memory-profiling.rst

==================
 Memory Profiling
==================

Ceph MON, OSD and MDS can generate heap profiles using
``tcmalloc``. To generate heap profiles, ensure you have
``google-perftools`` installed::

	sudo apt-get install google-perftools

The profiler dumps output to your ``log file`` directory (i.e.,
``/var/log/ceph``). See `Logging and Debugging`_ for details.
To view the profiler logs with Google's performance tools, execute the
following:: 

    google-pprof --text {path-to-daemon}  {log-path/filename}

For example::

    $ ceph tell osd.0 heap start_profiler
    $ ceph tell osd.0 heap dump
    osd.0 tcmalloc heap stats:------------------------------------------------
    MALLOC:        2632288 (    2.5 MiB) Bytes in use by application
    MALLOC: +       499712 (    0.5 MiB) Bytes in page heap freelist
    MALLOC: +       543800 (    0.5 MiB) Bytes in central cache freelist
    MALLOC: +       327680 (    0.3 MiB) Bytes in transfer cache freelist
    MALLOC: +      1239400 (    1.2 MiB) Bytes in thread cache freelists
    MALLOC: +      1142936 (    1.1 MiB) Bytes in malloc metadata
    MALLOC:   ------------
    MALLOC: =      6385816 (    6.1 MiB) Actual memory used (physical + swap)
    MALLOC: +            0 (    0.0 MiB) Bytes released to OS (aka unmapped)
    MALLOC:   ------------
    MALLOC: =      6385816 (    6.1 MiB) Virtual address space used
    MALLOC:
    MALLOC:            231              Spans in use
    MALLOC:             56              Thread heaps in use
    MALLOC:           8192              Tcmalloc page size
    ------------------------------------------------
    Call ReleaseFreeMemory() to release freelist memory to the OS (via madvise()).
    Bytes released to the OS take up virtual address space but no physical memory.
    $ google-pprof --text \
                   /usr/bin/ceph-osd  \
                   /var/log/ceph/ceph-osd.0.profile.0001.heap
     Total: 3.7 MB
     1.9  51.1%  51.1%      1.9  51.1% ceph::log::Log::create_entry
     1.8  47.3%  98.4%      1.8  47.3% std::string::_Rep::_S_create
     0.0   0.4%  98.9%      0.0   0.6% SimpleMessenger::add_accept_pipe
     0.0   0.4%  99.2%      0.0   0.6% decode_message
     ...

Another heap dump on the same daemon will add another file. It is
convenient to compare to a previous heap dump to show what has grown
in the interval. For instance::

    $ google-pprof --text --base out/osd.0.profile.0001.heap \
          ceph-osd out/osd.0.profile.0003.heap
     Total: 0.2 MB
     0.1  50.3%  50.3%      0.1  50.3% ceph::log::Log::create_entry
     0.1  46.6%  96.8%      0.1  46.6% std::string::_Rep::_S_create
     0.0   0.9%  97.7%      0.0  26.1% ReplicatedPG::do_op
     0.0   0.8%  98.5%      0.0   0.8% __gnu_cxx::new_allocator::allocate

Refer to `Google Heap Profiler`_ for additional details.

Once you have the heap profiler installed, start your cluster and
begin using the heap profiler. You may enable or disable the heap
profiler at runtime, or ensure that it runs continuously. For the
following commandline usage, replace ``{daemon-type}`` with ``mon``,
``osd`` or ``mds``, and replace ``{daemon-id}`` with the OSD number or
the MON or MDS id.


Starting the Profiler
---------------------

To start the heap profiler, execute the following:: 

	ceph tell {daemon-type}.{daemon-id} heap start_profiler

For example:: 

	ceph tell osd.1 heap start_profiler

Alternatively the profile can be started when the daemon starts
running if the ``CEPH_HEAP_PROFILER_INIT=true`` variable is found in
the environment.

Printing Stats
--------------

To print out statistics, execute the following:: 

	ceph  tell {daemon-type}.{daemon-id} heap stats

For example:: 

	ceph tell osd.0 heap stats

.. note:: Printing stats does not require the profiler to be running and does
   not dump the heap allocation information to a file.


Dumping Heap Information
------------------------

To dump heap information, execute the following:: 

	ceph tell {daemon-type}.{daemon-id} heap dump

For example:: 

	ceph tell mds.a heap dump

.. note:: Dumping heap information only works when the profiler is running.


Releasing Memory
----------------

To release memory that ``tcmalloc`` has allocated but which is not being used by
the Ceph daemon itself, execute the following:: 

	ceph tell {daemon-type}{daemon-id} heap release

For example:: 

	ceph tell osd.2 heap release


Stopping the Profiler
---------------------

To stop the heap profiler, execute the following:: 

	ceph tell {daemon-type}.{daemon-id} heap stop_profiler

For example:: 

	ceph tell osd.0 heap stop_profiler

.. _Logging and Debugging: ../log-and-debug
.. _Google Heap Profiler: http://goog-perftools.sourceforge.net/doc/heap_profiler.html