RFR: 8292989: Avoid dynamic memory in AsyncLogWriter

[Xin Liu]

On Wed, 31 Aug 2022 14:04:46 GMT, Johan Sjölén <duke 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.
>
> src/hotspot/share/logging/logAsyncWriter.cpp line 143:
> 
>> 141:     log_warning(logging)("AsyncLogging failed to create buffers. Falling back to synchronous logging.");
>> 142: 
>> 143:     if (buf0 != nullptr) {
> 
> `os::free(nullptr)` is safe, no need for a branch here.

okay! I will update it.

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