= 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: https://liburcu.org/

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

* GNU/Linux
* FreeBSD (tested 11.1)

It may work on NetBSD, OpenBSD and DragonFly BSD.

== Install

	# FreeBSD: pkg install liburcu

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

	# Install mwrap via RubyGems.org
	gem install mwrap

== Usage

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:

	https://80x24.org/mwrap/MwrapRack.html

== Known problems

* 32-bit machines are prone to overflow (WONTFIX)

== Mail archives and list:

	https://80x24.org/mwrap-public/
	nntp://80x24.org/inbox.comp.lang.ruby.mwrap

No subscription will ever be required to post, but HTML mail
will be rejected:

		mwrap-public@80x24.org

== Hacking

	git clone https://80x24.org/mwrap.git

Send all patches and pull requests (use "git request-pull" to format) to
the mailing list.  We do not use centralized or proprietary messaging
systems.

== License

GPL-2.0+ <https://www.gnu.org/licenses/gpl-2.0.txt>