Drive gradual reclaim by memory pressure instead of CPU idle#40778
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benhillis wants to merge 2 commits into
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Drive gradual reclaim by memory pressure instead of CPU idle#40778benhillis wants to merge 2 commits into
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Pull request overview
Updates the WSL2 guest mini-init memory reduction thread so Gradual cache reclaim is driven by memory pressure (PSI) rather than CPU idleness, enabling cold page-cache reclaim even during CPU-bound workloads while backing off under pressure.
Changes:
- Add procfs helpers to read full procfs snapshots and to compute aggregate busy vs idle CPU time from
/proc/stat. - Implement PSI-based gating for gradual reclaim using
/proc/pressure/memory(some avg10), with bounded reclaim steps while busy and full drain while idle. - Refactor the reduction thread into clear per-tick helpers for Gradual reclaim, DropCache, and Compaction, with best-effort procfs writes.
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| const int Result = TEMP_FAILURE_RETRY(read(Fd.get(), Buffer + Total, (Size - 1) - Total)); | ||
| if (Result < 0) | ||
| { | ||
| if (LogErrors) | ||
| { | ||
| LOG_ERROR("read({}) failed {}", Path, errno); | ||
| } | ||
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| return false; | ||
| } | ||
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| if (Result == 0) | ||
| { | ||
| break; | ||
| } | ||
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| Total += Result; |
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| // Best-effort: WriteToFile logs internally on failure. EAGAIN merely means the kernel could not evict | ||
| // the full amount this pass (pages were still freed), so it counts as reclaim. Never throw on a | ||
| // transient write error and tear down the long-lived reduction thread. | ||
| const int Status = WriteToFile(RECLAIM_PATH, Bytes.c_str()); | ||
| return (Status == 0) || (errno == EAGAIN); |
Replace the ring-buffer idle detector and user-CPU-only sampling in the mini-init memory reduction thread with a clearer, helper-based design: - Sample aggregate non-idle CPU time (user, system, irq, softirq, steal) so kernel-bound work keeps the VM out of the idle state, instead of looking at user time alone. - ReadProcFile reads a full procfs snapshot into a caller buffer (close-on-exec, partial-read safe); GetReclaimableCacheBytes / GetFreeMemoryBytes read the relevant counters through it. - Gradual mode reclaims cold page cache (cgroup memory.reclaim) above a fixed floor while CPU-idle, with a hysteresis margin so it does not churn near the floor. - DropCache mode stays gated on sustained CPU idle, drops once, and re-drops only after the reclaimable cache grows meaningfully. - Compaction is gated on free-memory growth so it runs only when there are newly-freed pages worth coalescing. RequestCgroupReclaim performs the memory.reclaim write best-effort: it treats the kernel's expected EAGAIN (some, but not all, pages evicted) as success without logging, and never throws so a transient write error cannot tear down the long-lived reduction thread. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
A CPU-bound workload can sit on gigabytes of cold page cache that a CPU-idle check would never reclaim. Read the PSI "some avg10" memory pressure from /proc/pressure/memory and reclaim cold cache toward the fixed floor whenever pressure is low, even while the VM is busy, backing off once the workload starts stalling on memory. A busy interval reclaims at most a bounded step (c_gradualStepBusyBytes) so a large backlog is drained gently; an idle interval drains the full excess at once. When PSI is unavailable (kernel built without CONFIG_PSI), gradual reclaim falls back to gating on CPU idle. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Closing for now. These changes are being submitted one PR at a time; this will be reopened against master once the preceding PR in the series has merged, so it shows only its own incremental change. The branch remains pushed. |
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Stacked PR 2 of 4 — builds on #40777. Diff shown is cumulative (helpers rework + this change).
What this does
A CPU-bound workload can sit on gigabytes of cold page cache that a CPU-idle check would never reclaim. This drives Gradual reclaim by memory pressure instead of CPU idleness:
some avg10memory pressure from/proc/pressure/memory.c_gradualStepBusyBytes) so a large backlog is drained gently; an idle interval drains the full excess.CONFIG_PSI), gradual reclaim falls back to gating on CPU idle.Stack