Re: [PATCH v3 31/35] lib: add memory allocations report in show_mem()

On Thu, 15 Feb 2024 19:32:38 -0500
Kent Overstreet [off-list ref] wrote:

quoted

But where are the benchmarks that are not micro-benchmarks. How much
overhead does this cause to those? Is it in the noise, or is it noticeable?  

Microbenchmarks are how we magnify the effect of a change like this to
the most we'll ever see. Barring cache effects, it'll be in the noise.

Cache effects are a concern here because we're now touching task_struct
in the allocation fast path; that is where the
"compiled-in-but-turned-off" overhead comes from, because we can't add
static keys for that code without doubling the amount of icache
footprint, and I don't think that would be a great tradeoff.

So: if your code has fastpath allocations where the hot part of
task_struct isn't in cache, then this will be noticeable overhead to
you, otherwise it won't be.

All nice, but where are the benchmarks? This looks like it will have an
affect on cache and you can talk all you want about how it will not be an
issue, but without real world benchmarks, it's meaningless. Numbers talk.

-- Steve