MATLAB: Matlab parfor saves and loads temporary variables during execution!

copyloadobjectParallel Computing Toolboxparallel pooparforsavesaveobjworker

The parallel pool implementation appears to save and load variables using effectively the normal "save" and "load" channels when making copies to be passed to the workers. This is really bad because if someone has a saveobj() implemented in a class object the state of the variable could be modified during execution and depending on the circumstances could lead to "unexplained" crashes.

Lets say I have a class object that needs a large amount of temporary data during execution. When saved I would naturally want to get rid of this temporary data. A convenient way to do this is to write a saveobj that clears the temporary data within the class. If, however, the object is saved and reloaded during execution as with parfor, there are problems.

Why is the copy not made internally?

Best Answer

I don't know why authoritatively, but it seems to me that this is only a danger if you are unaware of this behavior on the part of parfor. If you are aware of it, you would surely write a loadobj() method to restore the temporary data when reloaded. And, this would be better than broadcasting copies of the temporary data to all the workers. The latter is done serially and would take a lot more time.

Incidentally, have you verified that saveobj() is triggered in this scenario just as in an ordinary call to save()?

Related Solutions

MATLAB: Custom class property and method attributes

So Matlab doesn't support this type of thing, which can't really be too surprising. The best way I've found to get around this is to just redefine saveobj() and loadobj() methods, while having all "special" data stored in a dedicated struct that is purpose built for properties that can't be saved normally. For example, if is a property that reference another object, but you just want to save a "reference" (like through a UID), make the property transient, and saveobj() and loadobj() are then responsible for saving the UID to the dedicated save struct and attempting to find the old reference through the saved UID when loading.

MATLAB: Can classes be made unsaveable

No. When a class object is saved, the saveobj method for that class gets called, if it exists. If this method throws an error, it is caught, and thrown as a warning instead. Therefore the best you can do is generate a warning. If you don't want the object to be saved, then you can have the saveobj method return an empty array, which is saved instead:

classdef myclass
    ...
     methods
      function a = saveobj(obj)
        a = [];
        Warning 'myclass objects are unsaveable.'
      end
     end
 end

Best Answer

Related Solutions

MATLAB: Custom class property and method attributes

MATLAB: Can classes be made unsaveable

Related Question