Re: [boost] [xpressive] Performance Tuning?

Subject: Re: [boost] [xpressive] Performance Tuning?
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2009-07-02 08:17:10

Eric Niebler wrote:
> Dave Jenkins wrote:
> > Robert Stewart wrote:
> >> Whoa!  The performance just shot up to a mere 6X the custom
> >> code (from 175X).  That might well be fast enough to keep
> >> the Xpressive version because of its readability!
> >
> > Can you create thread-local match_results objects and reuse
> > them?  If so, I think you'll see parsing dwindle to almost
> > nothing and your semantic actions will account for the bulk
> > of the time spent.

At Dave's suggestion, I tried that.

> Dave, thanks for spotting the obvious perf problem I missed. I can
> confirm that reusing the match results object largely eliminates the
> remaining performance problem. I tried 3 different scenarios:
>
> 1) The original code
> 2) Static const regexes
> 3) Static const regexes with reused match results objects
>
> I ran each config for 1000000 iterations and got roughly
> these numbers:
>
> 1) ~950 sec
> 2) ~45 sec
> 3) ~9 sec
>
> So reusing the match results object (3) results in a 5x speedup
> over just using static const regexes (2). That almost
> completely erases any performance advantage of the hand-crafted
> parsing code.

Unfortunately, my results don't bear that out with the added overhead of locating the smatch via thread local storage.  I see no difference between default constructing an smatch each time and reusing an instance from TLS.

> I'll also point out this section of the docs:
> http://www.boost.org/doc/libs/1_39_0/doc/html/xpressive/user_s_guide.html#boost_xpressive.user_s_guide.tips_n_tricks
>
> Both of the above optimizations (reuse regexes and
> match_results objects) are recommended there.

I read through some of those quickly upon an initial read through the documentation.  I chose static regexes because I understood them to be faster.  I did not reuse the match_results<> because they aren't thread safe.  Dave's TLS suggestion hadn't occurred to me to reconcile the competing forces of reuse and no thread safety, of course.

Upon rereading, I see that you note that reusing a static regex improves performance, but I'd forgotten it by the time I needed the information.  May I suggest a "Performance Tuning" section that discusses such things apart from common pitfalls, etc. and stands out better in the TOC?  I didn't expect to find such information in a "Tips and Tricks" section, and forgotten that I had seen it when it was wanted.

_____
Rob Stewart                           robert.stewart_at_[hidden]
Software Engineer, Core Software      using std::disclaimer;
Susquehanna International Group, LLP  http://www.sig.com

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