Allocation profiling

Instead of detecting CPU-consuming code, the profiler can be configured to collect call sites where the largest amount of heap memory is allocated.

async-profiler does not use intrusive techniques like bytecode instrumentation or expensive DTrace probes which have significant performance impact. It also does not affect Escape Analysis or prevent from JIT optimizations like allocation elimination. Only actual heap allocations are measured.

The profiler features TLAB-driven sampling. It relies on HotSpot-specific callbacks to receive two kinds of notifications:

• when an object is allocated in a newly created TLAB (aqua frames in a Flame Graph);
• when an object is allocated on a slow path outside TLAB (brown frames).

This means not each allocation is counted, but only allocations every N kB, where N is the average size of TLAB. This makes heap sampling very cheap and suitable for production. On the other hand, the collected data may be incomplete, though in practice it will often reflect the top allocation sources.

Sampling interval can be adjusted with --alloc option. For example, --alloc 500k will take one sample after 500 KB of allocated space on average. However, intervals less than TLAB size will not take effect.

The minimum supported JDK version is 7u40 where the TLAB callbacks appeared.