Re: [Libmesh-devel] PerfLog isn't thread-safe

[Benjamin K. <ben...@na...>]

>> Cool, so that should be the last thread-unsafe thing left in
>> libmesh, right? ;)
> 
> Heh, funny.  As a matter of fact, I'm ashamed to say that if you grep
> through our source code for "This should also screw up multithreading
> royally" you'll find some of my own programming no-nos.  But at least
> my problems only affect the Clough-Tocher and Hermite elements; the
> PerfLog bug can crash any code with both tbb and perf-log turned on.


The how-to-instrument multithreaded code is the main point the petsc folks
point to when talking about why they can't easily thread.  At least in our
case it is as simple as --disable-perflog.  I think the immediate fix is to
disable performance logging at run-time unless --n_threads=1 is specified.

In the long term we could use parallel_reduce in place of parallel_for
whenever logged methods will be called.  The approach would be to declare a
local perf_log inside each object that gets called on a separate thread.
The join() method could then combine the results from the distinct
perf_logs.

The problem is I don't know how much overhead this will incur, and if it is
worth it.

Thoughts?

-Ben