[PATCH] D25585: Add interface for querying physical hardware concurrency

[Mehdi Amini via llvm-commits]

> On Oct 13, 2016, at 5:25 PM, Mehdi Amini <mehdi.amini at apple.com> wrote:
> 
>> 
>> On Oct 13, 2016, at 5:22 PM, Teresa Johnson <tejohnson at google.com <mailto:tejohnson at google.com>> wrote:
>> 
>> 
>> 
>> On Thu, Oct 13, 2016 at 5:14 PM, Mehdi Amini <mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>> wrote:
>> 
>> > On Oct 13, 2016, at 5:07 PM, Teresa Johnson <tejohnson at google.com <mailto:tejohnson at google.com>> wrote:
>> >
>> > tejohnson added inline comments.
>> >
>> >
>> > ================
>> > Comment at: include/llvm/Support/Threading.h:121
>> > +  /// host system, otherwise falls back to thread::hardware_concurrency().
>> > +  unsigned hardware_physical_concurrency();
>> > }
>> > ----------------
>> > mehdi_amini wrote:
>> >> I think we may want to name it `hardware_coarse_concurrency`. Because:
>> >>
>> >> - This looks like expressing better what we're looking after.
>> >> - Hyperthreading is in some sense "physical concurrency", but sharing some resources.
>> >> - Other platforms may have something in between.
>> >>
>> > I thought about that name after you mentioned it on the prior review thread. But I felt that saying "physical concurrency" is a better expression for what it is actually trying to give you.  I think of the hyperthreading concurrency as "logical concurrency", vs physical concurrency due to physical cores.
>> 
>> Hyperthreading is sharing some physical resources on the core, but have some other physical resources duplicated/dedicated, which is why I feel it is murky. But “good enough” as well...
>> Also, I don’t know enough the PowerPC or Sparc equivalent of hyper-threading to know how much they share/duplicate and what we would pick on these for instance.
>> 
>> Right it is a bit murky. One reason I didn't like "coarse" is that coarse-grained parallelism relates to how closely the tasks communicate/synchronize
> 
> Fair.
> In my mind “coarse” relates to the “size” of the task. But a few light and long-lived task could fit a “coarse” level of granularity.
> 
> I don’t have anything better to qualify these tasks (“heavy” is kind of what I’m looking for, but I can’t translate this into an API name…).

Asked Duncan (he’s good at naming usually), and he suggested:

lightweight_hardware_concurrency(); // maximum number of resource available for threading
heavyweight_hardware_concurrency();  // number of dedicated hardware resource available for threading
hardware_concurrency(); // default to one of the other above, or in between, depending on the platform.


— 
Mehdi
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20161013/2555303c/attachment-0001.html>