如果我使用的是 Heroku buildpack，是否需要使用 jemalloc 构建 Ruby 并使用 gem？

Question

Sample a Ruby (2.4.1) on Rails (4.2.8) app, configured with Puma and deployed on Heroku (cedar-16) . Currently developing on a Mojave 10.14.5 MBP. I'm on a mission to cut down memory usage and recently discovered jemalloc via this post . I reinstalled Ruby (using RVM) with jemalloc enabled as proved by the following output - -lpthread -ljemalloc -lgmp -ldl -lobjc . I proceeded to add the jemalloc gem and bundle installed without a problem. I've also set up the Heroku buildpack and configured necessary environment variables.

However, with the gem in my Gemfile, I'm unable to run any rake/rails commands - rails c returns the following error, promptly followed by my Mac's fans firing up:

ruby(45487,0x10739c5c0) malloc: *** malloc_zone_unregister() failed for 0x7fff94a3f000

foreman start gets stuck at * Preloading application . All issues vanish & app works fine if I remove the gem.

Since I'm not privy to how Heroku buildpacks work and how that specific one is programmed exactly, my questions are as follows:

1. With the buildpack added, do I really need the gem installed in my project?
2. With the buildpack added, do I need a Ruby version compiled using jemalloc?
3. If I do need the gem, how do I get rid of the aforementioned error?

Answer 1

malloc gem was last released 5 years ago and it bundles a pretty old version of jemalloc lib (v3.4 and currently we have v5.2). All the gem does, is adding jemalloc lib to LD_PRELOAD (or DYLD_INSERT_LIBRARIES on mac os) before executing ruby binary. It shouldn't be needed if ruby is built against jemalloc library with --with-jemalloc flag.

So to answer your questions:

1. With the buildpack added, do I really need the gem installed in my project?

No, buildpack will handle everything automatically for you while deploying to heroku. Actually, there might be a problem with conflicting jemalloc versions if you have the gem.

2. With the buildpack added, do I need a Ruby version compiled using jemalloc?

For local development? Technically no. But I would advise you to use exactly the same ruby version as you use on production to avoid suprises.

3. If I do need the gem, how do I get rid of the aforementioned error?

You don't need the gem. You can enable jemalloc by rebuilding your ruby with --with-jemalloc flag (as you did).

Here's a full list of steps:

# install jemalloc lib locally
brew install jemalloc

# reinstall ruby (with rbenv):
RUBY_CONFIGURE_OPTS=--with-jemalloc rbenv install 2.7.1
# or with rvm:
rvm reinstall 2.7.1 -C —with-jemalloc

# confirm ruby is using jemalloc (ruby >= 2.6)
ruby -r rbconfig -e "puts RbConfig::CONFIG['MAINLIBS']"
# or with ruby < 2.6
ruby -r rbconfig -e "puts RbConfig::CONFIG['LIBS']"

You might also need to rebuild gems with native extensions (though I'm not sure about this one): See this question for how to do it: How do I get a list of gems that are installed that have native extensions?