linux-kernel - Re: [PATCH] mm: remove global locks from mm/highmem.c

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Message-ID: <20070128151700.GA7644@elte.hu>
Date:	Sun, 28 Jan 2007 16:17:00 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Christoph Hellwig <hch@...radead.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andrew Morton <akpm@...l.org>
Subject: Re: [PATCH] mm: remove global locks from mm/highmem.c


* Christoph Hellwig <hch@...radead.org> wrote:

> On Sun, Jan 28, 2007 at 03:11:34PM +0100, Peter Zijlstra wrote:
> > Eradicate global locks.
> > 
> >  - kmap_lock is removed by extensive use of atomic_t, a new flush
> >    scheme and modifying set_page_address to only allow NULL<->virt
> >    transitions.
> 
> What's the point for this? [...]

scalability. I did lock profiling on the -rt kernel, which exposes such 
things nicely. Half of the lock contention events during kernel compile 
were due to kmap(). (The system had 2 GB of RAM, so 40% lowmem, 60% 
highmem.)

> [...] In doubt we just need to convert that caller to kmap_atomic.

the pagecache ones cannot be converted to kmap_atomic, because we can 
block while holding them. Plus kmap_atomic is quite a bit slower than 
this scalable version of kmap().

	Ingo

ps. please fix your mailer to not emit Mail-Followup-To headers. In Mutt
    you can do this via "set followup_to=no" in your .muttrc.
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