RFR: JDK-8307314: Implementation: Generational Shenandoah (Experimental) [v4]

[Y. Srinivas Ramakrishna]

On Fri, 2 Jun 2023 02:49:25 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> OpenJDK Colleagues:
>> 
>> Please review this proposed integration of Generational mode for Shenandoah GC under https://bugs.openjdk.org/browse/JDK-8307314.
>> 
>> Generational mode of Shenandoah is enabled by adding `-XX:+UnlockExperimentalVMOptions -XX:ShenandoahGCMode=generational` to a command line that already specifies ` -XX:+UseShenandoahGC`.  The implementation automatically adjusts the sizes of old generation and young generation to efficiently utilize the entire heap capacity.  Generational mode of Shenandoah resembles G1 in the following regards:
>> 
>> 1. Old-generation marking runs concurrently during the time that multiple young generation collections run to completion.
>> 2. After old-generation marking completes, we perform a sequence of mixed collections.  Each mixed collection combines collection of young generation with evacuation of a portion of the old-generation regions identified for collection based on old-generation marking information.
>> 3. Unlike G1, young-generation collections and evacuations are entirely concurrent, as with single-generation Shenandoah.
>> 4. As with single-generation Shenandoah, there is no explicit notion of eden and survivor space within the young generation.  In practice, regions that were most recently allocated tend to have large amounts of garbage and these regions tend to be collected with very little effort.  Young-generation objects that survive garbage collection tend to accumulate in regions that hold survivor objects.  These regions tend to have smaller amounts of garbage, and are less likely to be collected.  If they survive a sufficient number of young-generation collections, the “survivor” regions are promoted into the old generation.
>> 
>> We expect to refine heuristics as we gain experience with more production workloads.  In the future, we plan to remove the “experimental” qualifier from generational mode, at which time we expect that generational mode will become the default mode for Shenandoah.
>> 
>> **Testing**: We continuously run jtreg tiers 1-4 + hotspot_gc_shenandoah, gcstress, jck compiler, jck runtime, Dacapo, SpecJBB, SpecVM, Extremem, HyperAlloc, and multiple AWS production workload simulators. We test on Linux x64 and aarch64, Alpine x64 and aarch64, macOS x64 and aarch64, and Windows x64.
>
> Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Force PLAB sizes to align on card-table size

src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp line 1285:

> 1283:   if (unalignment != 0) {
> 1284:     word_size = word_size - unalignment + CardTable::card_size_in_words();
> 1285:   }

Probably not a big deal since this is only used when refilling a PLAB, which is an infrequent operation, but `mod` is an expensive operation, in general, and best to avoid in our code except in assertion checks (or even there given recent experiences with debug tests timing out). Since card size is a power of 2, may be we could use addition and masking instead. Something like defining the following inline in the CardTable class and using it everywhere where card alignment granularity is sought. There may even be a macro or method defined for this already perhaps:


(FOO + CardSize - 1) & ~((1 << LogCardSize) - 1)


One could even store the mask to avoid the arithmetic to produce the mask although it's pretty cheap.

This may turn out to be less expensive than mod, test, and branch, but as I said probably not a big deal here. We should make sure we don't overuse mods in our allocation paths much.

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PR Review Comment: https://git.openjdk.org/jdk/pull/14185#discussion_r1214658808