Memory compression¶

Compression can reduce the cost of retaining cold RAM when encoding savings outweigh storage, CPU, and recovery costs. Choose local ownership first; add a pool only when sharing across instances justifies coordination.

Target and current status¶

Per-instance compression is delivered experimentally on Linux x86_64 through the default-off vm.cold_ram_compression / --vm-cold-ram-compression: a runtime-owned userfaultfd pager with bounded local storage, no FUSE and no external pool. It is distinct from vm.ram_compression (FUSE file backing), and is eviction/refault probing rather than a read-access heat detector. Neither it nor pooled-server compression is a production-benefit claim. The macOS/HVF heap pool remains experimental and can fail dependent VMs when its service is lost; client-owned immutable backing is still a proposal.

Ownership and data flow¶

Each instance owns checkpoint save, compression, and restore independently of runtime optimization. A complete environment snapshot includes machine state and durability; runtime cold objects retain only current RAM fragments. A compressed checkpoint does not automatically reclaim live RAM.

The VM runtime owns cold-page selection and mapping changes; the codec transforms bytes, and backing carries them. memfd does not compress automatically. The target restores writable pages into instance-private RAM, even with shared objects.

Choices and tradeoffs¶

Local compression avoids service coordination but cannot share payloads across instances. A pool adds optional deduplication, shared storage, and host-wide budgets, at the cost of IPC, contention, and broader failure risks. Both need temporary-memory and recovery headroom, CPU budgets, and acceptable tails.

Direction and evidence¶

The Linux local pager publishes and validates objects before reclaiming RAM, then validates restoration through UFFD_COPY. Kernel-fault userfaultfd authority is required; compiled support does not grant permission and missing authority fails startup. It rejects dedup, file/FUSE backing, snapshot capture/restore, whole-VM offload, and external memory pools; see the local contract. Linux sealed memfd pool delivery remains proposed; a macOS sealed equivalent is not delivered. Start from the optimization architecture; memory evidence establishes no known production net gains or verified net physical-memory savings.