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

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Date:	Thu, 7 Jan 2010 08:19:56 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Christoph Lameter <cl@...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, Christoph Lameter wrote:
> >
> > depends on the workload; on a many-threads-java workload, you also get
> > it for write quite a bit (lots of malloc/frees in userspace in addition
> > to pagefaults).. at which point you do end up serializing on the
> > zeroing.
> >
> > There's some real life real big workloads that show this pretty badly;
> > so far the workaround is to have glibc batch up a lot of the free()s..
> > but that's just pushing it a little further out.
> 
> Again mmap_sem is a rwsem and only a read lock is held. Zeroing in
> do_anonymous_page can occur concurrently on multiple processors in the
> same address space. The pte lock is intentionally taken *after* zeroing to
> allow concurrent zeroing to occur.

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

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_..

(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).

			Linus
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