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Session name is given to each device via `--session`, which must be synchronized from a higher level, e.g. from start script or command & control entity.
36 lines
3.1 KiB
Markdown
36 lines
3.1 KiB
Markdown
# Shared Memory transport
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Shared memory transport for FairMQ. To try with existing devices, run the devices with `--transport shmem` option or configure channel transport in JSON (see examples/MQ/multiple-transports).
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The transport manages shared memory via boost::interprocess library. The transfer of the meta data, required to locate the content in the shared memory, is done via ZeroMQ. The transport supports all communication patterns where a single message is received by a single receiver. For multiple receivers for the same message, the message has to be copied.
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Devices track and cleanup shared memory on shutdown. For more information on the current shared memory segment and additional cleanup options, see following section.
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## Shared memory monitor
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The shared memory monitor tool, supplied with the shared memory transport can be used to monitor shared memory use and automatically cleanup shared memory in case of device crashes.
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With default arguments the monitor will run indefinitely with no output, and clean up shared memory segment if it is open and no heartbeats from devices arrive within a timeout period. It can be further customized with following parameters:
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`--session <arg>`: customize the name of the shared memory segment via the session name (default is "default").
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`--cleanup`: start monitor, perform cleanup of the memory and quit.
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`--self-destruct`: run until the memory segment is closed (either naturally via cleanup performed by devices or in case of a crash (no heartbeats within timeout)).
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`--interactive`: run interactively, with detailed segment details and user input for various shmem operations.
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`--timeout <arg>`: specifiy the timeout for the heartbeats from shmem transports in milliseconds (default 5000).
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The options can be combined, with the exception of `--cleanup` option, which will invoke the described behaviour independent of other options.
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Without the `--self-destruct` option, the monitor will run continuously, moitoring (and cleaning up if needed) consecutive topologies.
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Possible further implementation would be to run the monitor with `--self-destruct` with each topology.
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The FairMQShmMonitor class can also be used independently from the supplied executable (built from `runFairMQShmMonitor.cxx`), allowing integration on any level. For example invoking the monitor could be a functionality that a device offers.
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FairMQ Shared Memory currently uses following names to register shared memory on the system:
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`fmq_shm_<sessionName>_main` - main segment name, used for user data (session name can be overridden via `--session`).
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`fmq_shm_<sessionName>_management` - management segment name, used for storing management data.
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`fmq_shm_<sessionName>_control_queue` - message queue for communicating between shm transport and shm monitor (exists independent of above segments).
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`fmq_shm_<sessionName>_mutex` - boost::interprocess::named_mutex for management purposes (exists independent of above segments).
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`fmq_shm_<sessionName>_region_<index>` - names of unmanaged regions.
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`fmq_shm_<sessionName>_region_queue_<index>` - names of queues for the unmanaged regions.
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