RFR: 8319373: Serial: Refactor dirty cards scanning during Young GC [v8]

[Kim Barrett]

On Fri, 10 Nov 2023 13:33:12 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> Reading the new code directly is probably easier. The structure is quite similar to its counterpart in Parallel.
>> 
>> It's mostly perf-neutral, except when dirty cards are scarce. Using `card_scan.java` in [JDK-8310031](https://bugs.openjdk.org/browse/JDK-8310031), I observed ~40% reduction in young-gc pause time with `stride = 32 * 64`.
>
> Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   next-obj-card

Changes requested by kbarrett (Reviewer).

src/hotspot/share/gc/serial/cardTableRS.cpp line 354:

> 352: 
> 353:   static_assert(clean_card_val() == (CardValue)-1, "inv");
> 354:   constexpr Word clean_word = (Word)-1;

CardTable provides `clean_card_row_val()` for this sort of thing.

src/hotspot/share/gc/serial/cardTableRS.cpp line 370:

> 368:   // Word comparison
> 369:   while (i_card + sizeof(Word) <= end_card) {
> 370:     Word* i_word = reinterpret_cast<Word*>(i_card);

Maybe CardTable should provide a helper for accessing a "row" of cards?

src/hotspot/share/gc/serial/cardTableRS.cpp line 372:

> 370:     Word* i_word = reinterpret_cast<Word*>(i_card);
> 371:     if (*i_word != clean_word) {
> 372:       // Found a clean card in this word; fall back to per-CardValue comparison.

s/clean/non-clean/ or s/clean/dirty/.

src/hotspot/share/gc/serial/cardTableRS.cpp line 490:

> 488:     if (dirty_l == end_card) {
> 489:       // No dirty cards to iterate.
> 490:       break;

Maybe s/break/return/ ?

src/hotspot/share/gc/serial/cardTableRS.hpp line 56:

> 54:   static void clear_cards(CardValue* start, CardValue* end);
> 55: 
> 56:   static CardTable::CardValue* find_first_dirty_card(CardValue* const start_card,

CardTable qualification for return type isn't needed here, or a couple lines later.

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PR Review: https://git.openjdk.org/jdk/pull/16492#pullrequestreview-1735202108
PR Review Comment: https://git.openjdk.org/jdk/pull/16492#discussion_r1396202686
PR Review Comment: https://git.openjdk.org/jdk/pull/16492#discussion_r1396204908
PR Review Comment: https://git.openjdk.org/jdk/pull/16492#discussion_r1396198763
PR Review Comment: https://git.openjdk.org/jdk/pull/16492#discussion_r1396209000
PR Review Comment: https://git.openjdk.org/jdk/pull/16492#discussion_r1396189076