Checking whether more cache memory would fix anything before buying it
Correcting the client hashing mismatch rebalanced the ring and lifted hit rate substantially with no new hardware. The planned capacity doubling was cancelled, and a modest increase targeted at one ke…
Overview
A SaaS provider was about to double their Memcached footprint to improve a disappointing hit rate. AceMQ assessed the tier first to determine whether capacity was actually the constraint.
Challenge
Aggregate hit rate looked poor, which made more memory the obvious answer. But the aggregate hid two separate problems. A meaningful share of gets were for keys that are only ever read once — cache misses by design, which no amount of memory fixes. And key distribution across the ring was badly uneven because a hashing configuration difference between two client libraries meant some nodes were near capacity and evicting while others sat half empty.
Environment
Memcached tier on AWS accessed by services in two languages using different client libraries.
Approach
The assessment breaks hit rate down by key pattern instead of treating it as a single number, and measures actual key and memory distribution per node. That separates the genuinely uncacheable traffic from the workload that would benefit from more memory, and exposes distribution problems that adding nodes would not fix.
Solution
- 1Hit rate decomposed by key prefix and access pattern, separating single-read keys from genuinely reusable ones
- 2Per-node key count and memory distribution measured to expose ring imbalance
- 3Client library hashing configuration compared across both languages, identifying the mismatch causing the imbalance
- 4Working set size estimated per key class so capacity requirements rest on measurement rather than a doubling guess
- 5Uncacheable access patterns identified and returned to the application teams as candidates for removal from the cache path
- 6Sizing recommendation with expected hit rate outcome per option, including the option of adding no capacity at all
Outcome
Correcting the client hashing mismatch rebalanced the ring and lifted hit rate substantially with no new hardware. The planned capacity doubling was cancelled, and a modest increase targeted at one key class was scheduled instead.
Technologies
Related Use Cases
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