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.
But I agree fallback makes it hard.
> 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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