Integrated: 8317007: Add bulk removal of dead nmethods during class unloading

[Thomas Schatzl]

On Tue, 21 Nov 2023 16:14:42 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

> Hi all,
> 
>   please review this change that bulk-removes dead nmethods for the STW collectors instead of unregistering nmethod by nmethod. This significantly speeds up the class unloading phase.
>     
> For G1, this is almost 100% the code that has been removed from the review for JDK-8315503.
>     
> For Serial and Parallel GC, the code is new.
>     
> This change does not try to improve the situation for concurrent collectors - this would at first glance require extending the scope of the `CodeCache_lock` which I did not want to do. See the CR for more details.
>     
> Also, no parallelism for Parallel GC: the existing data structure for code root remembered set is a linked list. There is in almost all cases no point in trying to parallelize what is basically a traversal of the linked list (with each element not having a lot of work to do). I filed JDK-8320067. There should already be a significant speedup with this change, as each `unregister_nmethod` call previously traversed the entire linked list to find that element (i.e. complexity reduction of O(n^2) -> O(n)).
>     
> One more comment:
> 
> *  So it would be possible to merge the `ScavengableNMethod::prune_nmethods_not_into_young` which removes elements from the code root sets that do no longer need to be remembered (due to object movement) with removing dead/unlinked nmethods. However, this would mean to put that class unloading code to the end of gc as during phase 1 the new locations which are relevant for pruning uninteresting code root remembered set entries did not move yet. I wanted to keep determining the unlinking nmethods and class/code unloading them together in the code, but I could be convinced to move it.
>     
> Testing: tier1-4
> 
> Depends on #16759, please review first.
> 
> Thanks,
>   Thomas

This pull request has now been integrated.

Changeset: f5538195
Author:    Thomas Schatzl <tschatzl at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/f55381950266088cc0284754b16663675867ac87
Stats:     194 lines in 27 files changed: 129 ins; 43 del; 22 mod

8317007: Add bulk removal of dead nmethods during class unloading

Reviewed-by: ayang, iwalulya

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PR: https://git.openjdk.org/jdk/pull/16767