Re: [PATCH v2 2/3] powerpc/64: enhance memcmp() with VMX instruction for long bytes comparision

[Simon Guo <wei.guo.simon@gmail.com>]

Hi Cyril,
On Sat, Sep 23, 2017 at 12:06:48AM +1000, Cyril Bur wrote:
> On Thu, 2017-09-21 at 07:34 +0800, wei.guo.simon@gmail.com wrote:
> > From: Simon Guo <wei.guo.simon@gmail.com>
> > 
> > This patch add VMX primitives to do memcmp() in case the compare size
> > exceeds 4K bytes.
> > 
> 
> Hi Simon,
> 
> Sorry I didn't see this sooner, I've actually been working on a kernel
> version of glibc commit dec4a7105e (powerpc: Improve memcmp performance
> for POWER8) unfortunately I've been distracted and it still isn't done.
Thanks for sync with me. Let's consolidate our effort together :)

I have a quick check on glibc commit dec4a7105e. 
Looks the aligned case comparison with VSX is launched without rN size
limitation, which means it will have a VSX reg load penalty even when the 
length is 9 bytes.

It did some optimization when src/dest addrs don't have the same offset 
on 8 bytes alignment boundary. I need to read more closely.

> I wonder if we can consolidate our efforts here. One thing I did come
> across in my testing is that for memcmp() that will fail early (I
> haven't narrowed down the the optimal number yet) the cost of enabling
> VMX actually turns out to be a performance regression, as such I've
> added a small check of the first 64 bytes to the start before enabling
> VMX to ensure the penalty is worth taking.
Will there still be a penalty if the 65th byte differs?  

> 
> Also, you should consider doing 4K and greater, KSM (Kernel Samepage
> Merging) uses PAGE_SIZE which can be as small as 4K.
Currently the VMX will only be applied when size exceeds 4K. Are you
suggesting a bigger threshold than 4K?

We can sync more offline for v3.

Thanks,
- Simon