RFR: 8263976: Remove block allocation from BasicHashtable

[Ioi Lam]

On Mon, 22 Mar 2021 15:49:24 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

> From CR:
> The useful/general BasicHashtable uses a block allocation scheme to reportedly reduce fragmentation. When the StringTable and SymbolTable used to use this hashtable, performance benefits were reportedly observed because of the block allocation scheme. Since these tables were moved to the concurrent hashtables, the tables left that use the block allocation scheme are:
> 
> AdapterHandlerLibrary, ResolutionError, LoaderConstraints, Leak profiler bitset table and Placeholders. 3 of these tables are very small and never needed block allocation to prevent fragmentation at least. Also there are 3 KVHashtables, which are built from BasicHashtable. 2 are used during dumping and 1 is ID2KlassTable which appears small.
> 
> ModuleEntry, PackageEntry, Dictionary, G1RootSet for nmethods, and JvmtiTagMap tables didn't use the block allocation scheme.
> 
> Removing this removes 7 pointers per table, and for each ClassLoaderData, which has 3 tables, removes 21 pointers.
> 
> This change was performance tested on linux and windows.
> 
> It was also tested with tier1-6.

Looks good overall. Just some small nits.

src/hotspot/share/utilities/hashtable.cpp line 54:

> 52:   BasicHashtableEntry<F>* entry = ::new (NEW_C_HEAP_ARRAY(char, this->entry_size(), F))
> 53:                                         BasicHashtableEntry<F>(hashValue);
> 54:   assert(_entry_size % HeapWordSize == 0, "");

Is this assert still needed? What's its purpose?

src/hotspot/share/utilities/hashtable.cpp line 64:

> 62: 
> 63:   if (DumpSharedSpaces) {
> 64:     // Avoid random bits in structure padding so we can have deterministic content in CDS archive

Hmm, the sequence looks a little odd: the constructor initializes some fields, they are then zeroed here, and then initialized again below ....

Actually, I think you can get rid of the `if (DumpSharedSpaces)` block for now. I am not sure if it's needed. Deterministic CDS archive is broken anyway (https://bugs.openjdk.java.net/browse/JDK-8253495).

That way you can get rid of the entry->set_xxx below as well.

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Changes requested by iklam (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/3123