Only enable the memory controller in cgroup subtree_control instead of
all available controllers. cpu.stat is available in cgroup v2 without
enabling the cpu controller (base accounting), and enabling io/pids/cpu
controllers adds unnecessary per-operation kernel overhead to Scylla
processes - particularly the memory controller's per-page-cache-operation
accounting combined with io controller overhead during heavy I/O.
Additionally, restrict SystemResourceMonitor to the master process only.
System-wide metrics (CPU%, memory) are identical from any process, so
running a monitoring thread in each xdist worker was redundant and added
unnecessary SQLite write contention and thread scheduling noise.
Co-Authored-By: Claude Opus 4.6 (200K context) <noreply@anthropic.com>
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