Re: [PATCH v2 2/5] mm: Extends local cpu counter vm_diff_nodestat from s8 to s16

[Christopher Lameter]

On Tue, 19 Dec 2017, Michal Hocko wrote:

> > Well the reason for s8 was to keep the data structures small so that they
> > fit in the higher level cpu caches. The large these structures become the
> > more cachelines are used by the counters and the larger the performance
> > influence on the code that should not be impacted by the overhead.
>
> I am not sure I understand. We usually do not access more counters in
> the single code path (well, PGALLOC and NUMA counteres is more of an
> exception). So it is rarely an advantage that the whole array is in the
> same cache line. Besides that this is allocated by the percpu allocator
> aligns to the type size rather than cache lines AFAICS.

I thought we are talking about NUMA counters here?

Regardless: A typical fault, system call or OS action will access multiple
zone and node counters when allocating or freeing memory. Enlarging the
fields will increase the number of cachelines touched.

> Maybe it used to be all different back then when the code has been added
> but arguing about cache lines seems to be a bit problematic here. Maybe
> you have some specific workloads which can prove me wrong?

Run a workload that does some page faults? Heavy allocation and freeing of
memory?

Maybe that is no longer relevant since the number of the counters is
large that the accesses are so sparse that each action pulls in a whole
cacheline. That would be something we tried to avoid when implementing
the differentials.