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mwrap - LD_PRELOAD malloc wrapper + line stats for Ruby

mwrap is designed to answer the question:

Which lines of Ruby are hitting malloc the most?

mwrap wraps all malloc-family calls to trace the Ruby source location of such calls and bytes allocated at each callsite. As of mwrap 2.0.0, it can also function as a leak detector and show live allocations at every call site. Depending on your application and workload, the overhead is roughly a 50% increase memory and runtime.

It works best for allocations under GVL, but tries to track numeric caller addresses for allocations made without GVL so you can get an idea of how much memory usage certain extensions and native libraries use.

It requires the concurrent lock-free hash table from the Userspace RCU project:

It does not require recompiling or rebuilding Ruby, but only supports Ruby trunk (2.6.0dev+) on a few platforms:

It may work on NetBSD, OpenBSD and DragonFly BSD.


# FreeBSD: pkg install liburcu

# Debian-based systems: apt-get liburcu-dev

# Install mwrap via
gem install mwrap


mwrap works as an LD_PRELOAD and supplies a mwrap RubyGem executable to improve ease-of-use. You can set dump_path: in the MWRAP environment variable to append the results to a log file:

MWRAP=dump_path:/path/to/log mwrap RUBY_COMMAND

# And to display the locations with the most allocations:
sort -k1,1rn </path/to/log | $PAGER

You may also 'require "mwrap"' in your Ruby code and use Mwrap.dump, Mwrap.reset, Mwrap.each, etc.

However, mwrap MUST be loaded via LD_PRELOAD to have any effect in tracking malloc use. However, it is safe to keep "require 'mwrap'" in performance-critical deployments, as overhead is only incurred when used as an LD_PRELOAD.

The output of the mwrap dump is a text file with 3 columns:

total_bytes     call_count      location

Where location is a Ruby source location (if made under GVL) or an address retrieved by backtrace_symbols(3). It is recommended to use the sort(1) command on either of the first two columns to find the hottest malloc locations.

mwrap 2.0.0+ also supports a Rack application endpoint, it is documented at:

Known problems

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