Summary
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Deprecate the combination of the Parallel Scavenge and Serial Old garbage collection algorithms.

Non-Goals
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- It is not a goal to remove this GC combination.

- It is not a goal to deprecate any other GC combinations.

Motivation
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There is one combination of GC algorithms that we believe is very little used but requires a significant amount of maintenance effort: The pairing of the parallel young generation GC (called ParallelScavenge) and the serial old GC (called SerialOld). This combination must be specifically enabled by the user with the `-XX:+UseParallelGC -XX:-UseParallelOldGC` command line options.

This combination is unusual since it pairs the parallel young generation and serial old generation GC algorithms. We think this combination is only useful for deployments with a very large young generation and a very small old generation. In this scenario the full collection pause times might be bearable due to the small size of the old generation. In practice this is a very rare and risky deployment, since a slight shift in liveness for objects in the young generation will result in an `OutOfMemoryException`, since the old generation is significantly smaller than the young generation. The only advantage of this combination compared to using a parallel GC algorithm for both the young and old generations is slightly lower total memory usage. We believe that this small memory footprint advantage (at most ~3% of the Java heap size) is not enough to outweigh the costs of maintaining this GC combination.

Description
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In addition to deprecating the option combination `-XX:+UseParallelGC -XX:-UseParallelOldGC` we will also deprecate the option `-XX:UseParallelOldGC`, since its only use is to deselect the parallel old generation GC, thereby enabling the serial old generation GC.

As a result, any explicit use of the `UseParallelOldGC` option will display a deprecation warning. A warning will, in particular, be displayed when `-XX:+UseParallelOldGC` is used standalone (without `-XX:+UseParallelGC`) to select the parallel young and old generation GC algorithms.

The only way to select the parallel young and old generation GC algorithms without a deprecation warning will to specify only `-XX:+UseParallelGC` on the command line.

Alternatives
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The only change is the deprecation message; there is no loss of functionality. The existing collector called "Parallel", which combines the parallel young generation and parallel old generation algorithms, has almost the same behavior and should be a drop-in replacement.

Risks and Assumptions
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We assume that there are very few users of this GC combination. If there are still a significant number of users of this combination, deprecation may be reconsidered.