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List:       openjdk-hotspot-runtime-dev
Subject:    Re: RFR: 8301106 allow cds interned strings to be moved by gc [v5]
From:       Ioi Lam <iklam () openjdk ! org>
Date:       2023-03-29 16:58:51
Message-ID: ZxzhjRs1c71SrJw2HDw5qnPMYn9tdqnZXnen38sn3Xc=.e6c97dfe-2fb1-4994-aed6-7b3d7277a00a () github ! com
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> **Background:**
> 
> Currently, the archived java strings are mapped in the G1 "[closed \
> archive](https://github.com/openjdk/jdk/blob/574b48c6925ebfb31345fc46c7d23aa4153f99b0/src/hotspot/share/gc/g1/heapRegionType.hpp#L80-L92)" \
> region. This essentially pins all the strings in memory. 
> As a prerequisite for  \
> ([JDK-8296263](https://bugs.openjdk.org/browse/JDK-8296263)), this PR removes the \
> requirement of pinning the archived strings. This will allow the CDS archive heap \
> to be mapped in garbage collectors that do not support object pinning. 
> **Code changes:**
> 
> - The archived strings are referenced through an objArray (`_shared_strings_array`) \
>                 to keep them alive. As a result, it's no longer necessary to pin \
>                 them.
> - Since it's possible for the GC to move these strings, the `_shared_table` in \
> stringTable.cpp is modified to store a 32-bit index for each archived string. This \
> index is used to retrieve the archived string from `_shared_strings_array` at \
> runtime. 
> Note that CDS has a limit on the size of archived objArrays. When there's a large \
> number of strings, we use a two-level table. See the comments around  \
> `_shared_strings_array` in the header file. 
> **Testing**
> 
> Tiers 1 - 4

Ioi Lam has updated the pull request with a new target base due to a merge or a \
rebase. The incremental webrev excludes the unrelated changes brought in by the \
merge/rebase. The pull request contains 10 additional commits since the last \
revision:

 - fixed merge
 - Merge branch 'master' into 8301106-allow-cds-interned-strings-to-be-moved-by-gc
 - fixed "optimized" build
 - @ashu-mehra and @dholmes-ora review -- better assert and error checking for large \
                objects; added StringTable::verify_secondary_array_index_bits()
 - @dholmes-ora comments
 - renamed obsolete functions to ArchiveHeapLoader::is_in_use()
 - fixed typos in comment
 - cleanup2
 - cleanup
 - 8301106: Allow archived Java strings to be moved by the collector

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/12607/files
  - new: https://git.openjdk.org/jdk/pull/12607/files/1ea3dde0..934d67f7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12607&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12607&range=03-04

  Stats: 375841 lines in 3206 files changed: 225901 ins; 124148 del; 25792 mod
  Patch: https://git.openjdk.org/jdk/pull/12607.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/12607/head:pull/12607

PR: https://git.openjdk.org/jdk/pull/12607


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