Enum mmtk::util::metadata::vo_bit::helper::VOBitUpdateStrategy

enum VOBitUpdateStrategy {
    ClearAndReconstruct,
    CopyFromMarkBits,
}

The strategy to update the valid object (VO) bits.

Each stategy has its strength and limitation. We should choose a strategy according to the configuration of the VM binding.

Current experiments show that the CopyFromMarkBits strategy is faster while also makes the VO bits available during tracing. We also include the ClearAndReconstruct strategy because

1. It was the strategy described in the original paper that described the algorithm for filtering roots using VO bits for stack-conservative GC. See: Fast Conservative Garbage Collection published in OOPSLA’14 https://dl.acm.org/doi/10.1145/2660193.2660198
2. It does not require mark bits to be on the side. It will be needed if we implement in-header mark bits in the future.

Variants

ClearAndReconstruct

Clear all VO bits after stacks are scanned, and reconstruct the VO bits during tracing.

Pros:

• Proven to work by published paper.

Cons:

• VO bits are not available during tracing.

CopyFromMarkBits

Copy the mark bits metadata over to the VO bits metadata after tracing.

Pros:

• VO bits are available during tracing.
• Faster according to current experiment.

Cons:

• Requires marking bits to be on the side.

Implementations

impl VOBitUpdateStrategy

pub fn vo_bit_available_during_tracing(&self) -> bool

Return true if the VO bit metadata is available during tracing.

Trait Implementations

impl Debug for VOBitUpdateStrategy

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.