Re: [PATCH] powerpc: Use aligned stores in memset

[Rajalakshmi Srinivasaraghavan <raji@linux.vnet.ibm.com>]

On 10/03/2017 11:59 PM, Adhemerval Zanella wrote:
> I think one way to provide a slight better memcpy implementation for POWER8
> and still be able to circumvent the non-aligned on non-cacheable memory
> is to use tunables.
> 
> The branch azanella/memcpy-power8 [1] has a power8 memcpy optimization which
> uses unaligned load and stores that I created some time ago but never actually
> send upstream.  It shows better performance on both bench-memcpy and
> bench-memcpy-random (about 10% on latter) and mixed results on bench-memcpy-large
> (which it is mainly dominated by memory throughput and on the environment I am
> using, a shared PowerKVM instance, the results does not seem to be reliable).
> 
> It could use some tunning, specially on some the range I used for unrolling
> the load/stores and it also does not care for unaligned access on cross-page
> boundary (which tend to be quite slow on current hardware, but also on
> current page size of usual 64k also uncommon).
> 
> This first patch does not enable this option as a default for POWER8, it just
> add on string tests as an option.  The second patch changes the selection to:
> 
>    1. If glibc is configure with tunables, set the new implementation as the
>       default for ISA 2.07 (power8).
> 
>    2. Also if tunable is active, add the parameter glibc.tune.aligned_memopt
>       to disable the new implementation selection.
> 
> So programs that rely on aligned loads can set:
> 
> GLIBC_TUNABLES=glibc.tune.aligned_memopt=1
> 
> And then the memcpy ifunc selection would pick the power7 one which uses
> only aligned load and stores.
> 
> This is a RFC patch and if the idea sounds to powerpc arch mantainers I can
> work on finishing the patch with more comments and send upstream.  I tried
> to apply same unaligned idea for memset and memmove, but I could get any real
> improvement in neither.
> 
> [1]https://sourceware.org/git/?p=glibc.git;a=shortlog;h=refs/heads/azanella/memcpy-power8

Thanks for sharing the patches. At this point we are also working on
memcpy for power8 with a different approach and we are planning
to post it soon. We can choose the better performing version and
use your tunables patch too.

-- 
Thanks
Rajalakshmi S