RFR: 8339387: ZGC: Synchronize medium page allocation

[Axel Boldt-Christmas]

On Fri, 6 Sep 2024 07:14:11 GMT, Stefan Johansson <sjohanss at openjdk.org> wrote:

> Please review this change to synchronize medium page allocations in ZGC.
> 
> **Summary**
> In ZGC objects of a certain size class are allocated in medium sized pages. For each age there is a single medium page shared by all mutators. When this page gets full all thread that try to do a medium page allocation will try to allocate and install a new medium page, but only one will succeed. This can lead to a lot of unnecessary medium page allocation which in turn can lead to the unnecessary page cache flushing.
> 
> This change introduces synchronization to only a allow a single thread to allocate the medium page in the common case.  
> 
> **Testing**
> * Functional testing through mach5 tier1-7 using ZGC
> * Performance testing through aurora to verify no regression occur
> * Manual testing to verify performance
> * Manual testing to verify we avoid page cache flushing

lgtm.

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

Marked as reviewed by aboldtch (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/20883#pullrequestreview-2288903518