SoftReference incorrect javadoc?

[Michael Pollmeier]

Hi Per,

While testing different JVMs I realized that it's fixed in openjdk 11,
e.g. openjdk version "11.0.1" 2018-10-16 LTS (zulu build), maybe by this
commit: https://hg.openjdk.java.net/jdk/jdk/rev/6464882498b5
That's great to know, but is it worth updating the javadocs of older
versions?

To reproduce it I attached a simple SoftRefTest.java to easily reproduce
it. It allocates (only) softly referenced objects that occupy some heap,
occasionally printing counts (instantiated, finalized, free heap).

Usage:
javac SoftRefTest.java
java -Xms256m -Xmx256m SoftRefTest

Output:
100000 instances created; free=212M
200000 instances created; free=181M
300000 instances created; free=152M
400000 instances created; free=121M
500000 instances created; free=93M
600000 instances created; free=61M
700000 instances created; free=33M
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
        at Instance.<init>(SoftRefTest.java:27)
        at SoftRefTest.main(SoftRefTest.java:12)

Interpretation:
It doesn't free any of the only softly referenced objects, resulting in
an OutOfMemoryError, contradicting the 'guarantee' in the javadoc.

Workaround: if you additionally configure
`-XX:SoftRefLRUPolicyMSPerMB=0`, it finalizes them and doesn't run out
of memory.

Tested with:
openjdk version "1.8.0_212"
java version "1.8.0_144" (oracle)
openjdk version "9.0.4.1" (zulu build)
openjdk version "10.0.2" 2018-07-17  (zulu build)

Cheers
Michael


On 16/04/19 6:27 pm, Per Liden wrote:
> Hi Michael,
> 
> On 4/16/19 4:19 AM, David Holmes wrote:
>> Hi Michael,
>>
>> Re-directing to core-libs-dev and hotspot-gc-dev.
>>
>> Thanks,
>> David
>>
>> On 16/04/2019 12:14 pm, Michael Pollmeier wrote:
>>> Quoting
>>> https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/ref/SoftReference.html
>>>
>>>
>>>
>>>> All soft references to softly-reachable objects are guaranteed to have
>>> been cleared before the virtual machine throws an OutOfMemoryError
>>>
>>> That statement was true when soft references were first introduced in
>>> java 1.2, but from java 1.3.1 the jvm property
>>> `-XX:SoftRefLRUPolicyMSPerMB` was introduced.
> 
> That statement is still true. When the GC gets into a situation where it
> is having trouble satisfying an allocation, then SoftRefLRUPolicyMSPerMB
> will be ignored and all soft references will be cleared, before the GC
> gives up and throws an OOME.
> 
>>> It defaults to 1000 (milliseconds), meaning that if there’s only 10MB
>>> available heap, the garbage collector will free references that have
>>> been used more than 10s ago. I.e. everything else (including young
>>> softly reachable objects) will *not* be freed, leading to an
>>> OutOfMemoryError, contradicting the above quoted 'guarantee'.
>>>
>>> That's also the behaviour I observed on various JREs. Would you agree,
>>> i.e. should I propose an updated doc?
> 
> Could you elaborate on what kind of test you did to come to this
> conclusion? Preferably provide a re-producer. In OOME situations, if a
> SoftReference isn't cleared, it is typically because you have
> unknowingly made it strongly reachable.
> 
> cheers,
> Per
> 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: SoftRefTest.java
Type: text/x-java
Size: 1028 bytes
Desc: not available
URL: <https://mail.openjdk.org/pipermail/hotspot-gc-dev/attachments/20190417/76df5b8d/SoftRefTest.java>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 833 bytes
Desc: OpenPGP digital signature
URL: <https://mail.openjdk.org/pipermail/hotspot-gc-dev/attachments/20190417/76df5b8d/signature.asc>