RFR: 8332139: SymbolTableHash::Node allocations allocates twice the required memory

[Johan Sjölen]

On Mon, 13 May 2024 12:30:38 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

> The symbols are inline and allocated together with the ConcurrentHashTable (CHT) Nodes. The calculation used for the required size is `alloc_size = size + value.byte_size() + value.effective_length();`
> 
> Where
>   * `size == sizeof(SymbolTableHash::Node) == sizeof(void*) + sizeof(Symbol)`
>   * `value.byte_size() == dynamic_sizeof(Symbol) == sizeof(Symbol) + <bytes beyond object>`
>   * `value.effective_length() == dynamic_sizeof(Symbol) - sizeof(Symbol) == <bytes beyond object>`
> 
> So `alloc_size` ends up being `sizeof(void*) /* node metadata */ + 2 * dynamic_sizeof(Symbol)`
> 
> Because using the CHT with dynamically sized (and inlined)  types requires knowing about its implementation details I chose to make the functionality for calculating the the allocation size a property of the CHT. It now queries the CHT for the node allocation size given the dynamic size required for the VALUE. 
> 
> The only current (implicit) restriction regarding using  dynamically sized (and inlined) types in CHT is that the _value field C++ object ends where the Node object ends, so there is not padding bytes where the dynamic payload is allocated. (effectively `sizeof(VALUE) % alignof(Node) == 0` as long as there are no non-standard alignment fields in the Node metadata).  I chose to test this as a runtime assert that the _value ends where the Node object ends, instead of a static assert with the alignment as it seemed to more explicitly show the intent of the check. 
> 
> Running testing tier1-7

Hi Axel,

I don't understand why the patch isn't just `size_t alloc_size = size + value.effective_length()`, as this would be `sizeof(void*) + sizeof(Symbol) + sizeof(<the dynamic string at the end of the symbol>)`. Could you explain that, please?

Thank you.

src/hotspot/share/utilities/concurrentHashTable.inline.hpp line 1065:

> 1063:   assert(value_size >= sizeof(VALUE), "must include the VALUE");
> 1064:   return sizeof(Node) - sizeof(VALUE) + value_size;
> 1065: }

Style, suggestion:
```c++
template <typename CONFIG, MEMFLAGS F>
inline size_t ConcurrentHashTable<CONFIG, F>::get_dynamic_node_size(size_t value_size) {


That's 88 characters, well within what we accept.

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PR Review: https://git.openjdk.org/jdk/pull/19214#pullrequestreview-2096695407
PR Review Comment: https://git.openjdk.org/jdk/pull/19214#discussion_r1626185397