RFR: 8229517: Support for optional asynchronous/buffered logging

[Yasumasa Suenaga]

On Mon, 22 Mar 2021 22:12:14 GMT, Xin Liu <xliu at openjdk.org> wrote:

> This patch provides a buffer to store asynchrounous messages and flush them to
> underlying files periodically.

I think this PR is very useful for us!

> May we know more about LogMessageBuffer.hpp/cpp? We haven’t found a real use of it. That’s why we are hesitating to support LogFileOutput::write(LogMessageBuffer::Iterator msg_iterator). Further, we haven’t supported async_mode for LogStdoutOutput and LogStderrOutput either. It’s not difficult but needs to big code change.

`LogMessageBuffer` is used in `LogMessage`. For example, we can see it as following. Frame # 1 is `LogFileOutput::write(LogMessageBuffer::Iterator msg_iterator)`. IMHO we do not need to change LogStdout/errOutput, but it is better to change LogMessageBuffer.

#0  LogFileStreamOutput::write (this=this at entry=0x7ffff0002af0, msg_iterator=...)
    at /home/ysuenaga/github-forked/jdk/src/hotspot/share/logging/logDecorators.hpp:108
#1  0x00007ffff6e80e8e in LogFileOutput::write (this=this at entry=0x7ffff0002af0, msg_iterator=...)
    at /home/ysuenaga/github-forked/jdk/src/hotspot/share/logging/logFileOutput.cpp:314
#2  0x00007ffff6e876eb in LogTagSet::log (
    this=this at entry=0x7ffff7d4a640 <LogTagSetMapping<(LogTag::type)16, (LogTag::type)68, (LogTag::type)0, (LogTag::type)0, (LogTag::type)0, (LogTag::type)0>::_tagset>, msg=...) at /home/ysuenaga/github-forked/jdk/src/hotspot/share/logging/logTagSet.cpp:85
#3  0x00007ffff6a194df in LogImpl<(LogTag::type)16, (LogTag::type)68, (LogTag::type)0, (LogTag::type)0, (LogTag::type)0, (LogTag::type)0>::write (
    msg=...) at /home/ysuenaga/github-forked/jdk/src/hotspot/share/logging/logTagSet.hpp:150
#4  LogMessageImpl<(LogTag::type)16, (LogTag::type)68, (LogTag::type)0, (LogTag::type)0, (LogTag::type)0, (LogTag::type)0>::flush (
    this=0x7ffff58675d0) at /home/ysuenaga/github-forked/jdk/src/hotspot/share/logging/logMessage.hpp:79
#5  LogMessageImpl<(LogTag::type)16, (LogTag::type)68, (LogTag::type)0, (LogTag::type)0, (LogTag::type)0, (LogTag::type)0>::~LogMessageImpl (
    this=0x7ffff58675d0, __in_chrg=<optimized out>) at /home/ysuenaga/github-forked/jdk/src/hotspot/share/logging/logMessage.hpp:74
#6  InstanceKlass::print_class_load_logging (this=this at entry=0x800007430, loader_data=loader_data at entry=0x7ffff00f5200,
    module_entry=module_entry at entry=0x0, cfs=cfs at entry=0x0) at /home/ysuenaga/github-forked/jdk/src/hotspot/share/oops/instanceKlass.cpp:3647

src/hotspot/share/runtime/globals.hpp line 2033:

> 2031:           "Milliseconds between asynchronous log flushing")                 \
> 2032:                                                                             \
> 2033:   product(bool, AsyncLogging, false,                                        \

I think this option is not needed - `async` should be set to `false` by default, and we should control it through `-Xlog` option like other log output options (e.g. `filecount`).

src/hotspot/share/runtime/globals.hpp line 2036:

> 2034:           "Enble asynchronous GC logging")                                  \
> 2035:                                                                             \
> 2036:   product(size_t, GCLogBufferSize, 2*K,                                     \

This PR is for UL, not only GC log. So it should be renamed.

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PR: https://git.openjdk.java.net/jdk/pull/3135