On Mon, 22 Feb 2021, Jens Axboe wrote: > On 2/22/21 2:31 PM, Hugh Dickins wrote: > > On Thu, 18 Feb 2021, Jens Axboe wrote: > >> On 2/18/21 4:16 PM, Andrew Morton wrote: > >>> On Thu, 18 Feb 2021 14:36:31 -0700 Jens Axboe <ax...@kernel.dk> wrote: > >>> > >>>> Currently we cap the batch count at max(32, 2*nr_online_cpus), which > >>>> these > >>>> days is kind of silly as systems have gotten much bigger than in 2009 > >>>> when > >>>> this heuristic was introduced. > >>>> > >>>> Bump it to capping it at 256 instead. This has a noticeable improvement > >>>> for certain io_uring workloads, as io_uring tracks per-task inflight > >>>> count > >>>> using percpu counters. > > > > I want to quibble with the word "capping" here, it's misleading - > > but I'm sorry I cannot think of the right word. > > Agree, it's not the best wording. And if you can't think of a better > one, then I'm at a loss too :-) > > > The macro is max() not min(): you're making an improvement for > > certain io_uring workloads on machines with 1 to 15 cpus, right? > > Does "bigger than in 2009" apply to those? > > Right, that actually had me confused. The box in question has 64 threads, > so my effective count was 128, or 256 with the patch.
Ah, yes, so there I *was* confused in saying "1 to 15", the improvement was for "1 to 127" of course - thanks. > > > Though, io_uring could as well use percpu_counter_add_batch() instead? > > That might be a simpler/better choice! > > > (Yeah, this has nothing to do with me really, but I was looking at > > percpu_counter_compare() just now, for tmpfs reasons, so took more > > interest. Not objecting to a change, but the wording leaves me > > wondering if the patch does what you think - or, not for the > > first time, I'm confused.) > > I don't think you're confused, and honestly I think using the batch > version instead would likely improve our situation without potentially > changing behavior for everyone else. So it's likely the right way to go. You're too polite! But yes, if percpu_counter_add_batch() suits, great. > > Thanks Hugh! > > -- > Jens Axboe