On Mon, Dec 30, 2013 at 04:22:49PM -0800, Pravin Shelar wrote:
> On Mon, Dec 30, 2013 at 4:12 PM, Ben Pfaff <b...@nicira.com> wrote:
> > On Mon, Dec 30, 2013 at 03:56:38PM -0800, Pravin Shelar wrote:
> >> On Mon, Dec 30, 2013 at 3:32 PM, Ben Pfaff <b...@nicira.com> wrote:
> >> > 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.
> >> >
> >> Depending on kernel, max cpu id can be calculated by a system call or
> >> sysfs read.
> >
> > For future reference, which syscall or sysfs object is that?
> >
> sysconf(_SC_NPROCESSORS_CONF): number of possible CPU.
> sched_getaffinity(): by examining returned CPU mask.
> sysfs: /sys/devices/system/cpu/possible
> procfs: /proc/cpuinfo
Thanks. I did not know about most of those.
> > I'm leaning toward just putting a mutex around these variables, and
> > skipping the ovsthread_counter abstraction entirely for now.
>
> I am fine with optimizing it later.
OK.
Thanks for all the ideas.
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