Re: [PATCH bpf-next v3] selftests/bpf: Add LPM trie microbenchmarks

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Fri, Aug 8, 2025 at 7:21 AM Matt Fleming <matt@readmodwrite.com> wrote:
>
> On Thu, Jul 31, 2025 at 5:41 PM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > well, random-key update when the map is full is also quite different from
> > random-key update when the map is empty.
> >
> > Instead doing an update from user space do timed ops:
> > 1 start with empty map, update (aka insert) all keys sequentially
> > 2 lookup all sequentially
> > 3 delete all sequentially
> > 4 update (aka insert) all sequentially
> > 5 lookup random
> > 6 update random
> > 7 delete all random
> >
> > The elapsed time for 1 and 4 should be exactly the same.
> > While all others might have differences,
> > but all can be compared to each other and reasoned about.
>
> Having both sequential and random access for the benchmarks is fine,
> but as far as I can tell the scheme you propose is not how the bpf
> bench framework is implemented.
>
> Plus, handing off a map between subtests is brittle and prone to
> error. What if I just want to investigate the sequential access update
> time? The cost of the most expensive op (probably delete) is going to
> dwarf all over timings making it difficult to separate them and this
> scheme is going to be susceptible to noise if I can't crank up the
> number of iterations without altering the number of entries in the
> map. Microbenchmarks mitigate noise/run-to-run variance by doing a
> single op over and over again.

The bench has to repeat the operation obviously.
The point is that it cannot benchmark 'delete' in isolation.
And it cannot benchmark 'update of empty aka insert' in isolation.
So some operations have to be paired and when another one
can be done standalone then the delta is the performance for the other.
In the above 2,5,6 are repeatable. The others need to be paired.

7 with pure random is probably not a good idea, since it will
try to delete some keys that were already deleted and will skew the numbers.