RFR(S) 8015255: NPG: Don't waste fragment at the end of a VirtualSpaceNode before retiring it.

[Jon Masamitsu]

Looks good.  Some questions below.  No blockers to the integration.

> 1120   for (int i = (int)MediumIndex; i >= (int)ZeroIndex; --i) {
> 1121     ChunkIndex index = (ChunkIndex)i;

What do you think about code like this that uses ChunkIndex as the
for loop counter.

   for (ChunkIndex i = Zer Index; i < NumberOfFreeLists; i = 
next_chunk_index(i))

I'm asking because I like the way it looks but it's not perfect.

> 3674     // The chunk sizes must be multiples of eachother, or this will fail
> 3675     STATIC_ASSERT(MediumChunk % SmallChunk == 0);
> 3676     STATIC_ASSERT(SmallChunk % SpecializedChunk == 0);

The above is a requirement now?   I've never thought of it as a 
requirement.  It just
happened to be the case.  I'm not opposed to it, just wondering.

3724       vsn.get_chunk_vs(MediumChunk + SpecializedChunk); // Humongous chunks will be aligned up to MediumChunk + SpecializedChunk

Should the chunk size beMediumChunk + SpecializedChunk  / 2
to test the alignment up?

Jon


  



On 10/17/2013 5:56 AM, Mikael Gerdin wrote:
> Hi all,
>
> Please review this change to fix needless memory waste in Metaspace.
>
> Description of problem:
> When receiving an allocation request which cannot be satisfied with the
> current VirtualSpaceNode the remaining committed memory in the current node is
> wasted when a new node is created and the allocation is satisfied from the new
> node.
>
> Suggested fix:
> Before creating a new VirtualSpaceNode, "retire" the current node by
> allocating chunks of decreasing size from the committed memory present in the
> current node.
> To ensure that allocating chunks at the end of a node uses all committed
> memory I've increased the size alignment for humongous chunks to the smallest
> chunk size. I don't foresee this increasing memory waste since previously the
> memory difference between 2-word-aligned sizes and smallest chunk-aligned
> would go unused since no chunk would fit there.
>
> Testing:
> Added new unit tests for VirtualSpaceNode and the retire functionality.
> JPRT run (including the new unit tests)
>
> Webrev: http://cr.openjdk.java.net/~mgerdin/8015255/webrev.0/
> Buglink: https://bugs.openjdk.java.net/browse/JDK-8015255
>
> /Mikael