Hardware Assisted Garbage Collection

Question

Because of Generational Collection, I'd have to say that tracing and copying are not huge bottlenecks to GC.

What would help, is hardware-assisted READ barriers which take away the need for 'stop the world' pauses when doing stack scans and marking the heap.

Azul Systems has done this: http://www.azulsystems.com/products/compute_appliance.htm They gave a presentation at JavaOne on how their hardware modifications allowed for completely pauseless GC.

Another improvement would be hardware assisted write barriers for keeping track of remembered sets.

Generational GCs, and even more so for G1 or Garbage First, reduce the amount of heap they have to scan by only scanning a partition, and keeping a list of remembered sets for cross-partition pointers.

The problem is this means ANY time the mutator sets a pointer it also has to put an entry in the appropriate rememered set. So you have (small) overhead even when you're not GCing. If you can reduce this, you'd reduce both the pause times neccessary for GCing, and overall program performance.

One obvious solution was to have memory pointers which are larger than your available RAM, for example, 34bit pointers on a 32 bit machine. Or use the uppermost 8 bits of a 32bit machine when you have only 16MB of RAM (2^24). The Oberon machines at the ETH Zurich used such a scheme with a lot success until RAM became too cheap. That was around 1994, so the idea is quite old.

This gives you a couple of bits where you can store object state (like "this is a new object" and "I just touched this object"). When doing the GC, prefer objects with "this is new" and avoid "just touched".

This might actually see a renaissance because no one has 2^64 bytes of RAM (= 2^67 bits; there are about 10^80 ~ 2^240 atoms in the universe, so it might not be possible to have that much RAM ever). This means you could use a couple of bits in todays machines if the VM can tell the OS how to map the memory.

Yes. Look at the related work sections of these 2 papers:

https://research.microsoft.com/en-us/um/people/simonpj/papers/parallel-gc/index.htm http://www.filpizlo.com/papers/pizlo-ismm2007-stopless.pdf

Or at this one:

http://researcher.watson.ibm.com/researcher/files/us-bacon/Bacon12StallFree.pdf

There was an article on lambda the ultimate describing how you need a GC-aware virtual memory manager to have a really efficient GC, and VM mapping is done mostly by hardware these days. Here you are :)