(Rust binding) Repeated invocation of EltwiseFMAModAVX512 (with different data) in loop has unexpected performance regression about hexl

[joserochh]

Hello @Janmajayamall. Unfortunately I no longer have the machines to run HEXL at full (Using AVX512). I can tell you modular reduction works different depending on BitShift variable.

Look at functions on fma_mod that depends on BitShift here: https://github.com/intel/hexl/blob/development/hexl/eltwise/eltwise-fma-mod-avx512.cpp

BitShift definition happens here https://github.com/intel/hexl/blob/development/hexl/eltwise/eltwise-fma-mod.cpp

Would you have the same behavior using logq = 48 or 46? just curious.

Regards,
José Rojas

[faberga]

Hi @Janmajayamall,

You mentioned that you are trying to use the Intel Advanced Vector Extensions 512 Integer Fused Multiply Add (AVX512-IFMA52) instructions. These were introduced in the 3rd Gen Intel® Xeon® Scalable Processors (and onwards), so checking which CPU manufacturer and type you are using will be important.

The AVX512-IFMA52 should only be used for primes below 50–52 bits, assuming it suffices for your computation.

For more information on how HEXL uses the AVX512-IFMA52, please refer to:
https://www.intel.com/content/www/us/en/developer/articles/technical/introducing-intel-hexl.html

and

https://arxiv.org/pdf/2103.16400.pdf

Regards,
Flavio

[faberga]

@Janmajayamall
A description of the AVX512-IFMA52 intrinsics can be found here: https://www.intel.com/content/www/us/en/docs/intrinsics-guide/index.html#avx512techs=AVX512IFMA52&cats=Arithmetic

[Janmajayamall]

Would you have the same behavior using logq = 48 or 46? just curious.

modulus/elwise_fma_mod_2d/n=32768/logq=48/mod_size=1
                        time:   [9.2788 µs 9.3188 µs 9.3523 µs]

modulus/elwise_fma_mod_2d/n=32768/logq=48/mod_size=3
                        time:   [28.762 µs 28.882 µs 28.987 µs]

modulus/elwise_fma_mod_2d/n=32768/logq=48/mod_size=5
                        time:   [76.355 µs 76.662 µs 76.946 µs]

modulus/elwise_fma_mod_2d/n=32768/logq=48/mod_size=15
                        time:   [273.11 µs 276.14 µs 279.43 µs]

Yeah it behaves same for logq=48 and can confirm same for logq=46.

I don't suspect that this is due to calling code from rust (but will still compare by implementing same in C++).

If I understand correctly the line here sets Bitshift value to 52 and uses IFMA, right?

You mentioned that you are trying to use the Intel Advanced Vector Extensions 512 Integer Fused Multiply Add (AVX512-IFMA52) instructions. These were introduced in the 3rd Gen Intel® Xeon® Scalable Processors (and onwards), so checking which CPU manufacturer and type you are using will be important.

I am using C3 machine on GCP (4th Gen Intel Xeon Scalable processor) that supports AVX512-IFMA. I don't think there are additional configs I need to enable for hexl, or am I missing something?

I am curious whether you have some ideas around what can cause this?

Thanks!

[faberga]

Hi @Janmajayamall

The 4th Gen Intel Xeon Scalable processor does support AVX512-IFMA instructions. But, just in case, assuming you are using Linux, can you check with the command "lscpu".

As far as how to make use of HEXL in an FHE library, I would suggest you study the integration of HEXL with MS SEAL and/or with OpenFHE.