Re: [PATCH bpf-next 2/2] selftests/bpf: Add mmap/munmap benchmark for array maps

[Jiri Olsa <olsajiri@gmail.com>]

On Thu, Jul 23, 2026 at 02:58:45PM -0700, Song Liu wrote:
> On Thu, Jul 23, 2026 at 11:46 AM Jiri Olsa <olsajiri@gmail.com> wrote:
> [...]
> > > +static void *producer(void *input)
> > > +{
> > > +     while (true) {
> > > +             void *addr;
> > > +
> > > +             addr = mmap(NULL, ctx.mmap_sz, PROT_READ | PROT_WRITE,
> > > +                         MAP_SHARED, ctx.map_fd, 0);
> > > +             if (addr == MAP_FAILED) {
> > > +                     fprintf(stderr, "mmap failed: %d\n", -errno);
> > > +                     exit(1);
> > > +             }
> >
> > hi,
> > so now pages are populated only when accessed, benchmark shows
> > this nicely:
> >
> > before:
> >                 nr_threads: 1, map_size: 1048576
> >         arraymap-mmap:       throughput: 0.006 ą 0.000 M ops/s, latency: 172703.437 ns/op
> >
> > after:
> >                 nr_threads: 1, map_size: 1048576
> >         arraymap-mmap:       throughput: 0.082 ą 0.002 M ops/s, latency: 12212.778 ns/op
> >
> >
> > would it make sense to add some map access in here (each page I guess)
> > enabled by new option and measure the impact for accessed map ?
> > I assume latency should be equal in such case.
> 
> If we add a read per page, which is the worst case:
> 
> before:
>         nr_threads: 1, map_size: 1048576
> arraymap-mmap:       throughput: 0.032 ± 0.000 M ops/s, latency: 31248.846 ns/op
> 
>         nr_threads: 1, map_size: 8388608
> arraymap-mmap:       throughput: 0.003 ± 0.000 M ops/s, latency:
> 372565.622 ns/op
> 
>         nr_threads: 1, map_size: 67108864
> arraymap-mmap:       throughput: 0.000 ± 0.000 M ops/s, latency:
> 2061083.005 ns/op
> 
> after:
>         nr_threads: 1, map_size: 1048576
> arraymap-mmap:       throughput: 0.006 ± 0.000 M ops/s, latency:
> 170794.193 ns/op
> 
>         nr_threads: 1, map_size: 8388608
> arraymap-mmap:       throughput: 0.001 ± 0.000 M ops/s, latency:
> 1346389.229 ns/op
> 
>         nr_threads: 1, map_size: 67108864
> arraymap-mmap:       throughput: 0.000 ± 0.000 M ops/s, latency:
> 10700389.105 ns/op
> 
> So filling all the pages in the mmap is faster than faulting one page at a time
> in many page faults. IOW, there is a performance regression in this case.

would it make sense to add new map flag for that?

jirka