On 04/14/2011 08:42 PM, mark florisson wrote:
On 14 April 2011 20:29, Dag Sverre Seljebotn<d.s.seljeb...@astro.uio.no> wrote:
On 04/13/2011 11:13 PM, mark florisson wrote:
Although there is omp_get_max_threads():
"The omp_get_max_threads routine returns an upper bound on the number
of threads that could be used to form a new team if a parallel region
without a num_threads clause were encountered after execution returns
from this routine."
So we could have threadsvailable() evaluate to that if encountered
outside a parallel region. Inside, it would evaluate to
omp_get_num_threads(). At worst, people would over-allocate a bit.
Well, over-allocating could well mean 1 GB, which could well mean getting an
unecesarry MemoryError (or, like in my case, if I'm not careful to set
ulimit, getting a SIGKILL sent to you 2 minutes after the fact by the
cluster patrol process...)
But even ignoring this, we also have to plan for people misusing the
feature. If we put it in there, somebody somewhere *will* write code like
this:
nthreads = threadsavailable()
with parallel:
for i in prange(nthreads):
for j in range(100*i, 100*(i+1)): [...]
(Yes, they shouldn't. Yes, they will.)
Combined with a race condition that will only very seldomly trigger, this
starts to sound like a very bad idea indeed.
So I agree with you that we should just leave it for now, and do
single/barrier later.
omp_get_max_threads() doesn't have a race, as it returns the upper
bound. So e.g. if between your call and your parallel section less
OpenMP threads become available, then you might get less threads, but
never more.
Oh, now I'm following you.
Well, my argument was that I think erroring in that direction is pretty
bad as well.
Also, even if we're not making it available in cython.parallel, we're
not stopping people from calling omp_get_max_threads directly
themselves, which should be OK for the people who know enough to do this
safely...
Dag Sverre
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