[Openmp-commits] [PATCH] Allow machine hierarchy expansion

[Wilmarth, Terry L]

It does represent the machine hierarchy, when there is no oversubscription, but it is actually used as a way to map existing threads to the machine hierarchy for the purpose of performing a barrier. Adding a level when oversubscribed simply adds more tree structure for all the additional threads to communicate along.  One additional level duplicates the existing hierarchy as a second child of a new root, doubling the number of threads that the barrier can handle.

-----Original Message-----
From: hfinkel at anl.gov [mailto:hfinkel at anl.gov] 
Sent: Wednesday, May 27, 2015 1:16 PM
To: Peyton, Jonathan L; hfinkel at anl.gov
Cc: Wilmarth, Terry L; llvm-commits at cs.uiuc.edu; openmp-commits at dcs-maillist2.engr.illinois.edu
Subject: Re: [PATCH] Allow machine hierarchy expansion

In http://reviews.llvm.org/D9900#178569, @tlwilmar wrote:

> Oversubscription.  There's nothing to prevent the user from requesting an arbitrarily large number of threads.


But I thought that the machine hierarchy information represented the physical properties of the machine (cpus, cores, sockets, etc.). How does oversubscription factor into this? And how does that affect the number of levels?


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