JDK-8186958 - Behaviour of large values for numMapping in WeakHashMap.newWeakHashMap API

[Jaikiran Pai]

On 07/06/22 9:39 am, Stuart Marks wrote:
> Hi Jai,
>
> The error
>
>     java.lang.OutOfMemoryError: Java heap space
>
> indicates that the VM really has run out of memory. Presumably if you 
> increased the heap size, it would actually be able to allocate that 
> memory. You might have to add the /othervm test directive and add JVM 
> options to require a larger heap.
>
> The table size must be a power of two, so the largest table size that 
> will be allocated is 1 << 30 or 1073741824 as you noted. That will 
> take about 8GB of heap (in the no-compressed-OOP case). That's not 
> terribly large, but we might want to check to see if there are other 
> tests that require that much memory.
>
> As you also noted, WeakHashMap eagerly allocates its table whereas 
> LinkedHashMap and HashMap do not. I think this is an acceptable 
> behavior variation. Note that we had to avoid this case in 
> WhiteboxResizeTest:
>
> https://github.com/openjdk/jdk/blob/master/test/jdk/java/util/HashMap/WhiteBoxResizeTest.java#L167 
>
>
> We might have to make similar special cases here for WHM.
>
> I don't think we need to document this behavior difference. More 
> precisely: this kind of implementation variation doesn't need to be 
> specified. In the future we might change WHM to allocate lazily.
>
> The API should accommodate extremely large values of numMappings. Even 
> if it's larger than 1 << 30 and the table size is allocated at 1 << 
> 30, it's still possible to add numMappings mappings without resizing. 
> (Memory permitting, of course.) Doing so will violate the load factor 
> invariant, and it might result in more collisions than one would like, 
> but it should still work.

Thank you for those inputs, Stuart.


>
> I think we just need to decide whether we want to have a test that 
> allocates this much memory, and if so, to apply the necessary settings 
> to make sure the JVM has enough heap.

As a start, I've removed the large value testing in these basic tests 
and moved the PR out of the draft state.

-Jaikiran