What am I supposed to do about glibc rseq errors?

[tyteen4a03]

Seeing these errors on startup:

Your system has glibc support for rseq built in, which is not yet supported by tcmalloc-google and has critical performance implications. Please set the environment variable GLIBC_TUNABLES=glibc.pthread.rseq=0

What am I supposed to do about these? Shouldn't the Docker image set those by default?

[tianon]

Do you have a (minimal) reproducer?

[tianon]

Ah, just running with no arguments does the trick:

{"t":{"$date":"2025-03-13T22:25:44.642+00:00"},"s":"W",  "c":"CONTROL",  "id":8718500, "ctx":"initandlisten","msg":"Your system has glibc support for rseq built in, which is not yet supported by tcmalloc-google and has critical performance implications. Please set the environment variable GLIBC_TUNABLES=glibc.pthread.rseq=0","tags":["startupWarnings"]}

[tianon]

I found https://www.mongodb.com/community/forums/t/unclear-server-warnings-with-default-docker-image-of-mongo-8/301337, but I haven't seen anyone report any concrete issues from this warning, so it feels a little bit overaggressively worded, but it does seem like setting that is probably reasonable.

I'm worried we won't know/notice when we shouldn't be setting it anymore, so it would definitely be helpful to figure out what it means, what the implications are, and how we can know when it's safe to stop setting this so we can document it all (both for ourselves and for users).

[yosifkit]

This warning is irrelevant until a user sets tcmalloc.usingPerCPUCaches to true and that tcmalloc.tcmalloc.cpu_free is greater than 0.

That is what I was going to say, but I have it backwards, those are not user settings but MongoDB reporting the current state of the new tcmalloc. So, we need to follow the instructions on the MongoDB docs page for enabling per-CPU caches by setting GLIBC_TUNABLES for MongoDB 8+. I'll open a PR shortly.

[yosifkit]

PR is #723. If users want the fix sooner, they can add -e GLIBC_TUNABLES=glibc.pthread.rseq=0 on their docker run to get the updated/fixed behavior.