RFR: 8343789: Move mutable nmethod data out of CodeCache

[Dean Long]

On Fri, 22 Nov 2024 02:40:38 GMT, Dean Long <dlong at openjdk.org> wrote:

>> This change relocates mutable data (such as relocations, oops, and metadata) from the nmethod. The change follows the recent PR #18984, which relocated immutable nmethod data from the CodeCache.
>> 
>> The core idea remains the same: use the CodeCache for executable code while moving additional data to the C heap. The primary motivations are improving security and enhancing code density.
>> 
>> Although performance is not the main focus, testing on AArch64 CPUs, where code density plays a significant role, has shown a 1–2% performance improvement in specific scenarios, such as the CodeCacheStress test and the Renaissance Dotty benchmark.
>> 
>> The numbers. Immutable data constitutes **~30%** on the nmehtod. Mutable data constitutes **~8%** of nmethod. Example (statistics collected on the CodeCacheStress benchmark):
>> - nmethod_count:134000, total_compilation_time: 510460ms
>> - total allocation time malloc_mutable/malloc_immutable/CodeCache_alloc: 62ms/114ms/6333ms,
>> - total allocation size (mutable/immutable/nmentod): 64MB/192MB/488MB
>> 
>> Functional testing: jtreg on arm/aarch/x86.
>> Performance testing: renaissance/dacapo/SPECjvm2008 benchmarks.
>> 
>> Alternative solution (see comments): In the future, relocations can be moved to _immutable_data.
>
> src/hotspot/share/code/codeBlob.cpp line 103:
> 
>> 101:   // The mutable_data_size is either calculated by the nmethod constructor to account
>> 102:   // for reloc_info and additional data, or it is set here to accommodate only the relocation data.
>> 103:   _mutable_data_size = (mutable_data_size == 0) ? cb->total_relocation_size() : mutable_data_size;
> 
> This seems strange to treat relocations as special.  Wouldn't it be better to have the caller always pass in the correct value?

Or compute using something like required_mutable_data_space()?

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PR Review Comment: https://git.openjdk.org/jdk/pull/21276#discussion_r1853215761