RFR: 8311248: Refactor CodeCache::initialize_heaps to simplify adding new CodeCache segments [v7]

[Evgeny Astigeevich]

On Tue, 20 Feb 2024 12:54:59 GMT, Boris Ulasevich <bulasevich at openjdk.org> wrote:

>> src/hotspot/share/code/codeCache.cpp line 312:
>> 
>>> 310:   // last adjustment: leftovers from page alignment go to non_nmethod segment
>>> 311:   non_nmethod.size += non_profiled.size & alignment_mask(min_size);
>>> 312:   non_nmethod.size += profiled.size & alignment_mask(min_size);
>> 
>> Why do we need this code?
>
> Let us assume that with the previous calculations we have this layout:
> - {cache_size=18004KB, non_profiled.size = 6002KB, profiled.size = 6002KB, non_nmethod.size=6000KB}
> 
> With next align_down for profiled and non_profiled segments, we get:
> - {cache_size=18004KB, non_profiled.size = 6000KB, profiled.size = 6000KB, non_nmethod.size=6000KB}
> 
> My intention here is to put 4KB into the non_nmethod segment to make the sum of the segment sizes equal to cache_size:
> - {cache_size=18004KB, non_profiled.size = 6000KB, profiled.size = 6000KB, non_nmethod.size=6004KB}

Why has `non_nmethod` been chosen? I have not seen it running out of space. It usually has a lot of free space. Also in case of large pages, e.g. 2M, it can get even more space than needed.
I'd rather give additional space to `profiled` or `non_profiled`. As `profiled` is optional, to simplify the code we can have:

  non_nmethod.size = align_up(non_nmethod.size, min_size);
  profiled.size = align_down(profiled.size, min_size);
  profiled.size = align_down(cache_size - profiled.size - non_nmethod.size, min_size);


For you example, we will have:

{cache_size=18004KB, non_profiled.size = 6004KB, profiled.size = 6000KB, non_nmethod.size=6000KB}

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PR Review Comment: https://git.openjdk.org/jdk/pull/17244#discussion_r1496663761