🐺 A library to curb OOMs by running Go GC according to a user-defined policy.

Package watchdog runs a singleton memory watchdog in the process, which watches memory utilization and forces Go GC in accordance with a user-defined policy.

There three kinds of watchdogs:

1. heap-driven (watchdog.HeapDriven()): applies a heap limit, adjusting GOGC dynamically in accordance with the policy.
2. system-driven (watchdog.SystemDriven()): applies a limit to the total system memory used, obtaining the current usage through elastic/go-sigar.
3. cgroups-driven (watchdog.CgroupDriven()): discovers the memory limit from the cgroup of the process (derived from /proc/self/cgroup), or from the root cgroup path if the PID == 1 (which indicates that the process is running in a container). It uses the cgroup stats to obtain the current usage.

The watchdog's behaviour is controlled by the policy, a pluggable function that determines when to trigger GC based on the current utilization. This library ships with two policies:

1. watermarks policy (watchdog.NewWatermarkPolicy()): runs GC at configured watermarks of memory utilisation.
2. adaptive policy (watchdog.NewAdaptivePolicy()): runs GC when the current usage surpasses a dynamically-set threshold.

You can easily write a custom policy tailored to the allocation patterns of your program.

The recommended way to set up the watchdog is as follows, in descending order of precedence. This logic assumes that the library supports setting a heap limit through an environment variable (e.g. MYAPP_HEAP_MAX) or config key.

1. If heap limit is set and legal, initialize a heap-driven watchdog.
2. Otherwise, try to use the cgroup-driven watchdog. If it succeeds, return.
3. Otherwise, try to initialize a system-driven watchdog. If it succeeds, return.
4. Watchdog initialization failed. Log a warning to inform the user that they're flying solo.

Given the low-level nature of this component, some tests need to run in isolation, so that they don't carry over Go runtime metrics. For completeness, this module uses a Docker image for testing, so we can simulate cgroup memory limits.

The test execution and docker builds have been conveniently packaged in a Makefile. Run with:

$ make

The garbage collector that ships with the go runtime is pretty good in some regards (low-latency, negligible no stop-the-world), but it's insatisfactory in a number of situations that yield ill-fated outcomes:

1. it is incapable of dealing with bursty/spiky allocations efficiently; depending on the workload, the program may OOM as a consequence of not scheduling GC in a timely manner.
2. part of the above is due to the fact that go doesn't concern itself with any limits. To date, it is not possible to set a maximum heap size.
3. its default policy of scheduling GC when the heap doubles, coupled with its ignorance of system or process limits, can easily cause it to OOM.

For more information, check out these GitHub issues:

• golang/go#42805
• golang/go#42430
• golang/go#14735
• golang/go#16843
• golang/go#10064
• golang/go#9849

Dual-licensed: MIT, Apache Software License v2, by way of the Permissive License Stack.