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


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


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