RFR: 8305896: Alternative full GC forwarding [v42]

[Aleksey Shipilev]

On Thu, 11 May 2023 12:30:08 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> Currently, the full-GC modes of Serial, Shenandoah and G1 GCs are forwarding objects by over-writing the object header with the new object location. Unfortunately, for compact object headers ([JDK-8294992](https://bugs.openjdk.org/browse/JDK-8294992)) this would not work, because the crucial class information is also stored in the header, and we could no longer iterate over objects until the headers would be restored. Also, the preserved-headers tables would grow quite large.
>> 
>> I propose to use an alternative algorithm for full-GC (sliding-GC) forwarding that uses a special encoding so that the forwarding information fits into the lowest 32 bits of the header.
>> 
>> It exploits the insight that, with sliding GCs, objects from one region will only ever be forwarded to one of two possible target regions. For this to work, we need to divide the heap into equal-sized regions. This is already the case for Shenandoah and G1, and can easily be overlaid for Serial GC, by assuming either the whole heap as a single region (if it fits) or by using SpaceAlignment-sized virtual regions.
>> 
>> We also build and maintain a table that has N elements, where N is the number of regions. Each entry is two addresses, which are the start-address of the possible target regions for each source region.
>> 
>> With this, forwarding information would be encoded like this:
>>  - Bits 0 and 1: same as before, we put in '11' to indicate that the object is forwarded.
>>  - Bit 2: Used for 'fallback'-forwarding (see below)
>>  - Bit 3: Selects the target region 0 or 1. Look up the base address in the table (see above)
>>  - Bits 4..31 The number of heap words from the target base address
>> 
>> This works well for all sliding GCs in Serial, G1 and Shenandoah. The exception is in G1, there is a special mode called 'serial compaction' which acts as a last-last-ditch effort to squeeze more space out of the heap by re-forwarding the tails of the compaction chains. Unfortunately, this breaks the assumption of the sliding-forwarding-table. When that happens, we initialize a fallback table, which is a simple open hash-table, and set the Bit 2 in the forwarding to indicate that we shall look up the forwardee in the fallback-table.
>> 
>> All the table accesses can be done unsynchronized because:
>> - Serial GC is single-threaded anyway
>> - In G1 and Shenandoah, GC worker threads divide up the work such that each worker does disjoint sets of regions.
>> - G1 serial compaction is single-threaded
>> 
>> The c...
>
> Roman Kennke has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Some more @shipilev comments
>  - Update src/hotspot/share/gc/shared/slidingForwarding.hpp
>    
>    Co-authored-by: Aleksey Shipilëv <shipilev at amazon.de>

The updates look fine, thanks.

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Marked as reviewed by shade (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/13582#pullrequestreview-1422790198