linux-kernel - Re: [RFC][PATCH 6/8] mm: handle_speculative

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Date:	Thu, 7 Jan 2010 10:36:52 -0600 (CST)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Arjan van de Ven <arjan@...radead.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"minchan.kim@...il.com" <minchan.kim@...il.com>,
	"hugh.dickins" <hugh.dickins@...cali.co.uk>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [RFC][PATCH 6/8] mm: handle_speculative_fault()

On Thu, 7 Jan 2010, Linus Torvalds wrote:

> You're missing what Arjan said - the jav workload does a lot of memory
> allocations too, causing mmap/munmap.

Well isnt that tunable on the app level? Get bigger chunks of memory in
order to reduce the frequency of mmap operations? If you want concurrency
of faults then mmap_sem write locking currently needs to be limited.

> So now some paths are indeed holding it for writing (or need to wait for
> it to become writable). And the fairness of rwsems quite possibly then
> impacts throughput a _lot_..

Very true. Doing range locking (maybe using the split pte lock
boundaries, shifting some state from mm_struct into vmas) may be a way to
avoid hold mmap_sem for write in that case.

> (Side note: I wonder if we should wake up _all_ readers when we wake up
> any. Right now, we wake up all readers - but only until we hit a writer.
> Which is the _fair_ thing to do, but it does mean that we can end up in
> horrible patterns of alternating readers/writers, when it could be much
> better to just say "release the hounds" and let all pending readers go
> after a writer has had its turn).

Have a cycle with concurrent readers followed by a cycle of serialized
writers may be best under heavy load. The writers need to be limited in
frequency otherwise they will starve the readers.
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