Soft References... are they working as intended?

[Damon Hart-Davis]

Hi,

FWIW I usually combine SoftReferences with some other sort of explicit limit based on heap size to help avert this type of issue, and indeed use a number of different strategies, often involving some explicit LRU management.

I can supply code snippets if that would help!  B^>

Rgds

Damon


On 18 Aug 2012, at 13:06, John Hendrikx wrote:

> I've come to the conclusion that SoftReferences in the current hotspot 
> implementation are suffering from some problems.
> 
> I'm running the latest Java 7, with default gc settings and a very 
> modest heap space of 256 MB.
> 
> On this heap I have on the order of 50-60 large objects that are 
> referenced by SoftReference objects.  Each object is a few megabytes in 
> size (they are decoded JPEG images).
> 
> At any given time, only 10 of these images have strong references to 
> them, totalling no more than 50-60 MB of heap space, the other 200 MB of 
> space is only soft referenced.
> 
> It is said that SoftReferences are guaranteed to get cleared before heap 
> space runs out, yet in certain extreme circumstances one of the 
> following can happen:
> 
> 1) 90% of the time, when under high memory pressure (many images loaded 
> and discarded), the VM gets really slow and it seems that some threads 
> get stuck in an infinite loop.  What is actually happening is that the 
> GC will run for long periods in a row (upto a few minutes, consuming one 
> CPU core) before the program gets unstuck and it finally noticed it can 
> clear some SoftReference objects.
> 
> It is possible that the GC has trouble deciding which SoftReferences can 
> be cleared because many of them had (upto a few seconds ago) strong 
> references to them, which themselves may not have been marked as garbage 
> yet.
> 
> So it recovers, but it is taking so much time to do it that users will 
> think the program is stuck.
> 
> 2) The rest of the time it actually will throw an out of heap space 
> exception, despite there being SoftReference objects that could have 
> been cleared.  This usually happens after a long pause as well.
> 
> Can anyone confirm that these problems exists, and perhaps advice a 
> course of action?
> 
> I really don't want to have to 2nd guess the GC about which images 
> should be discarded, but it looks like I will have no choice but to 
> limit this Image cache manually to some reasonable value to avoid the GC 
> getting stuck for long periods.
> 
> Best regards,
> John Hendrikx
> 
> 
> 
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