Question regarding G1 option to run parallel Old generation garbage collection?

[Thomas Schatzl]

Hi,

On Thu, 2012-10-25 at 20:40 -0700, Srinivas Ramakrishna wrote:
> Thomas, while adding newly freed regions to the head of a common
> global free region list probably achieves much reuse, I was
> suggesting something even stronger and deliberate, in that a much
> smaller set of regions be preferred for repeated reuse for Eden
> allocation and for reuse as survivor regions, kind of
> segregating that smaller subset completely from regions used for
> tenuring objects.

What I mean is, that to exploit cache effects in the way you
suggest, these areas must be much smaller than you suggest; on
multi-GB ranges of memory you likely won't benefit from them any
more than now. NUMA awareness does not change that, especially
because you typically use NUMA on machines because you want to
use lots of memory.

I.e. just a quick calculation: with NUMA awareness, the amount
of memory touched per node/core is likely still too high to
benefit a lot from caching. E.g. a 64GB eden divided across 8
nodes still means you're touching 8GB/node with processors that
have a few MB of cache each. In the best case.

NUMA awareness does not improve upon cacheability (it might as a
secondary effect!), but its main point is to improve access
to memory beyond the caches imo. 

One idea where you exploit cache effects by giving each
thread/core a very small heap with eden (typically <= 1 MB,
i.e. something that fits nicely into the cache) that is
reused like you suggest over and over again is generally
referred to as "thread local heaps".

The problem is, that the threshold to get improvements is
much higher than with NUMA awareness. If a thread only
has such a small eden, you cannot stop all threads every time
it fills up any more, because that would naturally decrease
throughput too much. I.e. it needs thread local gcs (so that
every thread can collect its own heap without stopping the
others) and associated complexity in the VM to work
efficiently though.

This is not meant to discourage you about NUMA awareness, just
a try to clear up things (if there ever was something to clear
up). NUMA awareness does have its use and it improves
performance, but I don't think it mainly does so because of
improved cachability.

Thomas