Misc #19052: Increased memory usage (RSS) for Ruby when compiled by gcc

• Status: Open
• Assignee: -

[bosh-ecosystem-vmware (BOSH Ecosystem Team @ VMware)]

Hello! We have seen a large increase in memory usage as measured by resident set size (RSS) in our Ruby 3.1.2 programs when testing on Ubuntu Jammy (22.04). After further testing, we have narrowed down the problem as starting with Ubuntu Eoan (19.10). We first noticed this increase coming from a gem called eventmachine v1.2.7, which is a concurrency library that uses threads internally. That led us to try threads directly, and we came up with a simple Ruby program that reproduces the memory usage issue by spawning multiple threads through Thread.new.

We are unsure whether the problem extends beyond just Thread.new; it was the easiest problem to spot within our applications.

We are building Ruby from source using ruby-install, though have seen the problem in versions installed via apt.

We are seeking to understand the underlying cause of this memory bloat, and what we can do to prevent it.

Generally, we've been seeing similar total virtual memory allocated, only a difference in RSS usage.

On Ubuntu Disco (19.04) and prior, we see a relatively small increase in RSS upon spawning the threads, on the order of 20 kilobytes per thread. On Ubuntu Eoan and later, we see that RSS increases by 1 megabyte per thread.

When Ruby is compiled using clang instead of gcc on Ubuntu Eoan and later, the memory usage is comparable to older versions of Ubuntu.

We do not believe it is related to gcc version by itself, as we tried compiling Ruby using gcc 9 on Ubuntu Disco, and did not encounter the memory problem. On the theory that it may be a memory fragmentation issue, we tried both using the MALLOC_ARENA_MAX runtime environment variable, and compiling Ruby against jemalloc. Neither of these reduced RSS usage.

Here are the memory differences when simply calling Thread.new 10 times:

Configuration	RSS before spawning threads	RSS after spawning threads	Diff
Disco + gcc 8	13596 kB	13800 kB	204 kB
Disco + gcc 9	13880 kB	14088 kB	208 kB
Eoan + gcc 9	22268 kB	32672 kB	10404 kB
Eoan + clang	13976 kB	14180 kB	204 kB
Jammy + gcc 11	22208 kB	32608 kB	10400 kB
Jammy + clang	13668 kB	13872 kB	204 kB
Jammy + jemalloc	24212 kB	35216 kB	11004 kB
Jammy + apt	21308 kB	31712 kB	10404 kB

We have attached the program and a series of Dockerfiles that replicate the test environments. While the reproduction is using Docker files, we have seen this same behavior on actual VMs. To run a particular Docker config, use ./run.sh <DIRECTORY>, e.g. ./run.sh jammy-clang.

Does anyone understand why there is an increase in resident set size (RSS) when Ruby is compiled with gcc vs clang?
Is there additional configuration (flags, etc) that could be sent to gcc to get equivalent memory usage on Jammy that is seen when using Disco or clang?

[jeremyevans0 (Jeremy Evans)]

• Tracker changed from Bug to Misc
• ruby -v deleted (3.1.2)
• Backport deleted (2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN)