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53 lines
2.1 KiB
Plaintext
53 lines
2.1 KiB
Plaintext
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Threading in the libvirtd daemon
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To allow efficient processing of RPC requests, the libvirtd daemon
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makes use of threads.
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- The process leader. This is the initial thread of control
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when the daemon starts running. It is responsible for
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initializing all the state, and starting the event loop.
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Once that's all done, this thread does nothing except
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wait for the event loop to quit, thus indicating an orderly
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shutdown is required.
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- The event loop. This thread runs the event loop, sitting
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in poll() on all monitored file handles, and calculating
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and dispatching any timers that may be registered. When
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this thread quits, the entire daemon will shutdown.
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- The workers. These 'n' threads all sit around waiting to
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process incoming RPC requests. Since RPC requests may take
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a long time to complete, with long idle periods, there will
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be quite a few workers running.
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The use of threads obviously requires locking to ensure safety when
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accessing/changing data structures.
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- the top level lock is on 'struct qemud_server'. This must be
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held before acquiring any other lock
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- Each 'struct qemud_client' object has a lock. The server lock
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must be held before acquiring it. Once the client lock is acquired
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the server lock can (optionally) be dropped.
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- The event loop has its own self-contained lock. You can ignore
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this as a caller of virEvent APIs.
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The server lock is used in conjunction with a condition variable
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to pass jobs from the event loop thread to the workers. The main
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event loop thread handles I/O from the client socket, and once a
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complete RPC message has been read off the wire (and optionally
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decrypted), it will be placed on the 'dx' job queue for the
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associated client object. The job condition will be signalled and
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a worker will wakeup and process it.
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The worker thread must quickly drop its locks on the server and
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client to allow the main event loop thread to continue running
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with its other work. Critically important, is that now libvirt
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API call will ever be made with the server or client locks held.
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-- End
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