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Re: [libgomp, testsuite] Support parallel testing in libgomp (PR libgomp/66005)


On 05/08/2015 10:40 AM, Thomas Schwinge wrote:
> Hi!
> 
> On Thu, 7 May 2015 13:39:40 +0200, Jakub Jelinek <jakub@redhat.com> wrote:
>> On Thu, May 07, 2015 at 01:26:57PM +0200, Rainer Orth wrote:
>>> As reported in the PR, with the addition of all those OpenACC tests,
>>> libgomp make check times have skyrocketed since the testsuite is still
>>> run sequentially.
> 
> ACK.  And, thanks for looking into that!
> 
>>> Fixing this proved trivial: I managed to almost literally copy the
>>> solution from libstdc++-v3/testsuite/Makefile.am, with a minimal change
>>> to libgomp.exp so the generated libgomp-test-support.exp file is found
>>> in both the sequential and parallel cases.  This isn't an issue in
>>> libstdc++ since all necessary variables are stored in a single
>>> site.exp.
>>
>> It is far from trivial though.
>> The point is that most of the OpenMP tests are parallelized with the
>> default OMP_NUM_THREADS, so running the tests in parallel oversubscribes the
>> machine a lot, the higher number of hw threads the more.
> 
> Do you agree that we have two classes of test cases in libgomp: 1) test
> cases that don't place a considerably higher load on the machine compared
> to "normal" (single-threaded) execution tests, because they're just
> testing some functionality that is not expected to actively depend
> on/interfere with parallelism.  If needed, and/or if not already done,
> such test cases can be parameterized (OMP_NUM_THREADS, OpenACC num_gangs,
> num_workers, vector_length clauses, and so on) for low parallelism
> levels.  And, 2) test cases that place a considerably higher load on the
> machine compared to "normal" (single-threaded) execution tests, because
> they're testing some functionality that actively depends on/interferes
> with some kind of parallelism.  What about marking such tests specially,
> such that DejaGnu will only ever schedule one of them for execution at
> the same time?  For example, a new dg-* directive to run them wrapped
> through »flock [libgomp/testsuite/serial.lock] [a.out]« or some such?

Looks the thread got stuck. Anyway I've just noticed how slow libgomp.exp tests
are on a recent Intel Machine with 160 HT cores. I'm attaching graph with CPU
utilization and 'ps ax | grep expect' log file that shows which tests are running.
Roughly, after 10 minutes I see drop in utilization and then libgomp.exp is running
mainly serially.

So I believe splitting tests in libgomp.exp to serial and parallel would make sense.
Another another idea is to overwrite OMP_NUM_THREADS to a reasonable number which
will enable also parallel execution of parallel tests?

Thanks,
Martin

> 
>> If we go forward with some parallelization of the tests, we at least should
>> try to export something like OMP_WAIT_POLICY=passive so that the
>> oversubscribed machine would at least not spend too much time in spinning.
> 
> (Will again have the problem that DejaGnu doesn't provide infrastructure
> to communicate environment variables to boards in remote testing.)
> 
>> And perhaps reconsider running all OpenACC threads 3 times, just allow
>> user to select which offloading target they want to test (host fallback,
>> the host nonshm hack, PTX, XeonPHI in the future?), and test just that
>> (that is pretty much how OpenMP offloading testing works).
> 
> My rationale is: if you configure GCC to support a set of offloading
> devices (more than one), you'll also want to get the test coverage that
> indeed all these work as expected.  (It currently doesn't matter, but...)
> that's something I'd like to see improved in the libgomp OpenMP
> offloading testing (once it supports more than one architecture for
> offloading).
> 
>> For tests that
>> always want to test host fallback, I hope OpenACC offers clauses to force
>> the host fallback.
> 
> Yes.
> 
> 
> Grüße,
>  Thomas
> 

Attachment: make-check.tar.bz2
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