RFR: 8292989: Avoid dynamic memory in AsyncLogWriter [v2]

[Thomas Stuefe]

On Tue, 6 Sep 2022 16:11:40 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> Xin Liu has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   update according to feedbacks.
>
> src/hotspot/share/logging/logAsyncWriter.cpp line 75:
> 
>> 73:     p->set_message(dest);
>> 74:     _pos += sz;
>> 75:     return true;
> 
> That's pretty hacky... :-) And `set_message` only exists because you need to repair the message pointer here, right?
> 
> Proposal:
> 
> Make AsyncLogMessage self-contained and variable-sized by letting the message follow the header. Like this:
> 
> 
> class AsyncLogMessage {
>   LogFileStreamOutput* _output;
>   const LogDecorations _decorations;
>   const size_t _len; // optional, or could also be 32- maybe even 16 bit
> public:
>   size_t len_in_bytes() const { return align_up(sizeof(AsyncLogMessage) + _len); }
>   const char* message() { return (const char*) (this + 1); }
> };
> 
> 
> Then give it a placement new() and place it inside the Buffer right away instead of creating it on the stack and then copying it to the buffer. Just an idea.

Update: I looked at AsyncLogMessage and you are almost at what I propose anyway since the in-buffer version of AsyncLogMessage already follows my proposed layout. And your `AsyncLogMessage::size()` is similar to what I propose too. 

But the way you do it, you have two versions, one where the message follows the header, one where it is wherever. This means that `AsyncLogMessage::size()` is wrong for at least half the time.

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PR: https://git.openjdk.org/jdk/pull/10092