RFR: Add an option to stream region sampling data to a file

[Roman Kennke]

On Thu, 7 Oct 2021 15:34:08 GMT, William Kemper <wkemper at openjdk.org> wrote:

> This change adds two command line options to enable and configure a feature that has Shenandoah stream region sampling data to disk. These changes are complemented by changes in the [Shenandoah Visualizer](https://github.com/openjdk/shenandoah-visualizer) project to load and replay the logged region sampling data.

Looks ok, I only have some minor nits.

src/hotspot/share/gc/shenandoah/shenandoahHeapRegionCounters.cpp line 92:

> 90:     jlong last = _last_sample_millis;
> 91:     if (current - last > ShenandoahRegionSamplingRate &&
> 92:         Atomic::cmpxchg(&_last_sample_millis, last, current) == last) {

Neither the previous nor the new version seems to align correctly.

src/hotspot/share/gc/shenandoah/shenandoahHeapRegionCounters.hpp line 104:

> 102: 
> 103: #endif // SHARE_GC_SHENANDOAH_SHENANDOAHHEAPREGIONCOUNTERS_HPP
> 104: 

Bunch of unrelated new newlines.

src/hotspot/share/gc/shenandoah/shenandoahLogFileOutput.cpp line 3:

> 1: /*
> 2:  * Copyright (c) 2021, Amazon.com, Inc. All rights reserved.
> 3:  * Copyright (c) 2015, 2020, Oracle and/or its affiliates. All rights reserved.

Is this a copy of or derived from an existing source file? If not, only putting the 2021 header should be enough.

src/hotspot/share/gc/shenandoah/shenandoahLogFileOutput.hpp line 2:

> 1: /*
> 2:  * Copyright (c) 2013, 2021, Red Hat, Inc. All rights reserved.

Is this a new file? Then the 2021-Amazon header should be enough. If it's derived from an existing file, then the header is ok as it is.

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Marked as reviewed by rkennke (Lead).

PR: https://git.openjdk.java.net/shenandoah/pull/82