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Program freezes during an experiment

Hi, we are trying to program an experiment in Open Sesame and exporting it onto our JATOS server. Currently, we are stuck with a strange problem: when running the experiment by using a JATOS link, it freezes around block 4 (out of 16 blocks of 32 trials each). It stalls, then flashes a black box instead of the display, and then hangs, fully crashed.

The design of this experiment is identical to a previous experiment that we ran successfully on JATOS. Here are similarities and differences between the experiment that worked (Exp 1) and the one that is freezing (Exp 2).

Common to both Exp 1 and Exp 2:

  • used “Legacy” as the backend
  • used a series of 23 .png files for instructions that ranged in size from 20 – 300 Kb
  • left the default “disable collect garbage” clicked
  • ran two practice blocks of 16 trials each followed by 16 blocks of 32 trials each

Different in Exp 2 from Exp 1

We thought perhaps it is a memory issue on our JATOS server.  It has 2 Gb. We are in the process of upgrading it to 4 Gb.  But we tried running our experiment on the free server (, which we think has 4 Gb, and it crashed there too.

Experiment 2 ran successfully through to completion (multiple times) on Open Sesame itself (i.e., before importing it to JATOS) running on a MacBook Pro with 16 Gb of memory, but failed on a MacBook Pro with 8 Gb of memory.

The stimuli that we are using are not particularly large (1 – 7 Kb each).  So it seems surprising that it would be running into a memory limit.  But we thought perhaps as we reuse them trial after trial, they are getting reloaded into some buffer that isn’t being cleared.  That led us to wonder about the “disable garbage collection” option.  But sadly disabling that did not solve the problem.

Finally, we tried a version of our Exp. 2 with the PsychoPy backend, instead of Legacy backend, and the program still crashed.

Currently, we are unsure of what else to try, and would be very grateful if you have any ideas about what the problem might be or what further diagnostic steps we might take.  Thank you!


  • I can only say something for the JATOS side of the issue since I'm not professional in OpenSesame.

    We thought perhaps it is a memory issue on our JATOS server. It has 2 Gb. We are in the process of upgrading it to 4 Gb. But we tried running our experiment on the free server (, which we think has 4 Gb, and it crashed there too.

    Maybe. And you are correct, has 4 GB. Can you tell me when you run your experiment (please with timezone) on Then I can have a look in the logs whether there was anything unusual.



  • Kristian, thank you for replying so quickly! That would be great! I ran the experiment on from 2020/11/12 03:30 PM to 03:38 PM (Central Standard Time). If it helps, the result ID is 17224, and the worker ID is 16942.

    Please let me know if there's other information you need, thank you!

  • The server was fine at this time. Plenty of resources left. In the log was also nothing unusual: last entry was a jatos.appendResultData. After that nothing. I guess your problem is more of a browser / OpenSesame / OSWeb one.

  • I just posted about a similar issue. In my case the issue seems to occur for windows users (using different browsers) but not for mac users. Do you have any information on the users for which it crashes?

  • @QZ2020, let me rephrase my question: using a JATOS link, does the experiment freeze around block 4 both when running it from a MacBook Pro with 16 Gb of memory and from a MacBook Pro with 8 Gb of memory (mirroring the behaviour is Open Sesame itself)?

  • @kri Thank you for helping me look at the entry! Over the weekend, I re-built the experiment using in-line code, and it works now without freezing. Thank you!

  • @LouisaB Thank you for commenting! We have only run it on MacBook Pros and have not tested it on Windows yet. If I encounter any problem, I will be happy to let you know.

    Over the weekend, I did three things, and then I ran the experiment, which ran smoothly without freezing now.

    1) Update the Open Sesame software to its latest version.

    2) Change the backend from Legacy to Psycho Py.

    3) Use in-line code to present images.

    The tutorial above is helpful for figuring out how to present images (step 6).

    Good luck!

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