Re: [RFC PATCH 1/1] mm: batch page copies in folio_copy() and folio_mc_copy()

[David Laight <david.laight.linux@gmail.com>]

On Mon, 18 May 2026 10:43:22 +0200
"David Hildenbrand (Arm)" <david@kernel.org> wrote:

...
> > Another option is to leave memcpy() untouched for this series and add
> > a new copy_pages() helper that the folio copy path can use. It would
> > use ALTERNATIVE_2 that picks rep movsb on ERMS/FSRM and rep movsq on
> > REP_GOOD and per-page copy_page() loop as the final fallback.  
> 
> That would fit the clear_pages() design we have. But if that's avoidable, that
> would be nice.
> 

For full pages 'rep movsq' is likely to be 'almost the best' on all cpu.
The fixed overhead is amortised over a lot of copies so has little impact.
(My brain suggests a value of 30 clocks - ignoring P4 netburst.)
For Intel cpu the aligned destination will double throughput.

I did a load of benchmarking of 'rep movsb' on my Zen-5.
(I should be able to find the results again.)
The real oddity was copies where (something like):
   0 < (dst - src) & 4095 < 128
when the startup time was a lot longer and the copy ran massively slower. 

I need to run those tests on some other cpu.
However I don't have any older AMD ones (except a piledriver) or Intel
ones newer than an i7-7 (Kaby lake?).
(I need to get my Apollo Lake N3350 into the test set for comparison.)
From what I remember of some earlier benchmarking (which failed to
measure the fixed cost properly) even Sandy bridge handles 'rep movsb'
and 'rep movsq' the same way.

The problem with memcpy() is you want a hint from the source about the
likely length and any alignment assumptions.
Otherwise the costs of the conditionals become significant.

-- David