Would like option to report hwloc bindings

[eschnett]

I would like to have an option that make HPX applications report the actual hwloc bindings used. This should use hwloc to read the bindings from the OS for each thread.

[hkaiser]

We should rather change --hpx:print-bind to report the actual bindings...

[sithhell]

If --hpx:print-bind doesn't report the actual bindings, there is a severe bug somewhere between setting the affinity masks and binding them to the cores in use.

[hkaiser]

Either we have that bug and it needs to be fixed or we can close this ticket. Do we have any evidence for such a problem?

[sithhell]

I'll investigate this issue on Wednesday. I thought i fixed those problems. Will check on various machines with different CPUs. I am currently using hwloc 1.7.2. Might be that earlier hwloc versions have a bug.

[sithhell]

I can not reproduce this problem. Which version of hwloc are you using?

[eschnett]

The issue was originally that --hpx:print-bind examines the command line options, creates an HPX-internal representation of these, and then outputs these. It did not call hwloc_get_cpubind to find out the actual bindings. This led to several errors in the past, since what hwloc_set_cpubind actually did was different from what was reported.

I thus request that the code should call hwloc_get_cpubind to find out the actual bindings, and then report these.

The only call to hwloc_get_cpubind is in the "tests" directory. I thus assume that hpx:print-bind does not actually call hwloc_get_cpubind.

[eschnett]

One a phone call this past Wednesday, Hartmut suggested to revisit this issue once we had a new case where --hpx:print-bind outputs wrong information.

There is now such a case; see #981.

[eschnett]

The current code still doesn't use hwloc_get_cpubind to output the actual bindings. As before, only HPX's view of the world is output. Given that view was wrong multiple times in the past weeks, I still strongly suggest to use hwloc_get_cpubind to query the actual CPU bindings, and to output these. Errors in CPU bindings are difficult to detect by an unsuspecting user, and tools such as --hpx:print-bind must be reliable.

[pagrubel]

watching

[sithhell]

Am 29.10.2013 14:46 schrieb "Erik Schnetter" notifications@github.com:

The current code still doesn't use hwloc_get_cpubind to output the actual
bindings. As before, only HPX's view of the world is output.

That's not entirely true. The commit I made now uses the exact same masks
which are used to bind the threads. I ditched the code which did the whole
command line parsing and lead to errors. The only thing that could lead to
a wrong output is that hwloc fails to bind the threads correctly. I could
add code which queries the current binding again but I don't see any
additional value in that.

Given that view was wrong multiple times in the past weeks, I still
strongly suggest to use hwloc_get_cpubind to query the actual CPU bindings,
and to output these. Errors in CPU bindings are difficult to detect by an
unsuspecting user, and tools such as --hpx:print-bind must be reliable.

I agree that there were bugs in the way we reported and calculated the
thread affinities. This should be detectable now for the reasons described
above.

[sithhell]

As far as I can tell, we have a reliable solution right now. It is not 100% fool proof for future changes. Will move the final resolution to 1.0.0.