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List:       openjdk-hotspot-runtime-dev
Subject:    Re: RFR: 8292989: Avoid dynamic memory in AsyncLogWriter
From:       Xin Liu <xliu () openjdk ! org>
Date:       2022-08-31 18:14:24
Message-ID: CnMaGTjZtDPZPrKWgpFdpwCJWrX2cI-ZN3nQvG6DP3c=.94ddcc6c-d35a-4860-9b54-1a40ca1a7220 () github ! com
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On Wed, 31 Aug 2022 14:04:46 GMT, Johan Sjölén <duke@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.

-------------

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


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