Originally posted by: BerkUstun

I've been receiving weird segmentation violations with a MEX file that uses the CPLEX 12.4 Callable Library.

After a lot of debugging, I have found out that the segmentation violation does not occur in the body of my C code but either before or after it launches. In addition, the segmentation violation only occurs when I run this MEX file in parallel, and it seems to occur more frequently when I use more cores. The violation does not occur when I run my program sequentially.

One of the possible reasons that was suggested for the violation was that multiple cores may be trying to access the CPLEX license file at the same time - so I had a few questions about how this works. In particular, I'm wondering if I have to I need to access the CPLEX license everytime I run my MEX file (even after it is compiled)?
#CPLEXOptimizers
#DecisionOptimization

Originally posted by: BerkUstun

After some further debugging, I have discovered that the segmentation violations originate from the "putenv" function in C, which appears to be used to set the CPLEX license file for other APIs. Some follow-up questions:

• Seeing how the academic initiative only grants 12 CPLEX licenses, I'm wondering if I could get a segmentation violation when I have to run more than 12 instances of my MEX file at the same time.

• I'm somewhat unfamiliar with how the license file works. What exactly happens to the license file when I open / close a CPLEX environment? Is there a counter on it that increments / decrements? Or are there 12 license files that can be used at the same time?

#CPLEXOptimizers
#DecisionOptimization

Originally posted by: SystemAdmin

CPLEX accesses the license file when you call CPXopenCPLEX().
It is a known problem with license management that this is may not work when called in parallel.
In your MEX file, could you just put locks around the calls to CPXopenCPLEX()? That should fix the problems.
On Linux/Unix this would be something like that:

#include <pthread.h>
static pthread_mutex_t openCPLEXLock = PTHREAD_MUTEX_INITIALIZER;
 
void function_in_mex(...) {
   int status;
   CPXENVptr env;
   pthread_mutex_lock(&openCPLEXLock);
   env = CPXopenCPLEX(&status);
   pthread_mutex_unlock(&openCPLEXLock);
   /* Check status */
   ...
}

On Windows

static CRITICAL_SECTION openCPLEXLock;
 
/* openCPLEXLock should be initialized _once_ at startup */
 
void function_in_mex(...) {
   int status;
   CPXENVptr env;
   EnterCriticalSection (&openCPLEXLock);
   env = CPXopenCPLEX(&status);
   LeaveCriticalSection (&openCPLEXLock);
   /* Check status */
   ...
}

#CPLEXOptimizers
#DecisionOptimization

Originally posted by: BerkUstun

Thank you for this! The fix appears to work for now, though I will have to test it a few times to make sure.

In any case, I had a quick follow-up question. Could I have avoided this by declaring env as a static variable in my MEX code, and calling CPXopenCPLEX() before and after all of my MEX runs? I've tried this in MATLAB but for some reason I cannot manage to keep env as a static variable - I'm wondering whether this is an issue with MATLAB / my code or something that you guys intended?
#CPLEXOptimizers
#DecisionOptimization

Originally posted by: SystemAdmin

If you want to do multiple solves in parallel you will need multiple instances of CPXENVptr. So a single static variable will not do, no matter whether the programming environment allows that or not.
#CPLEXOptimizers
#DecisionOptimization

Originally posted by: BerkUstun

Does that mean that you can you only have as many CPXENVptr open as the number of licenses that you have (i.e. 12 on the Academic Initiative license?)
#CPLEXOptimizers
#DecisionOptimization

Originally posted by: SystemAdmin

The license file should not limit the number CPXENVptr instances that you can create simultaneously. You should be able to create as many CPXENVptr instances as you like (as long as the OS has resources, of course). Do you observe otherwise?
#CPLEXOptimizers
#DecisionOptimization

Originally posted by: BerkUstun

Just curious. I am still getting segmentation violations linked to the putenv() function. Right now, I've changed all the declarations of

void function_in_mex(...) 
{   CPXENVptr env     = NULL; 

int       status = 0; env               = CPXopenCPLEX (&status);   
// my code    
}

to

static            pthread_mutex_t openCPLEXLock = PTHREAD_MUTEX_INITIALIZER;   

void function_in_mex(...) 
{ 

int status        = 1; CPXENVptr env     = NULL;   

while ( status ) 
{ 
// this is not a good idea, though I've done it this way to ensure that CPLEX will open in the MEX file pthread_mutex_lock(&openCPLEXLock); env = CPXopenCPLEX (&status) pthread_mutex_unlock(&openCPLEXLock); 
}   
// my code    
}

Since I run the MEX file in parallel using a parfor loop in CPLEX, I've also made it a point to declare the openCPLEXLock as a global / static variable before the parfor loop.

I think this makes sense, though please let me know if it sets off any alarm. Otherwise, it might just be an issue with MATLAB.
#CPLEXOptimizers
#DecisionOptimization

Originally posted by: SystemAdmin

A global/static lock sounds like the right thing to do, at least from a C point of view. I don't know about matlab and MEX :-(
Do you have any chance to run the whole thing through valgrind and see if this gives more details? Or could you create a backtrace for the segmentation fault in a debugger and post that here?
#CPLEXOptimizers
#DecisionOptimization

Originally posted by: BerkUstun

Hmm I will post on the MATLAB forums as well then. Meanwhile, here is a recent stack trace from the segmentation fault... Let me know if you guys can figure anything out.

---

Segmentation violation detected at Mon Apr 23 13:06:41 2012

---

Configuration:
Crash Decoding : Disabled
Current Visual : None
Default Encoding: UTF-8
GNU C Library : 2.14.90 development
MATLAB Root : /home/software/matlab/matlab-2012a
MATLAB Version : 7.14.0.739 (R2012a)
Operating System: Linux 3.3.1-3.fc16.x86_64 #1 SMP Wed Apr 4 18:08:51 UTC 2012 x86_64
Processor ID : x86 Family 6 Model 26 Stepping 4, GenuineIntel
Virtual Machine : Java 1.6.0_17-b04 with Sun Microsystems Inc. Java HotSpot(TM) 64-Bit Server VM mixed mode
Window System : No active display

Fault Count: 1
Abnormal termination:
Segmentation violation

Register State (from fault):
RAX = 0000000000000000 RBX = 0000000000000011
RCX = 0000000000000030 RDX = 0000000000000011
RSP = 00007f576e6aaf28 RBP = 00007f576e6aafa0
RSI = 00007f576e6aafb0 RDI = 00007f5689596b80

R8 = 000000000000ffff R9 = 0000000000000060
R10 = 0000003b46489e20 R11 = 0000000000000011
R12 = 00007f576e6aafb0 R13 = 00007f5689596b80
R14 = 0000000000000050 R15 = 0000003b467b4598

RIP = 0000003b46488acc EFL = 0000000000010283

CS = 0033 FS = 0000 GS = 0000

Stack Trace (from fault):
0 0x00007f5786ae192e /home/software/matlab/matlab-2012a/bin/glnxa64/libmwfl.so+00370990 _ZN2fl4diag15stacktrace_base7captureERKNS0_14thread_contextEm+000158
1 0x00007f5786ae47d0 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwfl.so+00382928
2 0x00007f5786ae4b3b /home/software/matlab/matlab-2012a/bin/glnxa64/libmwfl.so+00383803 _ZN2fl4diag13terminate_logEPKcRKNS0_14thread_contextE+000171
3 0x00007f57859c6203 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcr.so+01253891 _ZN2fl4diag13terminate_logEPKcPK8ucontext+000067
4 0x00007f57859c30fd /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcr.so+01241341
5 0x00007f57859c479d /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcr.so+01247133
6 0x00007f57859c4925 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcr.so+01247525
7 0x00007f57859c4f01 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcr.so+01249025
8 0x00007f57859c53f5 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcr.so+01250293
9 0x0000003b4700f500 /lib64/libpthread.so.0+00062720
10 0x0000003b46488acc /lib64/libc.so.6+00559820
11 0x0000003b4643915a /lib64/libc.so.6+00233818
12 0x0000003b4643903d /lib64/libc.so.6+00233533 putenv+000109
13 0x00007f56c0ffe676 /home/software/cplex/cplex-12.4/cplex/matlab/cplexlink124.mexa64+02860662
14 0x00007f56c0e0a434 /home/software/cplex/cplex-12.4/cplex/matlab/cplexlink124.mexa64+00812084
15 0x00007f56c0e09af4 /home/software/cplex/cplex-12.4/cplex/matlab/cplexlink124.mexa64+00809716
16 0x00007f56c0e0930c /home/software/cplex/cplex-12.4/cplex/matlab/cplexlink124.mexa64+00807692
17 0x00007f56c0dc2c09 /home/software/cplex/cplex-12.4/cplex/matlab/cplexlink124.mexa64+00519177
18 0x00007f56c0dc2146 /home/software/cplex/cplex-12.4/cplex/matlab/cplexlink124.mexa64+00516422 mexFunction+000448
19 0x00007f577fbfecca /home/software/matlab/matlab-2012a/bin/glnxa64/libmex.so+00109770 mexRunMexFile+000090
20 0x00007f577fbfaf79 /home/software/matlab/matlab-2012a/bin/glnxa64/libmex.so+00094073
21 0x00007f577fbfbde1 /home/software/matlab/matlab-2012a/bin/glnxa64/libmex.so+00097761
22 0x00007f578566e063 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_dispatcher.so+00479331 _ZN8Mfh_file11dispatch_fhEiPP11mxArray_tagiS2_+000515
23 0x00007f5784f34476 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_interpreter.so+01987702
24 0x00007f5784ee5426 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_interpreter.so+01664038
25 0x00007f5784ee9be4 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_interpreter.so+01682404
26 0x00007f5784ee6333 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_interpreter.so+01667891
27 0x00007f5784ee7037 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_interpreter.so+01671223
28 0x00007f5784f50690 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_interpreter.so+02102928
29 0x00007f578566e063 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_dispatcher.so+00479331 _ZN8Mfh_file11dispatch_fhEiPP11mxArray_tagiS2_+000515
30 0x00007f577e81ba6f /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcos.so+01522287
31 0x00007f577e7b7124 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcos.so+01110308
32 0x00007f577e7b7bab /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcos.so+01113003
33 0x00007f577e7b7ebe /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcos.so+01113790
34 0x00007f577e7b918d /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcos.so+01118605
35 0x00007f577e7b92ad /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcos.so+01118893
36 0x00007f577e7b954c /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcos.so+01119564 _Z27omConstructObjectWithClientN4mcos9COSNameIDEiPPK11mxArray_tagPKNS_9COSClientE+000476
37 0x00007f577e8241bd /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcos.so+01556925
38 0x00007f577e89e563 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcos.so+02057571
39 0x00007f57856286a1 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_dispatcher.so+00194209 _ZN13Mfh_MATLAB_fn11dispatch_fhEiPP11mxArray_tagiS2_+000481
40 0x00007f5784f34476 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_interpreter.so+01987702
41 0x00007f5784ee5426 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_interpreter.so+01664038
42 0x00007f5784ee9be4 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_interpreter.so+01682404
43 0x00007f5784ee6333 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_interpreter.so+01667891
44 0x00007f5784ee7037 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_interpreter.so+01671223
45 0x00007f5784f50690 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_interpreter.so+02102928
46 0x00007f578566e063 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_dispatcher.so+00479331 _ZN8Mfh_file11dispatch_fhEiPP11mxArray_tagiS2_+000515
47 0x00007f577e816388 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcos.so+01500040
48 0x00007f577e7b7c22 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcos.so+01113122
49 0x00007f577e7b7ebe /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcos.so+01113790
50 0x00007f577e7b9c7c /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcos.so+01121404
51 0x00007f577e802236 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcos.so+01417782 _Z19omCallLoadobjMethodRN4mcos15COSInterfacePtrENS_8COSValueENS_14COSDataTypePtrE+000694
52 0x00007f577e802937 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcos.so+01419575
53 0x00007f577e73c676 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcos.so+00607862
54 0x00007f577e7f40eb /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcos.so+01360107
55 0x00007f577e7e5f41 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcos.so+01302337
56 0x00007f577e7e63fe /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcos.so+01303550
57 0x00007f577e7e73f8 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcos.so+01307640
58 0x00007f5786833654 /home/software/matlab/matlab-2012a/bin/glnxa64/libmx.so+00419412
59 0x00007f578683557a /home/software/matlab/matlab-2012a/bin/glnxa64/libmx.so+00427386
60 0x00007f5786835017 /home/software/matlab/matlab-2012a/bin/glnxa64/libmx.so+00426007
61 0x00007f5786834b18 /home/software/matlab/matlab-2012a/bin/glnxa64/libmx.so+00424728
62 0x00007f5786835017 /home/software/matlab/matlab-2012a/bin/glnxa64/libmx.so+00426007
63 0x00007f5786835017 /home/software/matlab/matlab-2012a/bin/glnxa64/libmx.so+00426007
64 0x00007f5786835017 /home/software/matlab/matlab-2012a/bin/glnxa64/libmx.so+00426007
65 0x00007f5786835017 /home/software/matlab/matlab-2012a/bin/glnxa64/libmx.so+00426007
66 0x00007f5786835a23 /home/software/matlab/matlab-2012a/bin/glnxa64/libmx.so+00428579 miGetCurrentItem+000387
67 0x00007f57868181db /home/software/matlab/matlab-2012a/bin/glnxa64/libmx.so+00307675 mxDeserializeWithTag+000315
68 0x00007f56d8bea1e0 /home/software/matlab/matlab-2012a/toolbox/distcomp/distcomp/distcompdeserialize.mexa64+00008672 mexFunction+000080
69 0x00007f577fbfecca /home/software/matlab/matlab-2012a/bin/glnxa64/libmex.so+00109770 mexRunMexFile+000090
70 0x00007f577fbfaf79 /home/software/matlab/matlab-2012a/bin/glnxa64/libmex.so+00094073
71 0x00007f577fbfbde1 /home/software/matlab/matlab-2012a/bin/glnxa64/libmex.so+00097761
72 0x00007f578566e063 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_dispatcher.so+00479331 _ZN8Mfh_file11dispatch_fhEiPP11mxArray_tagiS2_+000515
73 0x00007f5784f34476 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_interpreter.so+01987702
74 0x00007f5784ee5426 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_interpreter.so+01664038
75 0x00007f5784ee9be4 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_interpreter.so+01682404
76 0x00007f5784ee6333 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_interpreter.so+01667891
77 0x00007f5784ee7037 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_interpreter.so+01671223
78 0x00007f5784f50690 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_interpreter.so+02102928
79 0x00007f578566e063 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_dispatcher.so+00479331 _ZN8Mfh_file11dispatch_fhEiPP11mxArray_tagiS2_+000515
80 0x00007f5784f1365f /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_interpreter.so+01853023
81 0x00007f5784f12d49 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_interpreter.so+01850697
82 0x00007f5784e742ec /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_interpreter.so+01200876 inCallFcnWithTrap+000092
83 0x00007f5784ed9b6b /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_interpreter.so+01616747
84 0x00007f5784e73438 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwm_interpreter.so+01197112 _Z28inCallFcnWithTrapInDesiredWSiPP11mxArray_tagiS1_PKcbP15inWorkSpace_tag+000104
85 0x00007f5780267d79 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwiqm.so+02485625 _ZN3iqm15BaseFEvalPlugin7executeEP15inWorkSpace_tagRN5boost10shared_ptrIN14cmddistributor17IIPCompletedEventEEE+000457
86 0x00007f575d07909d /home/software/matlab/matlab-2012a/bin/glnxa64/libnativejmi.so+00696477 _ZN9nativejmi14JmiFEvalPlugin7executeEP15inWorkSpace_tagRN5boost10shared_ptrIN14cmddistributor17IIPCompletedEventEEE+000173
87 0x00007f575d0ab6e5 /home/software/matlab/matlab-2012a/bin/glnxa64/libnativejmi.so+00902885 _ZN3mcr3mvm27McrSwappingIqmPluginAdapterIN9nativejmi14JmiFEvalPluginEE7executeEP15inWorkSpace_tagRN5boost10shared_ptrIN14cmddistributor17IIPCompletedEventEEE+000629
88 0x00007f5780250482 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwiqm.so+02389122
89 0x00007f5780241264 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwiqm.so+02327140
90 0x00007f5785c32afc /home/software/matlab/matlab-2012a/bin/glnxa64/libmwbridge.so+00125692 _Z10ioReadLinebP8_IO_FILEPcS1_iPbRKN5boost8optionalIKP15inWorkSpace_tagEEb+000508
91 0x00007f5785c33165 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwbridge.so+00127333
92 0x00007f5785c37d0a /home/software/matlab/matlab-2012a/bin/glnxa64/libmwbridge.so+00146698
93 0x00007f5785c38165 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwbridge.so+00147813
94 0x00007f5785c389ce /home/software/matlab/matlab-2012a/bin/glnxa64/libmwbridge.so+00149966 mnParser+000702
95 0x00007f57859aade2 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcr.so+01142242 _ZN11mcrInstance30mnParser_on_interpreter_threadEv+000034
96 0x00007f578598d51a /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcr.so+01021210
97 0x00007f578598d598 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcr.so+01021336
98 0x00007f57861c0053 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwservices.so+00823379
99 0x00007f57861c0315 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwservices.so+00824085
100 0x00007f57861bebc1 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwservices.so+00818113
101 0x00007f577c328605 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwuix.so+00505349
102 0x00007f57862449a1 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwservices.so+01366433 _ZSt8for_eachIN9__gnu_cxx17__normal_iteratorIPN5boost8weak_ptrIN4sysq10ws_ppeHookEEESt6vectorIS6_SaIS6_EEEENS4_8during_FIS6_NS2_10shared_ptrIS5_EEEEET0_T_SH_SG_+000081
103 0x00007f5786245aab /home/software/matlab/matlab-2012a/bin/glnxa64/libmwservices.so+01370795
104 0x00007f57862435f9 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwservices.so+01361401 _Z25svWS_ProcessPendingEventsiib+000665
105 0x00007f578598c76f /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcr.so+01017711
106 0x00007f578598cc3b /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcr.so+01018939
107 0x00007f578598cd97 /home/software/matlab/matlab-2012a/bin/glnxa64/libmwmcr.so+01019287
108 0x0000003b47007d90 /lib64/libpthread.so.0+00032144
109 0x0000003b464f0f5d /lib64/libc.so.6+00986973 clone+000109
This error was detected while a MEX-file was running. If the MEX-file
is not an official MathWorks function, please examine its source code
for errors. Please consult the External Interfaces Guide for information
on debugging MEX-files.

If this problem is reproducible, please submit a Service Request via:
http://www.mathworks.com/support/contact_us/

A technical support engineer might contact you with further information.

Thank you for your help.
#CPLEXOptimizers
#DecisionOptimization

Originally posted by: SystemAdmin

The stacktrace suggests that the crash happens when a new Cplex instance is created in matlab .
Is it possible that you create Cplex instances concurrently in your matlab code?
#CPLEXOptimizers
#DecisionOptimization

Originally posted by: BerkUstun

Yes actually! I am using the CPLEX API for MATLAB, and the CPLEX Callable library in the MEX file.
#CPLEXOptimizers
#DecisionOptimization

Originally posted by: BerkUstun

Sorry should have added more detail:

Right now my experiment (an m-file) uses the CPLEX MATLAB API as well as a MEX function that uses the CPLEX Callable Library.

I'm guessing that the segmentation violation occurs when one experiment creates Cplex() objects in MATLAB while another experiment is using the MEX file?

Would this result in an error because the MATLAB API also uses the putenv() function (and is not thread safe)? And if so, is there any way around this that does not involve "timing" the experiments so that I do not create a Cplex() object?

Thank you so much for the insight so far! I feel like I'm finally getting somewhere.
#CPLEXOptimizers
#DecisionOptimization

Originally posted by: SystemAdmin

What really puzzles me is that CPXopenCPLEX() calls putenv(). How do you setup your licensing? Do you just point ILOG_LICENSE_FILE to the location of the license file or do you use a different sort of licensing setup?
The only way to serialize the various ways in which CPXopenCPLEX() may be called is to protect all of them by the same lock. Can you write two MEX functions that result in pthread_mutex_lock(&openCPLEXLock) and in pthread_mutex_unlock(&openCPLEXLock) respectively and call them before/after instantiating CPLEX objects in matlab?
#CPLEXOptimizers
#DecisionOptimization

Originally posted by: BerkUstun

Sorry - will get back to you soon about the licensing since all of this is set up on a computing cluster. I will also have to try the MEX functions, though I'm a little confused about what exactly I should do. Basically the idea is to replace any command in MATLAB such as

LP = Cplex()

with

mutexLock()
LP = Cplex()
mutexUnLock()

where mutexLock and mutexUnLock are the MEX files that call pthread_mutex_lock(&openCPLEXLock) and in pthread_mutex_unlock(&openCPLEXLock) respectively right?

Also another tidbit: I recently started to instantiate the LP objects outside of the parfor loop and have not experienced a segmentation violation since. I'm wondering whether this instantiating the objects within MATLAB may have been the issue all along?
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Originally posted by: SystemAdmin

>

> mutexLock()
> LP = Cplex()
> mutexUnLock()
>

>
> where mutexLock and mutexUnLock are the MEX files that call pthread_mutex_lock(&openCPLEXLock) and in pthread_mutex_unlock(&openCPLEXLock) respectively right?
>
Yes, that is what I meant to suggest.

> Also another tidbit: I recently started to instantiate the LP objects outside of the parfor loop and have not experienced a segmentation violation since. I'm wondering whether this instantiating the objects within MATLAB may have been the issue all along?
>
Our best guess so far was that parallel invocation of CPXopenCPLEX() (which is called implicitly by the Cplex() constructor in matlab) was the cause for your troubles. Any code change that avoids calling this function in parallel should fix the problem.
I guess you still have the locking in your MEX file for the cases in which you call CPXopenCPLEX() directly from the MEX file? If you instantiate Cplex outside the parfoor loop, can it happen that an instantiation of Cplex and a call to CPXopenCPLEX() from the MEX file may overlap? Or is the MEX file only invoked from inside the parfor loop? If the MEX file is called only from inside the parfor loop, the MEX file uses locks around explicit calls to CPXopenCPLEX() and you instantiate Cplex only outside the parfor loop in matlab then it looks to me as if you have eliminated parallel invocations of CPXopenCPLEX().
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Originally posted by: BerkUstun

Thanks for all your help on this.

Using the mutexLock() and mutexUnlock() functions as described above avoids the segmentation errors. Alternatively, creating the Cplex() objects before the parfor loop also works (mainly because it avoids constructing Cplex() objects within the parfor loop where the MEX file is running) also avoids the segmentation errors.

Just FYI about the licensing setup. I don't quite understanding how it works, but I asked our admins and it turns out that my computing cluster just sets an environmental variable that points to the ILOG_LICENSE_FILE - so I guess it's the standard setup. I suppose something weird may be going on with the way that parfor works?

If there's anything else you'd like to know let me know!
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Originally posted by: SystemAdmin

I have no clue why CPLEX would call putenv() if you use the ILOG_LICENSE_FILE environment variable to point CPLEX to the license file. But there is probably no way to figure what exactly is going on without me logging onto your cluster :-)
So if you agree I will not pursue this any further as it seems your code is working without problems right now.
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Originally posted by: BerkUstun

Of course! Thank you again for all your help. Everything runs very smoothly now.
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