RE: [PATCH v2 0/3] lib/string: optimized mem* functions

[David Laight]

From: Matteo Croce
> Sent: 11 July 2021 00:08
> 
> On Sat, Jul 10, 2021 at 11:31 PM Andrew Morton
> <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Fri,  2 Jul 2021 14:31:50 +0200 Matteo Croce <mcroce@xxxxxxxxxxxxxxxxxxx> wrote:
> >
> > > From: Matteo Croce <mcroce@xxxxxxxxxxxxx>
> > >
> > > Rewrite the generic mem{cpy,move,set} so that memory is accessed with
> > > the widest size possible, but without doing unaligned accesses.
> > >
> > > This was originally posted as C string functions for RISC-V[1], but as
> > > there was no specific RISC-V code, it was proposed for the generic
> > > lib/string.c implementation.
> > >
> > > Tested on RISC-V and on x86_64 by undefining __HAVE_ARCH_MEM{CPY,SET,MOVE}
> > > and HAVE_EFFICIENT_UNALIGNED_ACCESS.
> > >
> > > These are the performances of memcpy() and memset() of a RISC-V machine
> > > on a 32 mbyte buffer:
> > >
> > > memcpy:
> > > original aligned:      75 Mb/s
> > > original unaligned:    75 Mb/s
> > > new aligned:          114 Mb/s
> > > new unaligned:                107 Mb/s
> > >
> > > memset:
> > > original aligned:     140 Mb/s
> > > original unaligned:   140 Mb/s
> > > new aligned:          241 Mb/s
> > > new unaligned:                241 Mb/s
> >
> > Did you record the x86_64 performance?
> >
> >
> > Which other architectures are affected by this change?
> 
> x86_64 won't use these functions because it defines __HAVE_ARCH_MEMCPY
> and has optimized implementations in arch/x86/lib.
> Anyway, I was curious and I tested them on x86_64 too, there was zero
> gain over the generic ones.

x86 performance (and attainable performance) does depend on the cpu
micro-archiecture.

Any recent 'desktop' intel cpu will almost certainly manage to
re-order the execution of almost any copy loop and attain 1 write per clock.
(Even the trivial 'while (count--) *dest++ = *src++;' loop.)

The same isn't true of the Atom based cpu that may be on small servers.
Theses are no slouches (eg 4 cores at 2.4GHz) but only have limited
out-of-order execution and so are much more sensitive to instruction
ordering.

	David

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