RFR: 8255237: ZGC: Bulk free garbage pages during relocation set selection

[Per Liden]

On Tue, 27 Oct 2020 09:21:46 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> During relocation set selection, we immediately free garbage (empty) pages. On very large heap, which has a very high number of garbage pages we can make a significant number of calls to free pages, which prolongs the "Concurrent Select Relocation Set" phase. Freeing a page involves grabbing a lock, inserting the page into the page cache, and then try to satisfy any stalled allocation. By freeing empty pages in bulk, we can amortize the some of the cost of freeing a page and speed up this phase. With this patch we free (up to) 64 pages in bulk.
>> 
>> This PR depends on "8255234: ZGC: Bulk allocate forwarding data structures" and "8255235: ZGC: Allocate and initialize forwarding data structures in parallel", which are separate PRs (#804 and #805).
>> 
>> This patch was joint work with Albert (ayang).
>
> src/hotspot/share/gc/z/zHeap.cpp line 391:
> 
>> 389:       selector.register_garbage_page(page);
>> 390: 
>> 391:       // Reclaim garbage pages in bulk
> 
> Maybe adding a comment that this is an optimization?

Will fix.

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PR: https://git.openjdk.java.net/jdk/pull/806