Request for review (M): 8002144: G1: large number of evacuation failures may lead to large c heap memory usage
john.cuthbertson at oracle.com
Fri Feb 8 06:06:51 UTC 2013
Looks very good. Ship it!.
On 2/1/2013 7:10 AM, Bengt Rutisson wrote:
> Hi all,
> Could I have a couple of reviews for this change?
> For the evacuation failure handling in G1 we need to store the mark
> word for objects that we self-forward. We store these mark words in
> two GrowableArrays. The problem is that if we have a lot of objects
> that need to be self-forwarded we will add a lot of entries to the
> GrowableArrays. These arrays will double in size when they get full.
> Worst case these arrays can require more consecutive free memory than
> is available in the OS malloc heap.
> I have a test (ManyObjects) that runs out of native memory on Windows
> 32 bit platforms because of this issue. We want to double from 10MB to
> 20MB when we hit native out of memory.
> My fix for this will reduce the risk for native out of memory. Instead
> of doubling the size I introduce a chunked data structure that will
> only malloc one new chunk when it gets full. This requires less
> consecutive memory. It also has the nice side effect that we don't
> have to copy the entries when we need more memory.
> Without this fix I get native out of memory about every three runs of
> the test. With this fix the test has been running for several days and
> more than 5600 iterations.
> The chunk size is variable but has a max limit. I use 40 entries as
> initial size since this is what the GrowableArrays used. I picked
> 10000 as the maximum size. The value 10000 can probably be tuned
> further, but 100000 was too much (I still got native OOME) and 10000
> works fine.
> I have been comparing GC pause times with and without my fix for the
> ManyObjects test. I don't see any large differences in the pause
> times. This will only affect performance for runs that have a lot of
> evacuation failures. These runs will benefit form the fact that we
> don't have to do as much copying as before, but they will also do
> several more mallocs compared to before my fix. The runs I've done
> indicate that this evens out. I can't see any large differences.
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