Jon Masamitsu's Weblog
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Java
January 26, 2006
Why not a Grand Unified Garbage Collector?
the parallel collector
the low pause collector
the serial collector
Why is there more than one? Actually, the usual answer applies. Specialization often
results in
better performance. If you're interested in more particulars about our
garbage collectors, read on.
All three collectors are generational (young generation and tenured generation).
Let's do the easy comparison first, why a parallel collector and a serial
collector. Parallelism has overhead. Nuf said? Yeah, I used
to read comic books when I was a kid. If you don't understand that reference, ignore
it. I'm just older.
As you might infer from the names, the serial collector uses 1 thread to do the
GC work and the parallel collector uses multiple threads to do the same. As usual
multiple threads doing the same tasks have to synchronize. That's pretty much it.
On a single cpu machine the additional cost of the synchronization means that the
parallel collector is slower than the serial collector. On a two cpu machine and
a VM that has a small heap the parallel collector is as about fast as the serial
collector. With two cpu's and large heaps the parallel collector will usually do
better. We keep asking ourselves if we can get rid of the serial collector and
use the parallel collector in its place. The answer so far keeps coming back no.
More interesting is the case of the low pause collector versus the parallel collector.
Above I made the remark about specialization and better performance. This is actually
a case of more complexity and lesser performance.
These two collectors do the collection of the young generation using almost the exact same
techniques The differences in the collectors have to do with the collections of the
tenured generation. The low pause collector does parts of that collection
while the application continues to run. One way to do that
is to not move the live objects when collecting the dead objects. The application tends
to get confused if the objects it is using move around while the application is running.
The other two collectors
compact the heap during a collection of the tenured generation (i.e., live objects are
moved so as to occupy one contiguous region of the heap). The low pause collector collects
the dead objects and coalesces their space into blocks that are kept
in free lists. Maintaining free lists and doing allocations from them takes effort so
it's slower than having a heap that is compacted. Having applications run while a
collection is happening means that new objects can be allocated during a collection.
That leads to so more complexity. Also the collection of the tenured generation can
be interrupted for a collection of the young generation. More complexity still. The bottom line
is that the low pause collector has shorter GC pauses but it costs performance.
That performance difference is not huge but it's large enough to keep us from ditching
the parallel collector and always using the low pause collector.
And last but not least, can we replace the serial collector with the low pause
collector? Very tempting. The serial collector is used by default on desktop machines.
We expect those to have 1 or 2 cpus and to be running applications that need 10's
of megabytes of Java heap as opposed to 100's of megabytes. With small heaps the differences
in collection times tend to make less difference. Even if the low pause collector was
10% slower than the serial collector, the difference between, for example, 70 ms
and 77 ms often isn't large enough to matter. It would probably be a done deal except
that the low pause collector has a larger memory footprint. It has additional data
structures that it uses (for example to keep track of what references are being
changed by the the running application while a collection is on going). It also usually
needs a larger Java heap to run an application. Recall that the low pause collector
uses free lists to keep track of the available space in the heap. Fragmentation
can become a problem. The best bet is that we'll replace the serial collector with
the low pause collector some day but not just yet.