A Bounded Reclaim Actuator for PSI-Guided Compressed Memory: A Controlled Ablation
TL;DR
The results suggest that the benefit of zram enabled only after PSI indicates memory pressure depends on the foreground workload and its memory-access path, rather than a general improvement across workloads.
Abstract
When the aggregate working set of active processes exceeds physical RAM capacity, the machine experiences memory pressure. Applications may therefore slow down before the kernel kills a process. Linux provides several ways to observe and respond: Pressure Stall Information (PSI) can detect memory-related task stalls, zram can provide compressed in-memory swap space, and cgroup v2 can request memory reclamation within a selected control group. These facilities are often discussed together even though they act at different points in the pressure path. This paper examines that distinction with a controlled systems study. We compare three setups: zram enabled from startup; zram enabled only after PSI indicates memory pressure; and zram enabled from startup with a one-time 96 MiB cgroup reclaim request. We first selected the request size in a 16-case pilot, then ran 180 confirmatory cases, 60 cases for each setup, on nine 1-vCPU Linux virtual machines with compute and SQLite workloads. Compared with static zram, the bounded reclaim configuration reduced compute p99 response time by 6\%, while the SQLite result was statistically indistinguishable. Delayed activation had higher median p99 latency than both alternatives. These results suggest that the benefit depends on the foreground workload and its memory-access path, rather than a general improvement across workloads.