Re: [V4][PATCH 0/4]page fault retry with NOPAGE_RETRY

[Ying Han <yinghan@google.com>]

On Mon, Apr 13, 2009 at 12:57 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
>
> 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.

Yes, it should be "< 3%", i will make the change.

>> 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
>

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