RE: [PATCH v2 21/24] selftests/resctrl: Read in less obvious order to defeat prefetch optimizations

[Ilpo Järvinen]

On Wed, 31 May 2023, Shaopeng Tan (Fujitsu) wrote:

> Hi Ilpo,
> 
> > When reading memory in order, HW prefetching optimizations will interfere
> > with measuring how caches and memory are being accessed. This adds noise
> > into the results.
> > 
> > Change the fill_buf reading loop to not use an obvious in-order access using
> > multiply by a prime and modulo.
> > 
> > Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>
> > ---
> >  tools/testing/selftests/resctrl/fill_buf.c | 17 ++++++++++-------
> >  1 file changed, 10 insertions(+), 7 deletions(-)
> > 
> > diff --git a/tools/testing/selftests/resctrl/fill_buf.c
> > b/tools/testing/selftests/resctrl/fill_buf.c
> > index 7e0d3a1ea555..049a520498a9 100644
> > --- a/tools/testing/selftests/resctrl/fill_buf.c
> > +++ b/tools/testing/selftests/resctrl/fill_buf.c
> > @@ -88,14 +88,17 @@ static void *malloc_and_init_memory(size_t s)
> > 
> >  static int fill_one_span_read(unsigned char *start_ptr, unsigned char
> > *end_ptr)  {
> > -	unsigned char sum, *p;
> > -
> > +	unsigned int size = (end_ptr - start_ptr) / (CL_SIZE / 2);
> > +	unsigned int count = size;
> > +	unsigned char sum;
> > +
> > +	/*
> > +	 * Read the buffer in an order that is unexpected by HW prefetching
> > +	 * optimizations to prevent them interfering with the caching pattern.
> > +	 */
> >  	sum = 0;
> > -	p = start_ptr;
> > -	while (p < end_ptr) {
> > -		sum += *p;
> > -		p += (CL_SIZE / 2);
> > -	}
> > +	while (count--)
> > +		sum += start_ptr[((count * 59) % size) * CL_SIZE / 2];
>
> Could you please elaborate why 59 is used?

The main reason is that it's a prime number ensuring the whole buffer 
gets read. I picked something that doesn't make it to wrap on almost 
every iteration.

-- 
 i.