How actually card table and writer barrier works?

Question

I am reading some materials about garbage collection in Java in order to get to know more deeply what really happens in GC process.

I came across on the mechanism ca

Answer 1

For anyone who is looking for a simple answer:

In JVM, the memory space of objects is broken down into two spaces:

• Young generation (space): All new allocations (objects) are created inside this space.
• Old generation (space): This is where long lived objects exist (and probably die)

The idea is that, once an object survives a few garbage collection, it is more likely to survive for a long time. So, objects that survive garbage collection for more than a threshold, will be promoted to old generation. The garbage collector runs more frequently in the young generation and less frequently in the old generation. This is because most objects live for a very short time.

We use generational garbage collection to avoid scanning of the whole memory space (like Mark and Sweep approach). In JVM, we have a minor garbage collection which is when GC runs inside the young generation and a major garbage collection (or full GC) which encompasses garbage collection of both young and old generations.

When doing minor garbage collection, JVM follows every reference from the live roots to the objects in the young generation, and marks those objects as live, which excludes them from the garbage collection process. The problem is that there may be some references from the objects in the old generation to the objects in young generation, which should be considered by GC, meaning those objects in young generation that are referenced by objects in old generation should also be marked as live and excluded from the garbage collection process.

One approach to solve this problem is to scan all of the objects in the old generation and find their references to young objects. But this approach is in contradiction with the idea of generational garbage collectors. (Why we broke down our memory space into multiple generations in the first place?)

Another approach is using write barriers and card table. When an object in old generation writes/updates a reference to an object in the young generation, this action goes through something called write barrier. When JVM sees these write barriers, it updates the corresponding entry in the card table. Card table is a table, which each one of its entries correspond to 512 bytes of the memory. You can think of it as an array containing 0 and 1 items. A 1 entry means there is an object in the corresponding area of the memory which contains references to objects in young generation.

Now, when minor garbage collection is happening, first every reference from the live roots to young objects are followed and the referenced objects in young generation will be marked as live. Then, instead of scanning all of the old object to find references to the young objects, the card table is scanned. If GC finds any marked area in the card table, it loads the corresponding object and follows its references to young objects and marks them as live either.