RFR: 8292989: Avoid dynamic memory in AsyncLogWriter [v5]

[Thomas Stuefe]

On Sat, 10 Sep 2022 18:48:52 GMT, Xin Liu <xliu at openjdk.org> wrote:

>> Current implementation of AsyncLogWriter uses dynamic memory. There are 2 sources. 
>> 
>> 1. Overhead of pointer-based linked-list. 
>> 2. strdup of message contents
>> 
>> This implementation has impact on glibc/malloc. If allocation of logsites interleave with other allocation, it's hard to clean up all glibc arenas. This worsens fragmentation issue.
>> 
>> In this PR, we replace linked-list with `2 pre-allocated raw buffers`. AsyncLogWriter appends payload AsyncLogMessage to the serving buffer and avoids all dynamic allocation. Please note this effort won't eliminate mutex lock. We use ping-pong buffers to guarantee AsyncLogWriter is still non-blocking. A buffer serves as a FIFO queue like before. 
>> 
>> In addition, AsyncLogWriter doesn't enqueue meta messages anymore when it needs to report the number of discarded messages. This is archived using a temporary hashtable called `snapshot`. It copies the working hashtable with the protection of lock and reset it. After writing all regular messages, AsyncLogWriter writes meta messages from snapshot.
>
> Xin Liu has updated the pull request incrementally with one additional commit since the last revision:
> 
>   update per reviewer's feedback.
>   
>   Also add a unit test for AsyncLogWriter::Buffer.

src/hotspot/share/logging/logAsyncWriter.hpp line 68:

> 66: 
> 67:   // Messsage is the envelop of a log line and its associative data.
> 68:   // The size is variety-length with zero-terminated c-str. It is only valid when we create it using placement new

s/variety/variable

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