On Sat, Jan 21, 2017 at 09:02:25PM +0200, Ants Aasma wrote:
> On Sat, Jan 21, 2017 at 6:41 PM, Andreas Karlsson <andreas@proxel.se> wrote:
> > It might be worth looking into using the CRC CPU instruction to reduce this
> > overhead, like we do for the WAL checksums. Since that is a different
> > algorithm it would be a compatibility break and we would need to support the
> > old algorithm for upgraded clusters..
>
> We looked at that when picking the algorithm. At that point it seemed
> that CRC CPU instructions were not universal enough to rely on them.
> The algorithm we ended up on was designed to be fast on SIMD hardware.
> Unfortunately on x86-64 that required SSE4.1 integer instructions, so
> with default compiles there is a lot of performance left on table. A
> low hanging fruit would be to do CPU detection like the CRC case and
> enable a SSE4.1 optimized variant when those instructions are
> available. IIRC it was actually a lot faster than the naive hardware
> CRC that is used for WAL and about on par with interleaved CRC.
I am afraid that won't fly with most end-user packages, cause
distributions can't just built packages on their newest machine and then
users get SIGILL or whatever cause their 2014 server doesn't have that
instruction, or would they still work?
So you would have to do runtime detection of the CPU features, and use
the appropriate code if they are available. Which also makes regression
testing harder, as not all codepaths would get exercised all the time.
Michael
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