On Mon, Dec 30, 2013 at 03:05:56PM -0800, Pravin Shelar wrote:
> On Mon, Dec 30, 2013 at 2:44 PM, Ben Pfaff <b...@nicira.com> wrote:
> > On Mon, Dec 30, 2013 at 02:37:38PM -0800, Pravin Shelar wrote:
> >> On Mon, Dec 30, 2013 at 1:34 PM, Ben Pfaff <b...@nicira.com> wrote:
> >> > On Mon, Dec 30, 2013 at 11:10:22AM -0800, Pravin Shelar wrote:
> >> >> On Mon, Dec 30, 2013 at 10:49 AM, Ben Pfaff <b...@nicira.com> wrote:
> >> >> > On Mon, Dec 30, 2013 at 10:40:13AM -0800, Pravin Shelar wrote:
> >> >> >> On Fri, Dec 27, 2013 at 8:03 PM, Ben Pfaff <b...@nicira.com> wrote:
> >> >> >> > ovsthread_counter is an abstract interface that could be
> >> >> >> > implemented
> >> >> >> > different ways. The initial implementation is simple but less than
> >> >> >> > optimally efficient.
> >> >> >> >
> >> >> >> > Signed-off-by: Ben Pfaff <b...@nicira.com>
> >> >> >> > +void
> >> >> >> > +ovsthread_counter_inc(struct ovsthread_counter *c, unsigned long
> >> >> >> > long int n)
> >> >> >> > +{
> >> >> >> > + c = &c[hash_int(ovsthread_id_self(), 0) % N_COUNTERS];
> >> >> >> > +
> >> >> >> Does it make sense optimize this locking so that threads running on
> >> >> >> same numa-node likely share lock?
> >> >> >> we can use process id hashing to achieve it easily.
> >> >> >
> >> >> > Yes, that makes a lot of sense. How do we do it?
> >> >> >
> >> >> Use processor id (sched_getcpu()) to hash it. In case of
> >> >> sched_getcpu() is not available then we can read thread affinity using
> >> >> sched_getaffinity() and return assigned CPU, in properly optimized
> >> >> environment we can assume that a thread wold be pinned to one cpu
> >> >> only. But I am not sure of doing on platforms other than linux.
> >> >
> >> > That's reasonable.
> >> >
> >> > But, on second thought, I am not sure of the benefit from threads on
> >> > the same node sharing a lock. I see that there are benefits from
> >> > threads on different nodes having different locks, but I'm not sure
> >> > that using only one lock on a single node really saves anything. What
> >> > do you think?
> >>
> >> Then how about having per-cpu lock?
> >
> > That would be ideal but how would I do it?
>
> Create array of ovsthread_counter__ for all possible cpu. So
> N_COUNTERS would be variable equal to num of possible cpu. To
> increment use counter at index sched_getcpu().
You make it sound easy ;-) I think that's harder than it sounds. "All
possible cpus" is hard to figure out (is it possible from userspace?)
especially since the number of cpus can increase or decrease. I don't
know of a guarantee that cpus are numbered consecutively from 0 (from
1?) so we'd probably need a hash table instead of an array. We'd
probably need a mutex anyway because there's no guarantee that
the process doesn't get migrated between cpus while running this
code. And we'd still need a fallback for non-Linux.
Per-thread would be easier. I'd have just used a per-thread variable
here except that POSIX doesn't provide a way to get access to other
threads' data, which makes it hard to get the sum.
In the end, my goal here is to provide a useful abstraction and a
simple implementation that is likely to be correct. Then, later, if
profiling shows that this is an actual bottleneck, we can optimize
it. But I don't want to spend a bunch of time optimizing code that I
don't even know is important.
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