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in reply to Re: Our perl/xs/c app is 30% slower with 64bit 5.24.0, than with 32bit 5.8.9. Why?
in thread Our perl/xs/c app is 30% slower with 64bit 5.24.0, than with 32bit 5.8.9. Why?
It'd be a whole lot easier to diagnose if we knew what the application was doing. And easier still if we could see the code.
The first thing you need to do is profile both and work out where the time is being used. Once you know that, it'll be easier to reason about the cause.
Thanks for the reply. As the the other Anonymous Monk said, the app is a mix of perl/xs/c and is difficult to profile on Windows (normally I would use valgrind on linux). I did try Very Sleepy, but nothing stood out. Can you recommend a profiler for windows?
The app itself mainly deals with numeric data. Lots of double datatypes in C structs, with data manipulation in C, with perl providing an 'API' to make things easy for the end user, so something like this:
my $sum = $apple + $orange;
The $apple and $orange variables are objects (typically mapped to large double vectors), the vector calculation would be carried out in C etc.
While our performance benchmarks are representative of our real workloads, they are very broad in nature...and contain lots and lots of code... While all the tests perform worse, the ones that stand out most (ie, 80%+ worse) do create more perl/xs objects than typical, so perhaps that is where I should start looking?
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Re^3: Our perl/xs/c app is 30% slower with 64bit 5.24.0, than with 32bit 5.8.9. Why?
by BrowserUk (Patriarch) on Dec 21, 2016 at 21:34 UTC
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I did try Very Sleepy, but nothing stood out. Can you recommend a profiler for windows?
Hm. That's the one I use for profiling C code; and I've found it very effective. Effective to the point of detecting a difference between two identical opcodes where one causes a cache miss and the other doesn't.
I'd love to take a look at the output from identical runs with the two builds.
the ones that stand out most (ie, 80%+ worse) do create more perl/xs objects than typical, so perhaps that is where I should start looking?
I'd start by rebuilding the 5.24 without PERL_COPY_ON_WRITE & PERL_HASH_FUNC_ONE_AT_A_TIME_HARD individually and together and see what effect they have.
I believe (perhaps wrongly) that the first is a space for speed tradeoff which might be factor.
The second is an (IMO) unnecessary fix for a non-problem that substitutes a different, more time consuming hashing function for the one used in 5.8.9 for "security reasons". Try replacing PERL_HASH_FUNC_ONE_AT_A_TIME_HARD with PERL_HASH_FUNC_ONE_AT_A_TIME_OLD and see if that makes any difference.
Beyond those guesses, I'd need to see the profiler output.
With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
In the absence of evidence, opinion is indistinguishable from prejudice.
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open my $fh, "</usr/share/dict/words" or die;
my %h;
$h{$_}++ while <$fh>;
consumes the following number of CPU Mcycles under various perls:
5.8.9 1,245
5.18.0 1,143
5.20.0 1,113
5.22.0 1,163
5.24.0 1,089
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First: I did say "Beyond those guesses,". The information provided by the OP so far consists solely of the build parameters for the two builds. I compared those two sets and attempted to reason about possibilities.
A non-problem that allows you to trivially DoS any web server where input from the client
Hm. That problem was addressed way back in 2003/5.8.1 with something akin to this:
So what new problem was addressed by the 5.17 changes? (And has anyone ever seen a plausible demonstration of that "new problem"? Has there ever been a reported sighting of anyone exploiting that new problem in the field? If the change is so critical, why wasn't it back-ported to 5.10 and other earlier versions that are still being shipped with 95% of *nix distributions?)
Anyway, perl's hash handling has been getting faster, not slower in recent years.
Agreed. Not just hash handling, but just about every aspect of Perl (save maybe string handling) has gotten faster in recent builds. Congratulations.
However, over the years there have been some weird behaviours that only affected windows builds.
Once again I'll remind you that I was attempting to help the OP on the basis of the minimal information supplied; whilst asking him to provide more.
With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
In the absence of evidence, opinion is indistinguishable from prejudice.
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Are you sure these speedups are due to Perl's hash improvements, and not improvements in Perl's IO handling? Because that latter would have been my first guess. A more interesting comparison might be to time the script under two modes, one with a simple counter increment and one with the hash addition. The difference between these two running times would be more illuminating, I think.
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Beyond those guesses, I'd need to see the profiler output.
Ok, I've found the issue. As with these things, a very unexpected source..
pthread_mutex_lock
We use pthreads as our threading library and for some reason, the version of pthreads that comes with strawberry is massively slower than what we are currently using. Remove all the lock/unlocks, and the 64bit 5.24.0 is faster than 32bit 5.8.9.
Now to figure out why this version of the library is so slow...
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Hm. That's the one I use for profiling C code; and I've found it very effective. Effective to the point of detecting a difference between two identical opcodes where one causes a cache miss and the other doesn't.
Ok, you've inspired me to look at sleepy again. Do you have any tips on using sleepy? Due to it sampling, I assume that the test cases need to run for some time? Any specific compile options I should use?
I isolated some of the code for the memory test case (the 80%+ slow down), and it turns our that the 64bit 5.24 version is much faster than the 32bit 5.8.9 version on basic perl/xs/c object creation/destruction. I need to do more digging.
I've been writing other test cases, and I'm suspecting something in the xs layer.
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Do you have any tips on using sleepy?
Once you've narrowed the scope of the possibilities, bracket the suspect code with calls to getc() or similar so that you can attach teh profiler just before the suspect code and stop instrumenting immediately after.
Not so useful if you've no idea where to look; but very useful once you do.
I assume that the test cases need to run for some time?
It kind of depends on the nature of the code; but longer you run in the errant code the more likely things are to stand out.
Any specific compile options I should use?
I use MS compilers rather than gcc, so I'm not familiar with the latters options. However, you should basically stick to the same options you use for your production code. Anything else is just apples and oranges.
With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
In the absence of evidence, opinion is indistinguishable from prejudice.
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Re^3: Our perl/xs/c app is 30% slower with 64bit 5.24.0, than with 32bit 5.8.9. Why?
by talexb (Chancellor) on Dec 21, 2016 at 21:26 UTC
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