Hi Michael,
On Fri, May 18, 2018 at 12:13:52AM +1000, Michael Ellerman wrote:

wei.guo.simon@gmail.com writes:

quoted

From: Simon Guo <redacted>

This patch is based on the previous VMX patch on memcmp().

To optimize ppc64 memcmp() with VMX instruction, we need to think about
the VMX penalty brought with: If kernel uses VMX instruction, it needs
to save/restore current thread's VMX registers. There are 32 x 128 bits
VMX registers in PPC, which means 32 x 16 = 512 bytes for load and store.

The major concern regarding the memcmp() performance in kernel is KSM,
who will use memcmp() frequently to merge identical pages. So it will
make sense to take some measures/enhancement on KSM to see whether any
improvement can be done here.  Cyril Bur indicates that the memcmp() for
KSM has a higher possibility to fail (unmatch) early in previous bytes
in following mail.
	https://patchwork.ozlabs.org/patch/817322/#1773629
And I am taking a follow-up on this with this patch.

Per some testing, it shows KSM memcmp() will fail early at previous 32
bytes.  More specifically:
    - 76% cases will fail/unmatch before 16 bytes;
    - 83% cases will fail/unmatch before 32 bytes;
    - 84% cases will fail/unmatch before 64 bytes;
So 32 bytes looks a better choice than other bytes for pre-checking.

This patch adds a 32 bytes pre-checking firstly before jumping into VMX
operations, to avoid the unnecessary VMX penalty. And the testing shows
~20% improvement on memcmp() average execution time with this patch.

The detail data and analysis is at:
https://github.com/justdoitqd/publicFiles/blob/master/memcmp/README.md

Any suggestion is welcome.

Thanks for digging into that, really great work.

I'm inclined to make this not depend on KSM though. It seems like a good
optimisation to do in general.

So can we just call it the 'pre-check' or something, and always do it?

Sound reasonable to me.
I will expand the change to .Ldiffoffset_vmx_cmp case and test accordingly.

Thanks,
- Simon