RFR: 8353273: Reduce number of oop map entries in instances

[Frederic Parain]

On Mon, 31 Mar 2025 14:02:03 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

> In preparation for planned GC performance improvements (KLUT), I would like to reduce the average number of oop map entries.
> 
> For details, please see JBS issue text.
> 
> -----------------------
> 
> Patch results:
> 
> The patch brings a positive change of oop map size, reducing the likelihood of lengthy oop maps. Here the oop map size distribution over all JDK classes in the JDK image: 
> 
> Before:
> 
>    5395  - non-static oop maps (0 entries)
>    9330  - non-static oop maps (1 entries)
>    1449  - non-static oop maps (2 entries)
>     274  - non-static oop maps (3 entries)
>     218  - non-static oop maps (4 entries)
>      75  - non-static oop maps (5 entries)
>       7  - non-static oop maps (6 entries)
>       4  - non-static oop maps (7 entries)
> 
> 
> Now:
> 
>    5395  - non-static oop maps (0 entries)
>   10178  - non-static oop maps (1 entries)
>     933  - non-static oop maps (2 entries)
>     229  - non-static oop maps (3 entries)
>      16  - non-static oop maps (4 entries)
>       1  - non-static oop maps (5 entries)
> 
> 
> For example, `java.util.concurrent.ConcurrentHashMap$TreeNode` is changed from having 2 entries to having just one entry, which is nice for a class that may be instantiated a lot:
> 
> Before:
> 
> java.util.concurrent.ConcurrentHashMap$TreeNode {0x000000000d1dddc0}
>  - ---- non-static fields (9 words):
>  - final 'hash' 'I' @12
>  - final 'key' 'Ljava/lang/Object;' @16 
>  - volatile 'val' 'Ljava/lang/Object;' @20 
>  - volatile 'next' 'Ljava/util/concurrent/ConcurrentHashMap$Node;' @24       << last field of base class
>  - 'red' 'Z' @28                                                             << derived class starts here, non-oops lead
>  - 'parent' 'Ljava/util/concurrent/ConcurrentHashMap$TreeNode;' @32 
>  - 'left' 'Ljava/util/concurrent/ConcurrentHashMap$TreeNode;' @36 
>  - 'right' 'Ljava/util/concurrent/ConcurrentHashMap$TreeNode;' @40 
>  - 'prev' 'Ljava/util/concurrent/ConcurrentHashMap$TreeNode;' @44 
>  - non-static oop maps (2 entries): 16-24 32-44  
> 
> Now:
> 
> java.util.concurrent.ConcurrentHashMap$TreeNode {0x000000007e1de450}
>  - ---- non-static fields (9 words):
>  - final 'hash' 'I' @12 
>  - final 'key' 'Ljava/lang/Object;' @16 
>  - volatile 'val' 'Ljava/lang/Object;' @20 
>  - volatile 'next' 'Ljava/util/concurrent/ConcurrentHashMap$Node;' @24       << last field of base class
>  - 'parent' 'Ljava/util/concurrent/ConcurrentHashMap$TreeNode;' @28          << class starts here, oops lead
>  - 'left' 'Ljava/util/concurrent/ConcurrentHashMap$TreeNode;' @32 
>  - 'right' 'Ljava/util/concurrent/Concurre...

Looks good to me.
Thank you for improving instances' layouts.

Fred

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

Marked as reviewed by fparain (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/24330#pullrequestreview-2732971561