Experimental Physics and
Industrial Control System

Simon Reiter via Tech-talk <tech-talk at aps.anl.gov> · Mon, 20 Jul 2020 10:44:47 +0200

Using 'forkserver' and 'spawn' seem to work both. The creating is slower, but
this might not be problematic.

The only drawback at the moment is that the framework is still running on
Python 2. We already made some effort to move to Python 3, but this is not
final. And Python 2 does not provide the selection of the start method.

PS: I also managed to run into the same problem on Python 3.8 in a conda
environment with obviously running EPICS 3.17.6:

> Thread 1 (Thread 0x7f65d9f52740 (LWP 9303)):
> #0  __lll_lock_wait () at
> ../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:135
> #1  0x00007f65d9b4ae9b in _L_lock_883 () from /lib64/libpthread.so.0
> #2  0x00007f65d9b4ad68 in __GI___pthread_mutex_lock (mutex=0x7f65af498fc0
> <onceLock>) at ../nptl/pthread_mutex_lock.c:78
> #3  0x00007f65af467b06 in epicsThreadOnce () from
> /home/sreiter/anaconda3/envs/lab3/epics/lib/linux-x86_64/./libCom.so.3.17.6
> #4  0x00007f65af4c905b in ca_context_create () from
> /home/sreiter/anaconda3/envs/lab3/epics/lib/linux-x86_64/libca.so
> ...

I just do not see the epicsTimeGetCurrent() function call.

On Sat, 2020-07-18 at 08:59 +0200, Simon Reiter wrote:
> I also thought about using ’spwan’ to create an independent process. But I
> was wondering why this is not mentioned at all on the pyepics page. And in
> fact, in that case it is also not clear to me what the advantage is of using
> CAProcess instead of Process.
> 
> I will try this now and report here again.
> 
> > On Jul 17, 2020, at 18:55, Michael Davidsaver <mdavidsaver at gmail.com>
> > wrote:
> > 
> > On 7/17/20 9:33 AM, Michael Davidsaver wrote:
> > > The python subprocess module shouldn't let this happen though since it
> > > does
> > > fork() and then execv(), and the situation which I describe is only
> > > possible
> > > if fork() is not followed by execv().
> > 
> > Oh wait.  CAProcess (which I've not seen previously) is using
> > the multiprocessing module, which can do fork() without exec().
> > In fact, this is the default on *NIX.  So this is likely or
> > problem.  I think you'll need to use the 'spawn' or 'forkserver'
> > method.
> > 
> > https://docs.python.org/3.7/library/multiprocessing.html#contexts-and-start-methods
> > 
> > https://github.com/pyepics/pyepics/blob/1f795c647458435cca0cf11b984b34fc11180c25/epics/multiproc.py
> 
> 

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System