Mostly Software

SGen – Concurrent Mark



Like most garbage collectors, SGen is of the stop-the-world variety, which means that when the time has come to perform its work, it stops all managed threads in their tracks and only resumes them once it has finished.

Nursery collections are usually relatively short affairs, and can be sped up even further by using a smaller nursery size. Major collections, on the other hand, can take a long time because they must scan every single live object in the system. Long garbage collection pauses are clearly a serious problem for many applications, be they interactive or on the server, and garbage collector engineers make considerable efforts to shorten pause times.

One approach to this is to do parts of the major collection concurrently with the running program, aka the mutator. SGen’s major collector, mark-and-sweep, consists of two main phases, unsurprisingly called mark and sweep.

Thanks to Zoltan’s recent work—lazy sweep—SGen already does most of sweep’s work outside of major collection pauses. I have been working on the other part of the story for the past few weeks: concurrent mark.

How it works

Our implementation of concurrent mark is very similar to that of Sun JVM’s concurrent mark-and-sweep (CMS) collector.

A major garbage collection starts with an initial marking pause, during which all roots are scanned.[1] Then it restarts the world and in the background a worker thread performs the marking and scanning of major heap objects, in the usual manner. When the concurrent marking phase is done, the collector stops the world again to finish up the collection in the final marking pause.

Dealing with concurrent mutation

Consider the following code, which manipulates a linked list, swapping the order between the two nodes following a:

1   b = a->next;
2   c = b->next;
3   a->next = c;
4   b->next = c->next;
5   c->next = b;

Let’s assume that right after a->next has been set in line 3, the concurrent worker thread scans and marks the node referenced by a, then follows next and processes c. If at that point our program hasn’t come around to setting c->next to b in line 5, the garbage collector will never encounter b—it will have been lost because for a brief moment that node wasn’t part of the list.

We can recruit the help of our friend the write barrier to deal with this issue, but we need to extend its scope a little: instead of just recording stores that point from the major heap to the nursery, it must now also record stores pointing from the major heap to the major heap. In other words: all reference stores that happen in the major heap, no matter where they point to, must be recorded.[2]

With this new write barrier the store to c->next will record the node c as requiring a re-scan. This re-scanning of all modified major heap objects happens in the final marking pause, so it’s not further disturbed by the mutator.

What if the final marking pause happens before c->next is set in line 5, however? The node b might not be pointed to by any object in the heap, so we won’t find it that way, yet it is still live. It’s live because it’s referenced by the variable b on the stack, so the final marking pause must also re-scan all roots.[3]

Nursery collections

Nursery collections that happen while concurrent mark is running stop the world, but concurrent mark keeps chugging along. It never follows references into the nursery anyway, so there is little potential for conflict.

There is an issue with the card table, however: nursery collections clear the card table, but its data is required during the final marking pause, which might only occur a few nursery collections later. The solution to this is to maintain a separate card table that accumulates the marked bits from the “regular” card table, and to use this union table in the final scan.


This graph shows maximum pause times for a few of the programs in our GC benchmarks suite. All of the pause times are relative to Boehm’s, so values below 1 are better than Boehm’s. Note that the values correspond to the longest stop-the-world pause that occurred when running the program, not the median or some average.

The three SGen configurations are standard mark-and-sweep in blue, concurrent mark-and-sweep in green and concurrent mark-and-sweep in conjunction with cementing, in yellow.

The programs represented in this graph are those whose maximum pause times with Boehm were longer than 10ms. The ones with the longest pause times are on the left—on graph4 Boehm had a pause of 2.5s—which are those that benefit most from concurrent mark, as expected: the longest pause on graph4 is less than 180ms with concurrent mark. I have not yet investigated why graph8 is such an outlier.

  1. For concurrent mark the roots not only include the usual suspects like thread stacks and global variables, but also nursery objects. In fact, the initial marking pause performs a nursery collection, after which only pinned objects will be left in the nursery. Those are treated as roots, or rather the references they contain are.  ↩

  2. Since we use a single card table for both kinds of references, this will increase the number of cards marked that don’t have references pertinent to nursery collections. The card scanning overhead will therefore increase, but in practice this doesn’t seem to be a problem. An alternative would be to use two separate card tables, one for references to nursery objects and the other for references to major heap objects. That would be more expensive both in terms of memory usage as well as in write barrier performance.  ↩

  3. This entails doing a full mark-scan loop, of course, which might still take a very long time in pathological cases. Imagine a program that keeps a huge linked list around, referenced only by a single object on the major heap. At some point concurrent mark kicks in. The program now loads the reference to the list into a local variable and nulls the reference on the heap. The garbage collector will not be able to mark the list until it scans the stack in the final marking pause, and will now have to traverse the list with the world stopped.  ↩