Re: [PATCH RFC v2] mm: introduce copy_mc_highpages() and batch copy in folio_mc_copy()

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

On Mon, 19 Oct 2026 18:54:26 +0000
"Garg, Shivank" <shivankg@amd.com> wrote:

> On Mon, 2026-10-19 at 13:19 +0100, David Laight wrote:
> > On Mon, 19 Oct 2026 10:06:49 +0000
> > Shivank Garg <shivankg@amd.com> wrote:
> >   
> > > folio_mc_copy() copies a folio one page at a time with copy_mc_highpage(),
> > > so a 2MB folio makes 512 separate copy_mc_to_kernel() calls. Each page
> > > pays the startup cost of the string instruction ("REP; MOVSB" on x86
> > > CPUs with ERMS) plus a cond_resched().  
> > 
> > Don't you always want to use 'rep movsq' for these aligned copies (on any
> > cpu anyone cares about).
> > I'm pretty even on some that don't advertise ERMS. 
> > 
> > The fastest a copy loop is ever going to run (on anything but the latest cpu)
> > is one write every clock - so 8 bytes/clock.
> > A 4k page will then take at least 512 clocks.
> > Even an old sandy bridge cpu will (IIRC) copy 32 bytes/clock if the destination
> > is 32 byte aligned (not much later cpu double that for 64byte aligned destinations).
> > So the copy part takes at most 128 clocks.
> > That makes 'rep movsq' faster unless the fixed overhead is in the P4 netburst
> > range.
> > (I was using lfence when I did my performance measurements, it hides
> > the fixed latency of rep.)
> > 
> > I've never measured zen 1-4.
> > 
> > I also suspect the slow part of a 2M copy is all the D-cache misses
> > (especially on the read side).
> > 
> > OTOH the longer copy is likely to be better...
> >   
> 
> Thanks for taking a look and for the detailed explanation.
> 
> In my measurements, 'rep movsb' and 'rep movsq' perform about the
> same for aligned, longer copies.

On Intel cpu it makes absolutely no difference which one you use.
But some of the older zen do handle them differently.

> 
> Since copy_mc_to_kernel() takes an arbitrary length, always using
> 'rep movsq' would mean adding an alignment/length, etc. or adding a
> separate copy_mc_pages() (or something similar) for page-aligned copies.

If you know you are copying aligned buffers it really is better to
have a copy routine that can rely on that fact.
I suspect it is only x86 'rep movsb' implementations that don't really care.
Pretty much all the others are going to have code to check the alignment.

David

> 
> I think that would be worthwhile x86 change on its own, independent of
> this series. Here, I'm not changing the underlying copy mechanism and
> behavior other callers.
>  
> Would it be okay to have hardware specific changes separately?
> 
> 
> Thanks,
> Shivank
> 
> 
>