ruby requests more memory when there are plenty free heap slots

Question

We have a server running

• Sidekiq 4.2.9
• rails 4.2.8
• MRI 2.1.9

This server periodically produce some amount of importing from external API's, perform some calculations on them and save these values to the database.

About 3 weeks ago server started hanging, as I see from NewRelic (and when ssh'ed to it) - it consumes more and more memory over time, eventually occupying all available RAM, then server hangs.

I've read some articles about how ruby GC works, but still can't understand, why at ~5:30 AM heap size jumps from ~2.3M to 3M , when there's still 1M free heap slots available(GC settings are default)

similar behavior, 3:35PM:

So, the questions are:

1. how to make Ruby fill free heap slots instead of requesting new slots from OS ?
2. how to make it release free heap slots to the system ?

Answer 1

how to make Ruby fill free heap slots instead of requesting new slots from OS ?

Your graph does not have "full" fidelity. It is a lot to assume that GC.stat was called by Newrelic or whatnot just at the exact right time.

It is incredibly likely that you ran out of slots, heap grew and since heaps don't shrink in Ruby you are stuck with a somewhat bloated heap.

To alleviate some of the pain you can limit RUBY_GC_HEAP_GROWTH_MAX_SLOTS to a sane number, something like 100,000 will do, I am trying to lobby setting a default here in core.

Also

• Create a persistent log of jobs that run and time they ran (duration and so on), gather GC.stat before and after job runs

• Split up your jobs by queue, run 1 queue on one server and other queue on another one, see which queue and which job is responsible for the problem

• Profile various jobs you have using flamegraph or other profiling tools

• Reduce the amount of concurrent jobs you run as an experiment, or place a mutex between certain job types. It is possible that 1 "job a" at a time is OKish, and 20 concurrent "job a"s at a time will bloat memory.