Which RUBY_GC environment variables best minimize RSS for low-traffic workloads?

Recommended RUBY_GC settings for low‑traffic workloads

export RUBY_GC_HEAP_INIT_SLOTS=5000
export RUBY_GC_HEAP_FREE_SLOTS=2500
export RUBY_GC_HEAP_GROWTH_FACTOR=1.1
export RUBY_GC_MALLOC_LIMIT=4194304   # 4 MiB
export RUBY_GC_OLDMALLOC_LIMIT=2097152 # 2 MiB

Confirmed facts

• Ruby’s heap size is driven by RUBY_GC_HEAP_INIT_SLOTS, RUBY_GC_HEAP_FREE_SLOTS and RUBY_GC_HEAP_GROWTH_FACTOR; lowering these reduces the memory reserved for the heap and thus RSS.
• Allocation thresholds RUBY_GC_MALLOC_LIMIT and RUBY_GC_OLDMALLOC_LIMIT trigger minor and old‑space GC; decreasing them causes more frequent, lightweight collections that keep live data small.

Likely explanation

For workloads that allocate few objects, a small initial heap (≈5 000 slots) and a conservative growth factor (≈1.1) stop the heap from over‑allocating, while low malloc limits (≈4 MB malloc, ≈2 MB old‑malloc) provoke frequent, short GC cycles that reclaim memory quickly, shrinking the resident set size.

Steps to apply and verify

1. Set the environment variables as shown above before launching the Ruby process.
2. Run the application under a representative low‑traffic load (e.g., a few requests per minute).
3. Measure RSS with ps -o rss -p <pid> or /proc/<pid>/statm and record the value.
4. Check GC statistics, for example via GC.stat or GC::Profiler, to ensure GC runs more often but total GC time stays below an acceptable threshold (e.g., <5 % CPU).
5. If RSS does not drop as expected, gradually increase the malloc limits or growth factor and re‑measure.

Missing diagnostic detail

To confirm that these values are appropriate for your specific case, please provide the observed allocation rate or baseline RSS of the application under low‑traffic load; if the app allocates in bursts, higher limits may be needed to avoid excessive GC overhead.