XLA:CPU performance regression with the min alignment changed from 16 to 64

[snadampal]

I'm observing performance regressions for bert and bart model inference with jax mainline compared to jax-v0.4.34 on both x86 and arm64 cpu platforms. The performance drop is around 50%. I have root-caused it to the following PR (and commit) that updated the minimum alignment for the buffer from 16 to 64 to match Eigen.

I have tested two AWS EC2 instances:
c7i.xlarge (x86 architecture) and
c8g.xlarge (arm64 architecture)

I'm able to restore the performance by switching the alignment back to 16.
Can you please let me know what scenarios was this change helping?

PR: #16505
commit:

commit 7db61fe79207e457c89886048afa21484d827590
Author: Adam Banaś <[email protected]>
Date:   Thu Oct 10 04:06:07 2024 -0700

    [XLA:CPU] Change the minimum alignment of buffers to match Eigen
    
    PiperOrigin-RevId: 684386915

[snadampal]

hi @Adam-Banas , appreciate if you could comment on this issue.

[Adam-Banas]

Hi @snadampal, can you provide the script you're using to reproduce?

One reason for such a significant performance drop is that additional buffer copies are performed (instead of views). This can happen if XLA receives unaligned buffers.

To answer your question, there were two reasons for integrating this change:

• Performance boost was reported on some models with higher alignment (actually, even on Bert): [XLA:CPU] Increase minimum alignment of buffers for CPU #17444.
• Compatibility with TensorFlow, which expects tensors to be aligned with Eigen's minimum alignment.