* Ken Chen ([email protected]) wrote:
> On 10/18/07, Mathieu Desnoyers <[email protected]> wrote:
> > Good question indeed. How large is this memory footprint exactly ? If it
> > is as small as you say, I suspect that the real issue could be that
> > these variable are accessed by the scheduler critical paths and
> > therefore trash the caches.
>
> Maybe my wording was ambiguous, I meant to reduce cache line pollution
> when accessing these schedstat fields.
>
> With unsigned long, on x86_64, schedstat consumes 288 bytes for each
> sched_domain and 128 bytes in struct rq. On a extremely small system
> that has a couple of CPU sockets with one level of numa node, there
> will be 704 bytes per CPU for schedstat. Given the sparseness of
> them, we are probably talking about 11-12 cache line eviction on
> several heavily used scheduler functions. Reduce cache line pollution
> is the primary goal, actual memory consumption isn't really a concern.
>
Generally speaking, if such cache trashing is an issue, why don't we
make sure that each task struct member is declared in this structure
following its access frequency ? (except for #ifdef blocks, which should
stay together) It could then statistically save a lot of cachelines.
Or is it already the case ? It doesn't look like it when I see:
struct list_head ptrace_list;
Just beside the
struct mm_struct *mm, *active_mm;
pointers.
Mathieu
--
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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