Messages in this thread |  | | Subject | Re: aio: questions with ioctx_alloc() and large num_possible_cpus() | From | Mauricio Faria de Oliveira <> | Date | Wed, 5 Oct 2016 14:21:27 -0300 |
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Hi Kent,
Thanks for commenting. I understood more of the code in trying to make sense of your point, but there are some things still unclear about it; if you could help a bit more, please.
Can you describe how a single thread might not be able to use all the slots because 'up to about half of the reqs_available slots might be on other percpu reqs_available' ?
I see that the thread might be scheduled on different CPUs (say, only 2 possible CPUs) and perform get_reqs_available() on both -- but that only gives one req_batch to each CPU, and for req_batch to be half of reqs_available its denominator needs to be 2, which doesn't happen w/ num_possible_cpus() * 4 -- which is 8. So I'm a bit confused here.
atomic_set(&ctx->reqs_available, ctx->nr_events - 1); ctx->req_batch = (ctx->nr_events - 1) / (num_possible_cpus() * 4);
On 10/05/2016 03:34 AM, Kent Overstreet wrote: >> - why "num_possible_cpus() * 4", and why "max(nr_events, <it>)" ?
> For the scheme to work - percpu allocation of slots - we have to ensure that > there aren't too many unused slots stranded on other CPUs. The stranding is > limited to 1/4th of the slots [snip]
By 'unused slots' you mean the slots included in the batch allocated to a particular cpu but not actually used by a thread in that cpu? (e.g., get_reqs_available() called once, unused_slots == req_batch - 1)
Can you please detail a bit more how the limit to 1/4th of the slots is ensured because of "num_possible_cpus() * 4", and what is the scenario where the math is based on? I've been thinking and assuming values for a while now, and didn't figure out the point where / how it occurs.
Thanks for your support,
-- Mauricio Faria de Oliveira IBM Linux Technology Center
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