Integrated: 8340171: CDS: Enhance bitmap truncation

[Aleksey Shipilev]

On Mon, 16 Sep 2024 08:41:56 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> Current CDS archive ships multiple bitmaps. Archival/loading code makes a useful optimization: it truncates the bitmaps at the beginning when it is known the prefix contains many zeroes. The object sorting code additionally sorts the pointer-rich objects at the end of the archive, increasing the zero-prefix length.
> 
> This optimization works less efficiently after [JDK-8338912](https://bugs.openjdk.org/browse/JDK-8338912), which moved the (pointer-rich) roots array at the beginning of the archive. We need to enhance current bitmap handling mechanism to truncate bitmaps better.
> 
> This PR does two things: a) enables bitmap truncation for both leading and trailing zeros; b) sorts the oop-rich objects at the beginning, where the oop-rich roots array is.
> 
> CDS archive sizes:
> 
> 
> # Before JDK-8338912
> $ ls -la build/linux-x86_64-server-release/images/jdk/lib/server/*.jsa
> 15450112 build/linux-x86_64-server-release/images/jdk/lib/server/classes.jsa
> 15851520 build/linux-x86_64-server-release/images/jdk/lib/server/classes_nocoops.jsa
> 
> # After JDK-8338912: +8..12K regression in size
> $ ls -la build/linux-x86_64-server-release/images/jdk/lib/server/*.jsa
> 15462400 build/linux-x86_64-server-release/images/jdk/lib/server/classes.jsa
> 15859712 build/linux-x86_64-server-release/images/jdk/lib/server/classes_nocoops.jsa
> 
> # After this fix: recovered to pre-JDK-8338912 levels
> $ ls -la build/linux-x86_64-server-release/images/jdk/lib/server/*.jsa
> 15450112 build/linux-x86_64-server-release/images/jdk/lib/server/classes.jsa
> 15851520 build/linux-x86_64-server-release/images/jdk/lib/server/classes_nocoops.jsa
> 
> 
> Additional testing:
>  - [x] Linux x86_64 server fastdebug, `runtime/cds`, `tier1`
>  - [x]  Linux x86_64 server fastdebug, `all`

This pull request has now been integrated.

Changeset: f31f97dd
Author:    Aleksey Shipilev <shade at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/f31f97ddb6f1fca1a74761e3e3eeef497f8a7416
Stats:     35 lines in 3 files changed: 7 ins; 8 del; 20 mod

8340171: CDS: Enhance bitmap truncation

Reviewed-by: matsaave, iklam

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PR: https://git.openjdk.org/jdk/pull/21010