Debugging Copying in Immix Plans

Immix uses opportunitic copying, which means it does not always copy objects. So a bug related with copying may be non-deterministic with Immix plans.

One way to make copying more deterministic is to use the following options to change the copying behavior of Immix.

Option	Default Value	Note
`immix_always_defrag`	`false`	Immix only does defrag GC when necessary. Set to `true` to make every GC a defrag GC
`immix_defrag_every_block`	`false`	Immix only defrags the most heavily fragmented blocks. Set to `true` to make Immix defrag every block with equal chances
`immix_defrag_headroom_percent`	`2`	Immix uses 2% of the heap for defraging. We can reserve more headroom to copy more objects. 50% makes Immix behave like SemiSpace.

A common way to maximumally expose Immix copying bugs is to run with the following values:

#![allow(unused)]
fn main() {
// Set options with MMTkBuilder
builder.options.immix_always_defrag.set(true);
builder.options.immix_defrag_every_block.set(true);
builder.options.immix_defrag_headroom_percent.set(50);
}

These options can also be used along with stress GC options:

#![allow(unused)]
fn main() {
// Do a stress GC for every 10MB allocation
builder.options.stress_factor.set(10485760);
}

Options can also be set using environment variables.

export MMTK_IMMIX_ALWAYS_DEFRAG=true
export MMTK_IMMIX_DEFRAG_EVERY_BLOCK=true
export MMTK_IMMIX_DEFRAG_HEADROOM_PERCENT=50
export MMTK_STRESS_FACTOR=10485760

Keyboard shortcuts

MMTk User Guide

Debugging Copying in Immix Plans