xref: /external/chromium_org/third_party/tcmalloc/vendor/README

IMPORTANT NOTE FOR 64-BIT USERS
-------------------------------
There are known issues with some perftools functionality on x86_64
systems.  See 64-BIT ISSUES, below.


TCMALLOC
--------
Just link in -ltcmalloc or -ltcmalloc_minimal to get the advantages of
tcmalloc -- a replacement for malloc and new.  See below for some
environment variables you can use with tcmalloc, as well.

tcmalloc functionality is available on all systems we've tested; see
INSTALL for more details.  See README_windows.txt for instructions on
using tcmalloc on Windows.

NOTE: When compiling with programs with gcc, that you plan to link
with libtcmalloc, it's safest to pass in the flags

 -fno-builtin-malloc -fno-builtin-calloc -fno-builtin-realloc -fno-builtin-free

when compiling.  gcc makes some optimizations assuming it is using its
own, built-in malloc; that assumption obviously isn't true with
tcmalloc.  In practice, we haven't seen any problems with this, but
the expected risk is highest for users who register their own malloc
hooks with tcmalloc (using gperftools/malloc_hook.h).  The risk is
lowest for folks who use tcmalloc_minimal (or, of course, who pass in
the above flags :-) ).


HEAP PROFILER
-------------
See doc/heap-profiler.html for information about how to use tcmalloc's
heap profiler and analyze its output.

As a quick-start, do the following after installing this package:

1) Link your executable with -ltcmalloc
2) Run your executable with the HEAPPROFILE environment var set:
     $ HEAPPROFILE=/tmp/heapprof <path/to/binary> [binary args]
3) Run pprof to analyze the heap usage
     $ pprof <path/to/binary> /tmp/heapprof.0045.heap  # run 'ls' to see options
     $ pprof --gv <path/to/binary> /tmp/heapprof.0045.heap

You can also use LD_PRELOAD to heap-profile an executable that you
didn't compile.

There are other environment variables, besides HEAPPROFILE, you can
set to adjust the heap-profiler behavior; c.f. "ENVIRONMENT VARIABLES"
below.

The heap profiler is available on all unix-based systems we've tested;
see INSTALL for more details.  It is not currently available on Windows.


HEAP CHECKER
------------
See doc/heap-checker.html for information about how to use tcmalloc's
heap checker.

In order to catch all heap leaks, tcmalloc must be linked *last* into
your executable.  The heap checker may mischaracterize some memory
accesses in libraries listed after it on the link line.  For instance,
it may report these libraries as leaking memory when they're not.
(See the source code for more details.)

Here's a quick-start for how to use:

As a quick-start, do the following after installing this package:

1) Link your executable with -ltcmalloc
2) Run your executable with the HEAPCHECK environment var set:
     $ HEAPCHECK=1 <path/to/binary> [binary args]

Other values for HEAPCHECK: normal (equivalent to "1"), strict, draconian

You can also use LD_PRELOAD to heap-check an executable that you
didn't compile.

The heap checker is only available on Linux at this time; see INSTALL
for more details.


CPU PROFILER
------------
See doc/cpu-profiler.html for information about how to use the CPU
profiler and analyze its output.

As a quick-start, do the following after installing this package:

1) Link your executable with -lprofiler
2) Run your executable with the CPUPROFILE environment var set:
     $ CPUPROFILE=/tmp/prof.out <path/to/binary> [binary args]
3) Run pprof to analyze the CPU usage
     $ pprof <path/to/binary> /tmp/prof.out      # -pg-like text output
     $ pprof --gv <path/to/binary> /tmp/prof.out # really cool graphical output

There are other environment variables, besides CPUPROFILE, you can set
to adjust the cpu-profiler behavior; cf "ENVIRONMENT VARIABLES" below.

The CPU profiler is available on all unix-based systems we've tested;
see INSTALL for more details.  It is not currently available on Windows.

NOTE: CPU profiling doesn't work after fork (unless you immediately
      do an exec()-like call afterwards).  Furthermore, if you do
      fork, and the child calls exit(), it may corrupt the profile
      data.  You can use _exit() to work around this.  We hope to have
      a fix for both problems in the next release of perftools
      (hopefully perftools 1.2).


EVERYTHING IN ONE
-----------------
If you want the CPU profiler, heap profiler, and heap leak-checker to
all be available for your application, you can do:
   gcc -o myapp ... -lprofiler -ltcmalloc

However, if you have a reason to use the static versions of the
library, this two-library linking won't work:
   gcc -o myapp ... /usr/lib/libprofiler.a /usr/lib/libtcmalloc.a  # errors!

Instead, use the special libtcmalloc_and_profiler library, which we
make for just this purpose:
   gcc -o myapp ... /usr/lib/libtcmalloc_and_profiler.a


CONFIGURATION OPTIONS
---------------------
For advanced users, there are several flags you can pass to
'./configure' that tweak tcmalloc performace.  (These are in addition
to the environment variables you can set at runtime to affect
tcmalloc, described below.)  See the INSTALL file for details.


ENVIRONMENT VARIABLES
---------------------
The cpu profiler, heap checker, and heap profiler will lie dormant,
using no memory or CPU, until you turn them on.  (Thus, there's no
harm in linking -lprofiler into every application, and also -ltcmalloc
assuming you're ok using the non-libc malloc library.)

The easiest way to turn them on is by setting the appropriate
environment variables.  We have several variables that let you
enable/disable features as well as tweak parameters.

Here are some of the most important variables:

HEAPPROFILE=<pre> -- turns on heap profiling and dumps data using this prefix
HEAPCHECK=<type>  -- turns on heap checking with strictness 'type'
CPUPROFILE=<file> -- turns on cpu profiling and dumps data to this file.
PROFILESELECTED=1 -- if set, cpu-profiler will only profile regions of code
                     surrounded with ProfilerEnable()/ProfilerDisable().
PROFILEFREQUENCY=x-- how many interrupts/second the cpu-profiler samples.

TCMALLOC_DEBUG=<level> -- the higher level, the more messages malloc emits
MALLOCSTATS=<level>    -- prints memory-use stats at program-exit

For a full list of variables, see the documentation pages:
   doc/cpuprofile.html
   doc/heapprofile.html
   doc/heap_checker.html


COMPILING ON NON-LINUX SYSTEMS
------------------------------

Perftools was developed and tested on x86 Linux systems, and it works
in its full generality only on those systems.  However, we've
successfully ported much of the tcmalloc library to FreeBSD, Solaris
x86, and Darwin (Mac OS X) x86 and ppc; and we've ported the basic
functionality in tcmalloc_minimal to Windows.  See INSTALL for details.
See README_windows.txt for details on the Windows port.


PERFORMANCE
-----------

If you're interested in some third-party comparisons of tcmalloc to
other malloc libraries, here are a few web pages that have been
brought to our attention.  The first discusses the effect of using
various malloc libraries on OpenLDAP.  The second compares tcmalloc to
win32's malloc.
  http://www.highlandsun.com/hyc/malloc/
  http://gaiacrtn.free.fr/articles/win32perftools.html

It's possible to build tcmalloc in a way that trades off faster
performance (particularly for deletes) at the cost of more memory
fragmentation (that is, more unusable memory on your system).  See the
INSTALL file for details.


OLD SYSTEM ISSUES
-----------------

When compiling perftools on some old systems, like RedHat 8, you may
get an error like this:
    ___tls_get_addr: symbol not found

This means that you have a system where some parts are updated enough
to support Thread Local Storage, but others are not.  The perftools
configure script can't always detect this kind of case, leading to
that error.  To fix it, just comment out (or delete) the line
   #define HAVE_TLS 1
in your config.h file before building.


64-BIT ISSUES
-------------

There are two issues that can cause program hangs or crashes on x86_64
64-bit systems, which use the libunwind library to get stack-traces.
Neither issue should affect the core tcmalloc library; they both
affect the perftools tools such as cpu-profiler, heap-checker, and
heap-profiler.

1) Some libc's -- at least glibc 2.4 on x86_64 -- have a bug where the
libc function dl_iterate_phdr() acquires its locks in the wrong
order.  This bug should not affect tcmalloc, but may cause occasional
deadlock with the cpu-profiler, heap-profiler, and heap-checker.
Its likeliness increases the more dlopen() commands an executable has.
Most executables don't have any, though several library routines like
getgrgid() call dlopen() behind the scenes.

2) On x86-64 64-bit systems, while tcmalloc itself works fine, the
cpu-profiler tool is unreliable: it will sometimes work, but sometimes
cause a segfault.  I'll explain the problem first, and then some
workarounds.

Note that this only affects the cpu-profiler, which is a
gperftools feature you must turn on manually by setting the
CPUPROFILE environment variable.  If you do not turn on cpu-profiling,
you shouldn't see any crashes due to perftools.

The gory details: The underlying problem is in the backtrace()
function, which is a built-in function in libc.
Backtracing is fairly straightforward in the normal case, but can run
into problems when having to backtrace across a signal frame.
Unfortunately, the cpu-profiler uses signals in order to register a
profiling event, so every backtrace that the profiler does crosses a
signal frame.

In our experience, the only time there is trouble is when the signal
fires in the middle of pthread_mutex_lock.  pthread_mutex_lock is
called quite a bit from system libraries, particularly at program
startup and when creating a new thread.

The solution: The dwarf debugging format has support for 'cfi
annotations', which make it easy to recognize a signal frame.  Some OS
distributions, such as Fedora and gentoo 2007.0, already have added
cfi annotations to their libc.  A future version of libunwind should
recognize these annotations; these systems should not see any
crashses.

Workarounds: If you see problems with crashes when running the
cpu-profiler, consider inserting ProfilerStart()/ProfilerStop() into
your code, rather than setting CPUPROFILE.  This will profile only
those sections of the codebase.  Though we haven't done much testing,
in theory this should reduce the chance of crashes by limiting the
signal generation to only a small part of the codebase.  Ideally, you
would not use ProfilerStart()/ProfilerStop() around code that spawns
new threads, or is otherwise likely to cause a call to
pthread_mutex_lock!

---
17 May 2011