Parallel processing works or not

[Motim]

Hello,
Recently I had several small video clips in the play list.
Each one was dragged to the time-line, made some editing, then set to export it.
In the “Advanced” setting checked the “Parallel processing”.
The first job started, then loaded a second clip to the time line, some small editing and sent it to export.
There was no “Parallel processing”, had to trick it, stopped the first job, the second started, then back to the first one, clicked to run it.
Only then both jobs were running. So did with a third clip, then all the three were running
Why is that ?
Answers will be appreciated.
Thanks
Motim

SC version 21.06.29
OS win 10 pro, 20H2 x64,
CPU INTEL CORE i5 7400 3.8GHz,
8GB RAM

[shotcut]

This is not a bug. Export > Advanced > Video > Parallel processing is not multiple jobs; it is frame-threading for a single job. See how this checkbox is located within the Video tab and not a general option? Please do not make every post a bug. Is it better behavior to ask first in #help-how-to
I strongly recommend against the trick you are doing to make multiple jobs run unless you have more than 32 GB RAM!

How to Reduce Memory Usage

Here are ways to reduce the memory usage in general: Save and restart Shotcut Close other browser tabs. Close other applications. Reboot (does all of the above). Here are ways to reduce the memory usage of your project: Reduce the resolution of the Video Mode. Reduce the number of tracks. The number of tracks will increase the amount of caching to prevent files from being closed and reopened repeatedly from one frame to the next. Here are some ways to reduce the memory usage while editing: …

[Motim]

Hello,
Thanks for the answer.
Sorry for posting to a wrong place.
I was thinking that parallel processing means running several jobs at the same time.
So if I check it, does it shorten exporting time of one job ? In other words, when check it and when not.
And please, just for my knowledge, what is “Frame-Threading” ?
Answers will be appreciated.
Thanks
Motim

[Elusien]

Yes, in general it speeds up the export of a single job. “frame threading” means that while one CPU thread works on encoding one frame another thread works on encoding a different frame in parallel, hence speeding up the export.

[shotcut]

Please be aware that this option can cause a problem in the video output. It is fairly low risk because there has been much debugged, but it is still not perfect. Also it requires more memory. For these reasons it does not default to be on.