Atlas: diagnose memory pressure and working set overflow

TL;DR: Cache evictions and page faults mean the working set exceeds RAM. Confirm with FTDC-style metrics, then reduce the working set or scale up. Confirm Correlate with query patterns: unindexed queries that scan large ranges churn the cache. Check the slow query list; a few cache-busting shapes are often the whole story.

The error

latency climbs under load, throughput drops, but CPU is not saturated.

The fix

Memory pressure

Symptom: latency climbs under load, throughput drops, but CPU is not saturated.

Diagnose

Atlas Metrics: cache eviction rate, page faults, and resident memory vs the tier's RAM. High evictions with a hot working set means MongoDB keeps throwing useful data out of cache to make room.

Confirm

Correlate with query patterns: unindexed queries that scan large ranges churn the cache. Check the slow query list; a few cache-busting shapes are often the whole story.

Fix, in order

1. Index the cache-busting queries so they touch pages instead of scanning them.
2. Reduce document bloat the hot paths fetch (project only needed fields).
3. Scale the tier up so RAM covers the working set. There is no tuning that substitutes for enough RAM when the hot data genuinely exceeds it.

Verify

Eviction rate drops to near zero under the same load, p95 latency recovers, and the improvement survives a full traffic cycle.

When to use this

• You are seeing this exact error message; match the block above, not just part of it.
• The failing call matches the scenario in the title: Atlas.
• You want the fastest verified fix before digging through logs.

When not to use this

• Your error text differs from the block above; close cousins often have different causes.
• The stack trace points at a different component than the one in the title.
• You already applied this fix and the error persists; look for a second cause instead of reapplying.

Compatibility

• Not pinned to a specific version; follows current Atlas behavior.

Cache and memory metrics per https://www.mongodb.com/docs/atlas/monitor-cluster-metrics/, verified 2026-09-26.