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Updated the documentation about shared memory.
 Changes to be committed: modified: docs/launching-apps/localhost.rst modified: docs/tuning-apps/networking/shared-memory.rst Signed-off-by: xbw <[email protected]>
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‎docs/launching-apps/localhost.rst‎

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and destination MPI processes can share memory (e.g., via SYSV or
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POSIX shared memory mechanisms).
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Shared memory MPI communication
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-------------------------------
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.. error:: TODO This should really be moved to the networking section.
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The ``sm`` BTL supports two modes of shared memory communication:
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#. **Two-copy:** Otherwise known as "copy-in / copy-out", this mode is
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where the sender copies data into shared memory and the receiver
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copies the data out.
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This mechanism is always available.
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#. **Single copy:** In this mode, the sender or receiver makes a
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single copy of the message data from the source buffer in one
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process to the destination buffer in another process. Open MPI
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supports three flavors of shared memory single-copy transfers:
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* `Linux KNEM <https://knem.gitlabpages.inria.fr/>`_. This is a
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standalone Linux kernel module, made specifically for HPC and MPI
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libraries to enable high-performance single-copy message
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transfers.
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Open MPI must be able to find the KNEM header files in order to
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build support for KNEM.
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* `Linux XPMEM <https://github.com/hjelmn/xpmem>`_. Similar to
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KNEM, this is a standalone Linux kernel module, made specifically
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for HPC and MPI libraries to enable high-performance single-copy
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message transfers. It is derived from the Cray XPMEM system.
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Open MPI must be able to find the XPMEM header files in order to
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build support for XPMEM.
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* Linux Cross-Memory Attach (CMA). This mechanism is built-in to
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modern versions of the Linux kernel. Although more performance
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than the two-copy shared memory transfer mechanism, CMA is the
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lowest performance of the single-copy mechanisms. However, CMA
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is likely the most widely available because it is enabled by
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default in several modern Linux distributions.
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Open MPI must be built on a Linux system with a recent enough
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Glibc and kernel version in order to build support for Linux CMA.
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Which mechanism is used at run time depends both on how Open MPI was
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built and how your system is configured. You can check to see which
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single-copy mechanisms Open MPI was built with via two mechanisms:
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#. At the end of running ``configure``, Open MPI emits a list of
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transports for which it found relevant header files and libraries
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such that it will be able to build support for them. You might see
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lines like this, for example:
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.. code-block:: text
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Shared memory/copy in+copy out: yes
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Shared memory/Linux CMA: yes
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Shared memory/Linux KNEM: no
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Shared memory/XPMEM: no
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The above output indicates that Open MPI will be built with 2-copy
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(as mentioned above, 2-copy is *always* available) and with Linux
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CMA support. KNEM and XPMEM support will *not* be built.
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#. After Open MPI is installed, the ``ompi_info`` command can show
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which ``smsc`` (shared memory single copy) components are
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available:
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.. code-block:: text
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shell$ ompi_info | grep smsc
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MCA smsc: cma (MCA v2.1.0, API v1.0.0, Component v5.1.0)
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This Open MPI installation only supports the Linux CMA single-copy
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mechanism.
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.. note:: As implied by the SMSC component names, none of them are
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supported on macOS. macOS users will use the two-copy mechanism.

‎docs/tuning-apps/networking/shared-memory.rst‎

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Shared Memory
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=============
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.. error:: TODO This section needs to be converted from FAQ Q&A style
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to regular documentation style.
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What is the sm BTL?
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-------------------
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The sm BTL
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----------
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The ``sm`` BTL is a low-latency, high-bandwidth mechanism for
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transferring data between two processes via shared memory. This BTL
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/////////////////////////////////////////////////////////////////////////
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How do I specify use of sm for MPI messages?
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--------------------------------------------
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Specifying the Use of sm for MPI Messages
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-----------------------------------------
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Typically, it is unnecessary to do so; OMPI will use the best BTL available
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for each communication.
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/////////////////////////////////////////////////////////////////////////
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How can I tune these parameters to improve performance?
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-------------------------------------------------------
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Tuning Parameters to Improve Performance
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----------------------------------------
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Mostly, the default values of the MCA parameters have already
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been chosen to give good performance. To improve performance further
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to resource congestion, but you can increase this parameter to
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pre-reserve space for more fragments.
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* ``btl_sm_backing_directory``: Directory to place backing files for
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shared memory communication. This directory should be on a local
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filesystem such as /tmp or /dev/shm (default: (linux) /dev/shm,
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(others) session directory)
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/////////////////////////////////////////////////////////////////////////
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Where is the shared memory mapped on the filesystem?
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Shared Memory Mechanisms
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------------------------
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The ``sm`` BTL supports two modes of shared memory communication:
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#. **Two-copy:** Otherwise known as "copy-in / copy-out", this mode is
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where the sender copies data into shared memory and the receiver
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copies the data out.
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This mechanism is always available.
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#. **Single copy:** In this mode, the sender or receiver makes a
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single copy of the message data from the source buffer in one
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process to the destination buffer in another process. Open MPI
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supports three flavors of shared memory single-copy transfers:
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* `Linux KNEM <https://knem.gitlabpages.inria.fr/>`_. This is a
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standalone Linux kernel module, made specifically for HPC and MPI
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libraries to enable high-performance single-copy message
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transfers.
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Open MPI must be able to find the KNEM header files in order to
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build support for KNEM.
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* `Linux XPMEM <https://github.com/hjelmn/xpmem>`_. Similar to
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KNEM, this is a standalone Linux kernel module, made specifically
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for HPC and MPI libraries to enable high-performance single-copy
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message transfers. It is derived from the Cray XPMEM system.
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Open MPI must be able to find the XPMEM header files in order to
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build support for XPMEM.
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* Linux Cross-Memory Attach (CMA). This mechanism is built-in to
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modern versions of the Linux kernel. Although more performance
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than the two-copy shared memory transfer mechanism, CMA is the
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lowest performance of the single-copy mechanisms. However, CMA
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is likely the most widely available because it is enabled by
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default in several modern Linux distributions.
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Open MPI must be built on a Linux system with a recent enough
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Glibc and kernel version in order to build support for Linux CMA.
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Which mechanism is used at run time depends both on how Open MPI was
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built and how your system is configured. You can check to see which
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single-copy mechanisms Open MPI was built with via two mechanisms:
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#. At the end of running ``configure``, Open MPI emits a list of
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transports for which it found relevant header files and libraries
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such that it will be able to build support for them. You might see
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lines like this, for example:
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.. code-block:: text
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Shared memory/copy in+copy out: yes
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Shared memory/Linux CMA: yes
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Shared memory/Linux KNEM: no
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Shared memory/XPMEM: no
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The above output indicates that Open MPI will be built with 2-copy
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(as mentioned above, 2-copy is *always* available) and with Linux
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CMA support. KNEM and XPMEM support will *not* be built.
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#. After Open MPI is installed, the ``ompi_info`` command can show
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which ``smsc`` (shared memory single copy) components are
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available:
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.. code-block:: text
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shell$ ompi_info | grep smsc
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MCA smsc: cma (MCA v2.1.0, API v1.0.0, Component v5.1.0)
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This Open MPI installation only supports the Linux CMA single-copy
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mechanism.
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.. note:: As implied by the SMSC component names, none of them are
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supported on macOS. macOS users will use the two-copy mechanism.
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/////////////////////////////////////////////////////////////////////////
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.. error:: TODO Is this correct?
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Shared Memory Mapping on the Filesystem
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---------------------------------------
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The file will be in the OMPI session directory, which is typically
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something like ``/tmp/openmpi-sessions-USERNAME@HOSTNAME/*``.
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The file itself will have the name
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``shared_mem_pool.HOSTNAME``. For example, the full path could be
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``/tmp/openmpi-sessions-johndoe@node0_0/1543/1/shared_mem_pool.node0``.
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The default location of the file is in the ``/dev/shm directory``. If ``/dev/shm``
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does not exist on the system, the default location will be the OMPI session
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directory. The path is typically something like:
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``/dev/shm/sm_segment.nodename.user_id.job_id.my_node_rank``.
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For example, the full path could be: ``/dev/shm/sm_segment.x.1000.23c70000.0``.
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.. error:: TODO The filename above will certainly be wrong.
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You can use the MCA parameter ``btl_sm_backing_directory`` to place the session
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directory in a non-default location.
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To place the session directory in a non-default location, use the MCA parameter
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``orte_tmpdir_base``.
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.. note:: The session directory is defined in PMIx. You can
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use ``--pmixmca orte_tmpdir_base "/path/to/somewhere"`` to place the session
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directory in a non-default location.
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.. error:: TODO The MCA param name above is definitely wrong.
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.. note:: Even when using single-copy methods like CMA, a shared memory file is still
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created for managing connection metadata.

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