2007-09-30

Data-wide wide-finder

Running a test with just breaking into lines without the matching, compared to the non-MPI version with matching runs on the 100x file in 380ms user time rather than 720ms. So there is work in both parts of the string processing.

Parallelising the scan for newlines to work on 8 chars at a time gives a reduction to around 220ms.

Restoring the matching, and parallelising the scan for the first character makes the CPU run at 800MHz.

A quick google search tells me that sudo dpkg-reconfigure gnome-applets enables me to use the gnome panel applet to lock the frequency at 2200MHz, so with that set, I can compare results between the linear and SIMD parallel forms. Typical results are around 400ms better for the parallelised version on the laptop:
fortinbras:$ time bin/tbray4 datasets/hundred-o10k.ap
matches: 110000
real 0m10.420s
user 0m0.688s
sys 0m0.268s

fortinbras:$ time bin/tbray6 datasets/hundred-o10k.ap
matches: 110000
real 0m8.622s
user 0m0.284s
sys 0m0.248s
This makes user+sys time go from 950ms to 532ms; if this process was CPU limited, that's as good as you could possibly get from running two cores.

Sorting out a few machine word dependencies so it runs on one of the Netra Solaris boxes (Linux/Solaris code is at tbray6.cpp; I was assuming size_t would be 64 bit on all 64 OSes), the results are:
tercel-2:$ time bin/tbray4 datasets/hundred-o10k.ap
matches: 110000

real 0m7.593s
user 0m5.638s
sys 0m1.894s

tercel-2:$ time bin/tbray6 datasets/hundred-o10k.ap
matches: 110000

real 0m5.915s
user 0m4.027s
sys 0m1.855s
Here we get a much more significant speed up - about 25% faster overall time, due to a different balance between CPU and disk.

These simple benchmarks indicate a few things to me:

Not all gains from parallelism require concurrent processes - more transistors can mean wider data paths as well as concurrent cores. If I were using the architecture specific SIMD registers on the AMD64, then the paths would be twice as wide again.

A slower CPU with a 'server class' disk outperforms a faster CPU with a 'consumer class' disk in this test, but shows up potential to parallelise the code.

I like playing with bit-twiddly code on the weekends. I don't like having to debug bit-twiddly code in mission critical applications for my customers to a tight deadline. The optimisations should be in the regex library, rather than complicating user code - for example, this code avoids reading past the end of the buffer only because it has a pattern longer than the word size.

I'm also wondering whether some kind of hash based wide-scan can detect two or more characters of a pattern well enough to give an improvement, rather than just a wide-scan for first one - there's probably only a couple of bits of information per character in those log files.


Pete

ETA:
fortinbras:$ grep -v "^ *\(\($\|[}{] *$\)\|\(//.*\)\)" src/tbray6.cpp | wc -l
85

It's hairy but it's not huge.

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