit spinlock by default?

How about NUMA awareness for the spinlocks? Larger backoff periods for 
off node lock contentions please.

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