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http://www.uhv.edu/ac/newsletters/writing/grammartip2009.07.01.htm (and several other sites) give hints that 'onto' is best used if you can also add 'up' just before it and still make sense. In many cases in the code base, we really want the two-word form, or even a simplification to just 'on' or 'to'. * docs/hacking.html.in: Use correct 'on to'. * python/libvirt-override.c: Likewise. * src/lxc/lxc_controller.c: Likewise. * src/util/virpci.c: Likewise. * daemon/THREADS.txt: Use simpler 'on'. * docs/formatdomain.html.in: Better usage. * docs/internals/rpc.html.in: Likewise. * src/conf/domain_event.c: Likewise. * src/rpc/virnetclient.c: Likewise. * tests/qemumonitortestutils.c: Likewise. * HACKING: Regenerate. Signed-off-by: Eric Blake <eblake@redhat.com>
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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================================
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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 wakup 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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