Hi John --
A little bit of follow-up to what Michael says here ...
The "nemesis" he refers to is the internal name of the particular
Qthreads scheduler we use when CHPL_LOCALE_MODEL=flat. Our understanding
is that the nemesis scheduler currently doesn't move qthreads (and by
extension, Chapel tasks) from pthread to pthread, which would break the
use of pthread local storage inside the remote caching implementation.
But there are significant caveats here:
* We use a different Qthreads scheduler when CHPL_LOCALE_MODEL=numa,
and that one definitely does move qthreads (thus Chapel tasks) from
pthread to pthread.
* We can't guarantee that we'll always use "nemesis" with the flat
locale model.
* We can't guarantee that, even if we do keep using it, "nemesis" will
continue to not move qthreads (thus Chapel tasks) from pthread to
pthread.
Taken together, this basically says that although we haven't observed
remote caching failures with qthreads, that shouldn't be taken as
evidence that it definitely does work now or will work in the future.
greg
On 2/18/2015 2:31 PM, Michael Ferguson wrote:
Hi -
One more thing about the --cache-remote feature, just to be clear and
for future reference:
The remote caching depends on pthread local storage, and Chapel task
movement among worker pthreads in Qthreads-based tasking could break
it. So far we haven't seen this happen, but we cannot guarantee it
won't. Symptoms of a failure could include silent wrong answers or
segfaults, either of which could be solid or intermittent/sporadic.
I *think* that this problem won't come up with the nemesis qthreads
scheduler, but we need to do some careful analysis before we can
declare the --cache-remote feature safe to use with qthreads.
Cheers,
-michael
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