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

[Mehdi Amini via llvm-commits]

> On Oct 13, 2016, at 5:22 PM, Teresa Johnson <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…).



> , and I while ThinLTO backend tasks would qualify, I don't think that's why they don't scale well beyond the # physical cores (but rather the memory intensiveness as you pointed out in the other thread). I'll go with this for now.
> 
> 
> 
> —
> Mehdi
> 
> 
> 
> 
> 
> 
> -- 
> Teresa Johnson |	 Software Engineer |	 tejohnson at google.com <mailto:tejohnson at google.com> |	 408-460-2413 <tel:408-460-2413>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20161013/020925c5/attachment.html>