Re: [boost] unordered_map 32bit VS 64bit

Subject: Re: [boost] unordered_map 32bit VS 64bit
From: Grund, Holger (ISGT) (Holger.Grund_at_[hidden])
Date: 2012-01-25 14:05:01

> I'm porting a project from Linux 32bit to Linux 64bit. While doing so I
> notice that I have a dramatic performance degradation with
> unordered_map.
> My platform is Ubuntu 10.4.3 64bit running on Intel Xeon X3220 but, I
> tested similar program on WindowsXP SP2 64bit which running on Intel i5
> and
> I get the same degradation. My boost version is 1_46_1.
> I use the following program in order to test unordered_map - Hop I
> didn't
> make any copy mistakes :-).
> 
> #include <stdio.h>
> #include <sys/time.h>
> #include <boost/unordered_map.hpp>
> 
> int main(int argc, char *argv[])
> {
>    struct timeval tv_before, tv_after;
>    boost::unordered_map<boost::int_32_t, boost::int32_t> boost_map;
> 
>    boost_map[1] = 2;
>    boost_map[3] = 4;
> 
>    gettimeofday(&tv_before, NULL);
>    for(int i = ; i < 100000; i++)
>       boost_map.find(1);
>     gettimeofday(&tv_after, NULL);
> 
>    printf("Boost time is %f \n",
>            (double)(((tv_after.tv_sec - tv_before.tv_sec) * 1000000 +
> (double)(tv_after.tv_usec - tv_before.tv_usec)) / 100000));
> 
>    return 0;
> }
> 
> I use the following compile commands:
> 64bit - g++ -O3 perf_test -o perf_test_64bit
> 32bit - g++ -m32 -O3 perf_test -o perf_test_32bit
> 
> When I run the 32bit program I get 0.5 sec
> When I run the 64bit program I get 1.54 sec
> 
> Any ideas ???
> 
My guess would be that there is 64-bit division for transforming the hash to a bucket index, which is killing perf (though one would wonder why there are no peeps for an instructions like MOD(1,x) in the codegen).

I guess, you could try out by changing the bucket count and hash code to a uint32_t in the 64-bit build.

Most compilers will transform a div by constant to a mul. Maybe some toolchains would replace a div by bucketcount with a MUL when doing value-range PGO and numbers are reasonably stable (i.e. there is some geometric growth with a factor of ~2 or so, so that there's only a handful of divisors).

-hg

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