Whippet: A new production embeddable garbage collector

Guile has a new garbage collector coming! The Whippet collector improve
throughput and scalability of Guile programs, and is built in such a way that
it can be swapped into most any language runtime that uses that BDW-GC. With
minimal adaptations in user code, you get a collector that's competitive with
the industry-standard Boehm-Demers-Weiser conservative collector, but which
scales better for multiple allocation threads. As incremental changes are made
in Guile to integrate Whippet, we will also gain the ability to compact the
heap, even while keeping conservative scanning of the C stack. This talk
presents the structure of whippet and some performance numbers for how it
improves Guile program behavior.

The Whippet garbage collector provides an abstract interface that user programs
(like Guile) can use to allocate memory, and also provides a number of
collectors that implement that API. One of them is the BDW-GC collector: this
will be the first step in Guile's switch to Whippet, to change to use the
Whippet API but keep the same GC implementation. Compile-time flags choose the
collector implementation, and in the next step, Guile will switch over to
Whippet's main collector, an Immix-derived mark-region collector. This
collector has a few modes, including a heap-conservative mode that mimics
BDW-GC, as well as a stack-conservative mode that allows evacuation and
compaction of objects that aren't referenced by conservative roots.

The talk will include a quick overview of Immix-style collectors, for context.

Whippet scales: it has been carefully designed to avoid contention during
allocation, and avoid locking except during collection. Collection is
optionally parallel and optionally generational. We'll examine the performance
and practical impacts of these choices, in synthetic test harnesses and in real
Guile programs.

Whippet is designed to be included into a user's C source code, as it needs
compile-time configuration. The talk will show examples of the size of the
collector and its memory efficiency when compared to BDW-GC and to a semispace
collector.

Slides available at https://www.slideshare.net/igalia/whi...

(c) FOSDEM 2023
4 & 5 February 2023
https://fosdem.org/2023/schedule/even...