RFR: 8272773: Investigate making card table size configurable

[Thomas Schatzl]

On Thu, 7 Oct 2021 11:04:09 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> Hi,
>> 
>> Please review the changes to make CardTable entry size configurable. The changes primarily consists of:
>> 
>> 1. Addition of a cmdline flag **GCCardSizeInBytes** to make the card size startup time configurable.
>> 2. Setting the card size based on the flag in G1, Parallel and Serial GC memory initialization paths.
>> 3. Setting BlockOffsetTable size and ObjectStartArray size based on the card size.
>
> src/hotspot/share/gc/g1/heapRegion.cpp line 96:
> 
>> 94:   // Initialize card size based on the region size.
>> 95:   // Maximum no. of cards per region is 2^16.
>> 96:   CardTable::initialize_card_size(1 << (region_size_log - 16));
> 
> Please make a constant out of the `16`; the reason is the size of the `G1CardSetArray::EntryDataType` being 16 bit.

Another question we need to answer is whether smaller card size than 512 gains anything - if not, I recommend keeping to use 512 at minimum, as otherwise the card table will obviously use much more memory for no gain.

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PR: https://git.openjdk.java.net/jdk/pull/5838