Heap Size Calculator
Calculate recommended heap size for a process based on workload and object allocation rate.
Inputs
Maximum resident heap objects at peak load
Multiplier for allocator fragmentation (typically 1.2-2.0)
Extra space for GC working room or allocation bursts
Recommended Heap Size
951MB
Recommended Heap Size
0.93GB
Fragmentation Overhead
153.6MB
GC/Allocator Headroom
285.3MB
Step by step
Peak live objects
512 MB
= 512 MB
With fragmentation
512 × 1.3
= 665.6 MB
Recommended heap (with headroom)
665.6 / (1 - 0.3)
= 951 MB (0.93 GB)
How it works
The recommended heap size must exceed the peak live object set by enough margin to accommodate allocator fragmentation (wasted space between allocated blocks) and garbage collector or allocator headroom (space needed for concurrent allocation during GC pauses or to avoid frequent compaction). A common rule of thumb is heap = peak_live × fragmentation_factor / (1 - headroom_fraction), targeting 2-3x the peak live set.
Formula
Recommended heap
heap = (peak_live × fragmentation_factor) / (1 - headroom%)
- L
- peak live object size
- f
- fragmentation factor
- h
- headroom fraction
Frequently Asked Questions
What is a good fragmentation factor for malloc?
For glibc malloc under typical workloads, fragmentation is usually 1.2-1.5x. For languages with GC (Java, Go), the GC compacts the heap periodically so fragmentation is lower, but you need more headroom for GC working space.
What happens if the heap is too small?
Frequent garbage collections (GC thrashing) in managed languages, or failed allocations (OOM) in C/C++. The process spends increasing time in memory management rather than useful work, severely degrading throughput.