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

[Mehdi Amini via llvm-commits]

> On Oct 13, 2016, at 5:07 PM, Teresa Johnson <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.

— 
Mehdi