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Date: Mon, 13 Apr 2009 12:57:50 -0700 (PDT)
From: Linus Torvalds <torvalds@linux-foundation.org>
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To: Ying Han <yinghan@google.com>
cc: linux-mm@kvack.org, linux-kernel <linux-kernel@vger.kernel.org>,
       akpm <akpm@linux-foundation.org>, Ingo Molnar <mingo@elte.hu>,
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       Lee Schermerhorn <lee.schermerhorn@hp.com>,
       Nick Piggin <npiggin@suse.de>, Wu Fengguang <fengguang.wu@intel.com>
Subject: Re: [V4][PATCH 0/4]page fault retry with NOPAGE_RETRY
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On Mon, 13 Apr 2009, Ying Han wrote:
> 
> Benchmarks:
> case 1. one application has a high count of threads each faulting in
> different pages of a hugefile. Benchmark indicate that this double data
> structure walking in case of major fault results in << 1% performance hit.
> 
> case 2. add another thread in the above application which in a tight loop
> of mmap()/munmap(). Here we measure loop count in the new thread while other
> threads doing the same amount of work as case one. we got << 3% performance
> hit on the Complete Time(benchmark value for case one) and 10% performance
> improvement on the mmap()/munmap() counter.
> 
> This patch helps a lot in cases we have writer which is waitting behind all
> readers, so it could execute much faster.

Hmm. I normally think of "<<" as "much smaller than", but the way you use 
it makes me wonder. In particular, "<< 3%" sounds very odd. If it's much 
smaller than 3%, I'd have expected "<< 1%" again. So it probably isn't.

> benchmarks from Wufengguang:
> Just tested the sparse-random-read-on-sparse-file case, and found the
> performance impact to be 0.4% (8.706s vs 8.744s) in the worst case.
> Kind of acceptable.

Well, have you tried the obvious optimization of _not_ doing the RETRY 
path when atomic_read(&mm->counter) == 1?

After all, if it's not a threaded app, and it doesn't have a possibility 
of concurrent mmap/fault, then why release the lock?

		Linus