Re: [RFC/PATCH 2/3] UDP memory usage accounting: accounting unit and variable

[Andi Kleen <andi@firstfloor.org>]

Satoshi OSHIMA <satoshi.oshima.fk@hitachi.com> writes:

> This patch introduces global variable for UDP memory accounting.
> The unit is page.

The global variable doesn't seem to be very MP scalable, especially
if you change it for each packet. This will be a very hot cache line,
in the worst case bouncing around a large machine.

Possible alternatives:
- Per CPU variables
- You only change the global on socket creation time (by pre allocating a large
amount) or when the system comes under memory pressure.
- Batching of the global updates for multiple packets [that's a variant
of the previous one, might be still too costly though]

Also for such variables it's usually good to cache line pad them on SMP
to avoid false sharing with something else.

-Andi