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SubjectRe: [patch 2/4] net: percpufy frequently used vars -- struct proto.memory_allocated
On Tue, Mar 07, 2006 at 06:14:22PM -0800, Andrew Morton wrote:
> Ravikiran G Thirumalai <kiran@scalex86.org> wrote:
> >
> > - if (atomic_read(sk->sk_prot->memory_allocated) < sk->sk_prot->sysctl_mem[0]) {
> > + if (percpu_counter_read(sk->sk_prot->memory_allocated) <
> > + sk->sk_prot->sysctl_mem[0]) {
>
> Bear in mind that percpu_counter_read[_positive] can be inaccurate on large
> CPU counts.
>
> It might be worth running percpu_counter_sum() to get the exact count if we
> think we're about to cause something to fail.

The problem is percpu_counter_sum has to read all the cpus cachelines. If
we have to use percpu_counter_sum everywhere, then might as well use plain
per-cpu counters instead of batching ones no?

sysctl_mem[0] is about 196K and on a 16 cpu box variance is 512 bytes, which
is OK with just percpu_counter_read I hope. Maybe, on very large cpu counts,
we should just change the FBC_BATCH so that variance does not go quadratic.
Something like 32. So that variance is 32 * NR_CPUS in that case, instead
of (NR_CPUS * NR_CPUS * 2) currently. Comments?
-
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