RFR: 8251330: Reorder CDS archived heap to speed up relocation [v7]

[Calvin Cheung]

On Tue, 12 Mar 2024 16:26:28 GMT, Matias Saavedra Silva <matsaave at openjdk.org> wrote:

>> We should reorder the archived heap to segregate the objects that don't need marking. This will save space in the archive and improve start-up time
>> 
>> This patch reorders the archived heap to segregate the objects that don't need marking. The leading zeros present in the bitmaps thanks to the reordering can be easily removed, and this the size of the archive is reduced.
>> 
>> Given that the bitmaps are word aligned, some leading zeroes will remain. Verified with tier 1-5 tests.
>> 
>> Metrics, courtesy of @iklam :
>> ```calculate_oopmap: objects = 15262 (507904 bytes, 332752 bytes don't need marking), embedded oops = 8408, nulls = 54
>> Oopmap = 15872 bytes
>> 
>> calculate_oopmap: objects = 4590 (335872 bytes, 178120 bytes don't need marking), embedded oops = 46487, nulls = 29019
>> Oopmap = 10496 bytes
>> 
>> (332752 + 178120) / (507904 + 335872.0) = 0.6054592688106796 
>> More than 60% of the space used by the archived heap doesn't need to be marked by the oopmap. 
>> 
>> $ java -Xshare:dump -Xlog:cds+map | grep lead
>> [3.777s][info][cds,map] - heap_oopmap_leading_zeros: 143286
>> [3.777s][info][cds,map] - heap_ptrmap_leading_zeros: 50713
>> 
>> So we can reduce the "bm" region by (143286 + 50713) / 8 = 24249 bytes.
>> 
>> Current output:
>> $ java -XX:+UseNewCode -Xshare:dump -Xlog:cds+map | grep lead 
>> [5.339s][info][cds,map] - heap_oopmap_leading_zeros:      26
>> [5.339s][info][cds,map] - heap_ptrmap_leading_zeros:      8
>
> Matias Saavedra Silva has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Nits

Looks good. I just have a minor comment below.
Also, the copyright year of some files need to be updated.

src/hotspot/share/cds/filemap.cpp line 1581:

> 1579: 
> 1580:   // We want to keep track of how many zeros were removed
> 1581:   size_t new_zeros = map->find_first_set_bit(0);

I think this statement could be enclosed with `DEBUG_ONLY` since the `new_zeros` is only used in the `assert `statement following it.

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Marked as reviewed by ccheung (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/17350#pullrequestreview-1931842261
PR Review Comment: https://git.openjdk.org/jdk/pull/17350#discussion_r1521889703