Re: [PATCH net-next v10 05/14] netdev: netdevice devmem allocator

Andrew Lunn
2024-06-05T00:52:45Z

> How is the compiler going to know which path is going to be taken the most?
> There's two main paths in the ring buffer logic. One when an event stays on
> the sub-buffer, the other when the event crosses over to a new sub buffer.
> As there's 100s of events that happen on the same sub-buffer for every one
> time there's a cross over, I optimized the paths that stayed on the
> sub-buffer, which caused the time for those events to go from 250ns down to
> 150 ns!. That's a 40% speed up.
>
> I added the unlikely/likely and 'always_inline' and 'noinline' paths to
> make sure the "staying on the buffer" path was always the hot path, and
> keeping it tight in cache.
>
> How is a compiler going to know that?

It might have some heuristics to try to guess unlikely/likely, but
that is not what we are talking about here.

How much difference did 'always_inline' and 'noinline' make? Hopefully
the likely is enough of a clue it should prefer to inline whatever is
in that branch, where as for the unlikely case it can do a function
call.

But compilers is not my thing, which is why i would reach out to the
compiler people and ask them, is it expected to get this wrong, could
it be made better?

Andrew