GC leaks debugging
Fri Apr 8 15:48:00 GMT 2011
>> The last good dump I have from the test after 12 million cycles (it
>> then got killed) has nothing like File stuff at all. A also saw other
>> suspicious objects, but they all disappeared later on. The collecter
>> really works wel!
>> Next hypothesis:
>> From analyzing graphs from the logs and comparing them to those of the
>> OpenJDK, I get the feeling that the collector looses control by not
>> collecting often enough.
>> The heap is quite unused/free, and remains so during the process. It
>> seems that at some point, the heap fills up very quickly, and then the
>> collector decides to expand the heap instead of collecting (the
>> algorithm for deciding this seems rather complicated). However, a
>> larger heap also causes the collector to collect less frequently. So
>> the next time the heap fills up rapidly, it again decides to expand
>> the heap, again causing less frequent collections. And so on. I´ll
>> post the graph data in a separate post if you want it.
> That makes sense as an explanation.
> It looks, then, as though there isn't a leak at all. The collector
> does what it's supposed to do. There is always the risk of this with
> any non-compacting dynamic memory allocator.
Yes, a non-moving collector has its limits, but I have still hope for
>> And the next hypothesis:
>> Perhaps the program allocates many different (possibly large) sizes,
>> which remain on the free list, but cannot be used because the next
>> objects requested are slightly bigger. I have to study this somewhat
> I wonder.
>> Just two questions:
>> 1. What is a reasonable number of heap sections? I have 131 here.
>> 2. What is a reasonable number of free lists? I have 60, which have
>> 13,000+ entries.
I found from the code that 60 is the upper bound. And 13,000 seems to
be a lot to me. Altogether, the free space is about 700 Mb, of a 900
Hans, is there a limit on the length of the free lists?
Adjacent blocks of the same size are coalesced; is there also
coalescing of blocks of different sizes at some point?
I tried a build with with USE_MUNMAP, hoping that would help the
collector clean up free-list, but I am still testing.
> Paging Hans Boehm. can you suggest ways to get the system to GC more
> frequently? Would doing so avoid this scenario?
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