[nsd-users] Run-time effects of new SIMD code

[Anand Buddhdev]

On 17/07/2024 16:57, Jeroen Koekkoek wrote:

Hi Jeroen,

> That is indeed correct. At runtime the CPUID instruction is used to
> detect which extensions are offered and it chooses based on that. When
> compiling on x86_64 with SSE4.2 and AVX2 enabled, the binary will
> actually contain 3 parsers, the most optimal one is used at runtime.

Thanks for this clarification. NSD doesn't log this info when starting, 
and "nsd -v" also doesn't reveal which parser it has chosen. I think 
such information will be useful to both users and developers, especially 
when trying to debug an issue. Do you think you could add this to the 
output of "nsd -v" (something like which parsers have been compiled, and 
which one is being chosen)?

So a question: as a packager and distributor, do I need to build on a 
processor that has both SSE4.2 and AVX2 instructions enabled? And if one 
or both of these are not enabled, will the resulting build contain fewer 
parsers?

> Optimized versions can be disabled with --disable-westmere (SSE4.2) and
> --disable-haswell (AVX2) if desired btw.

Thanks for providing these options. But it would help users to know the 
effects of enabling/disabling these options, so that they can make an 
informed choice.

Another question: if a user has a binary containing all three parsers, 
and wants to disable one or more of them at run-time, how is the user 
supposed to do this?

Actually, I just went to look at doc/README, which has notes on how to 
compile NSD, but not all the enable/disable options are fully 
documented, so a user cannot make informed choices about whether to 
enable/disable certain features in their builds. I think I'll open a 
separate issue about this.

Regards,
Anand