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

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Date:	Tue, 5 Jan 2010 19:27:07 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
cc:	Minchan Kim <minchan.kim@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>, cl@...ux-foundation.org,
	"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 Wed, 6 Jan 2010, KAMEZAWA Hiroyuki wrote:
>
> My host boots successfully. Here is the result.

Hey, looks good. It does have that 3% trylock overhead:

      3.17%  multi-fault-all  [kernel]                  [k] down_read_trylock

but that doesn't seem excessive.

Of course, your other load with MADV_DONTNEED seems to be horrible, and 
has some nasty spinlock issues, but that looks like a separate deal (I 
assume that load is just very hard on the pgtable lock).

That said, profiles are hard to compare performance with - the main thing 
that matters for performance is not how the profile looks, but how it 
actually performs. So:

> Then, the result is much improved by XADD rwsem.
> 
> In above profile, rwsem is still there.
> But page-fault/sec is good. I hope some "big" machine users join to the test.

That "page-fault/sec" number is ultimately the only thing that matters. 

> Here is peformance counter result of DONTNEED test. Counting the number of page
> faults in 60 sec. So, bigger number of page fault is better.
> 
> [XADD rwsem]
> [root@...extal memory]#  /root/bin/perf stat -e page-faults,cache-misses --repeat 5 ./multi-fault-all 8
> 
>  Performance counter stats for './multi-fault-all 8' (5 runs):
> 
>        41950863  page-faults                ( +-   1.355% )
>       502983592  cache-misses               ( +-   0.628% )
> 
>    60.002682206  seconds time elapsed   ( +-   0.000% )
> 
> [my patch]
> [root@...extal memory]#  /root/bin/perf stat -e page-faults,cache-misses --repeat 5 ./multi-fault-all 8
> 
>  Performance counter stats for './multi-fault-all 8' (5 runs):
> 
>        35835485  page-faults                ( +-   0.257% )
>       511445661  cache-misses               ( +-   0.770% )
> 
>    60.004243198  seconds time elapsed   ( +-   0.002% )
> 
> Ah....xadd-rwsem seems to be faster than my patch ;)

Hey, that sounds great. NOTE! My patch really could be improved. In 
particular, I suspect that on x86-64, we should take advantage of the 
64-bit counter, and use a different RW_BIAS. That way we're not limited to 
32k readers, which _could_ otherwise be a problem.

So consider my rwsem patch to be purely preliminary. Now that you've 
tested it, I feel a lot better about it being basically correct, but it 
has room for improvement.

				Linus
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