Re: Questions about checksum feature in 9.3

On Sun, Sep 15, 2013 at 8:13 AM, Kevin <kevo(at)gatorgraphics(dot)com> wrote:
> My attempts to compile it vectorized on OS X seemed to have failed since I don't find a vector instruction in the .o file even though the options -msse4.1 -funroll-loops -ftree-vectorize should be supported according to the man page for Apple's llvm-gcc.

I'm not sure what version of LLVM Apple is using for llvm-gcc. I know
that clang+llvm 3.3 can successfully vectorize the checksum algorithm
when -O3 is used.

> So, has anyone compiled checksum vectorized on OS X? Are there any performance data that would indicate whether or not I should worry with this in the first place?

Even without vectorization the worst case performance hit is about
20%. This is for a workload that is fully bottlenecked on swapping
pages in between shared buffers and OS cache. In real world cases it's
hard to imagine it having any measurable effect. A single core can
checksum several gigabytes per second of I/O without vectorization,
and about 30GB/s with vectorization.

Regards,
Ants Aasma
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