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Question

This is what I get when I run my app through instruments. It's processing a lot of core data. I can see this obviously isn't right, but I don't really know what this screenshot is telling me. What appears to be the problem? If you're interested, the code, condensed, is this:

    NSFetchRequest *oldFetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *oldEntryEntity = [NSEntityDescription entityForName:@"Entry"
                                                      inManagedObjectContext:oldContext];

    [oldFetchRequest setEntity:oldEntryEntity];

    int numberOfEntries = [oldContext countForFetchRequest:oldFetchRequest error:nil];

    int batchSize = 4;
    [oldFetchRequest setFetchLimit:batchSize];
    int offset = 0;

    while (numberOfEntries - offset > 0) {
        @autoreleasepool {
        [oldFetchRequest setFetchOffset:offset];
        NSError *error;
        NSArray *entries = [oldContext executeFetchRequest:oldFetchRequest error:&error];

        for (NSManagedObject *entry in entries) {
        @autoreleasepool {
            Entry *newEntry = [NSEntityDescription insertNewObjectForEntityForName:@"Entry"
                                                            inManagedObjectContext:newContext];

            newEntry.entryID = [entry valueForKey:@"entryID"];

            NSMutableOrderedSet *newMediaSet = [[NSMutableOrderedSet alloc] init];

            NSOrderedSet *mediaSet = [entry valueForKey:@"media"];

            int i = 0;

            for (NSManagedObject *media in mediaSet) {

                Media *newMedia = [NSEntityDescription insertNewObjectForEntityForName:@"Media"
                                                                inManagedObjectContext:newContext];

                newMedia.isInPhotoLibrary = [media valueForKey:@"isInPhotoLibrary"];
                newMedia.positionInEntry = [NSNumber numberWithDouble:i + 1];

                MediaImageData *imageData = [NSEntityDescription insertNewObjectForEntityForName:@"MediaImageData"
                                                                          inManagedObjectContext:newContext];

                imageData.data = [media valueForKey:@"originalImage"];

                newMedia.imageData = imageData;

                newMedia.entry = newEntry;
                [newMediaSet addObject:newMedia];

                i++;
            }

            newEntry.media = newMediaSet;


        }
        }

        [newContext save:&error];

        offset = offset + batchSize;
   }
}

EDIT: I followed the advice given here, wrapping it in @autoreleasepools (which i've edited in above), and it still crashed after this happened:

It obviously got further into the code than the first time, but it's still crashing.

Answer 1

Without discussing the efficiency of your code:
Wrap your code in @autoreleasepool {} for both the outer (while) and inner (for) loops.
Otherwise, you're just allocating a whole lot of memory and freeing it only when the outer event loop reaches it's end of loop.

Answer 2

Try wrapping the insides of your fast enumeration ( for loop) in @autoreleasepool , that should help. You can also run some sort of counter to save your context inside the loop when changes reach dangerous memory consumption.

You sure do have a lot of objects though, think about optimising your data structure.

Answer 3

First, a bit of advice on posting Core Data related questions. There is almost no such thing as too much information. When you omit details, it makes it very difficult for people to help you because there are so many possible interactions.

Just looking at the code you posted, it leaves so many questions.

I would want to see the information sorted by overall size, not number of allocations. That will tell you more readily where the memory is being used. As it is, I can't see where the memory is used in that picture (then again - old eyes - when I enlarge it, it gets too blurry).

If you think it has to do with core data, why are you not running the core data instruments?

On to some diagnostic assistance.

Is your new MOC connected directly to the PSC or does it have a parent MOC. If it has a parent MOC, then the saves are just shoved into the parent MOC, and will stay in memory until the saves are persisted to the database.

How many items are in your mediaSet? It's possible to have a lot of those, which could chew up memory.

What parameters have you applied to the FetchRequest and the MOCs? Based on those options, the MOCs could be retaining objects. A way to check this is to log the counts of the retained/inserted/deleted/etc sets for each MOC. By logging those, you can see if either MOC is holding onto stuff you don't think it should be.

Then you can know if you need to refresh/reset your MOC.

You could cheat, and call [moc reset] after saving, but it would be nice to see the numbers before/after doing that so you can convince yourself where the memory is.

Do more debugging. Give yourself more information so you can make good decisions... and it will give us better information to assist you as well.

EDIT

Oooops. I removed the comments about possibly not being too much memory. My old eyes did me no favors looking at the tiny print on that image.

EDIT

Thanks. How do i log the counts of the retained/inserted/deleted sets for each MOC? – Andrew

The MOC has several properties ( registeredObjects , insertedObjects , deletedObjects , updatedObjects ) that provide a set of objects matching those categories. Simply NSLog the count of objects in each category. You can see, for each MOC, how many objects the MOC is keeping track of over time.

Also, enable the core data instruments. They are really informative.