RFR: 8327152: NMT: use BitMap for committed memory regions in summary mode

[Afshin Zafari]

On Fri, 1 Mar 2024 22:25:14 GMT, Afshin Zafari <azafari at openjdk.org> wrote:

> In summary mode, we don't show/use the stack traces. So, it is possible to use `BitMap` for sub-regions of a reserved region. A bit is used for every page in the reserved region and is 1 when committed and 0 when uncommitted.
> No need to handle split/merge/exclude and any other similar operations on sub-regions. We just set/clear the bits accordingly.
> 
> To find the actual amount of committed/uncommitted memory, we just count the 1 bits before the operation and adjust the request size appropriately. For example:
> 
>                                               1         2
> Bit index:                          01234567890123456789012
> Current state of a reserved region: 11111000000111110000111
> commit sub-region [8,15):                   ^-----^       
> already-committed = 4
> actual-committed = (15 - 8) - 4 = 3

The GHA error is not related to this PR. It failed on creating sysroot on RISCV.

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PR Comment: https://git.openjdk.org/jdk/pull/18090#issuecomment-1975963291