mirror of
https://gitlab.com/libvirt/libvirt.git
synced 2024-12-27 08:05:24 +00:00
5c991c8628
* daemon/THREADING.txt: Rename... * daemon/THREADS.txt: ...to match qemu thread notes. * daemon/Makefile.am (EXTRA_DIST): Reflect rename.
53 lines
2.1 KiB
Plaintext
53 lines
2.1 KiB
Plaintext
|
|
Threading in the libvirtd daemon
|
|
================================
|
|
|
|
To allow efficient processing of RPC requests, the libvirtd daemon
|
|
makes use of threads.
|
|
|
|
- The process leader. This is the initial thread of control
|
|
when the daemon starts running. It is responsible for
|
|
initializing all the state, and starting the event loop.
|
|
Once that's all done, this thread does nothing except
|
|
wait for the event loop to quit, thus indicating an orderly
|
|
shutdown is required.
|
|
|
|
- The event loop. This thread runs the event loop, sitting
|
|
in poll() on all monitored file handles, and calculating
|
|
and dispatching any timers that may be registered. When
|
|
this thread quits, the entire daemon will shutdown.
|
|
|
|
- The workers. These 'n' threads all sit around waiting to
|
|
process incoming RPC requests. Since RPC requests may take
|
|
a long time to complete, with long idle periods, there will
|
|
be quite a few workers running.
|
|
|
|
The use of threads obviously requires locking to ensure safety when
|
|
accessing/changing data structures.
|
|
|
|
- the top level lock is on 'struct qemud_server'. This must be
|
|
held before acquiring any other lock
|
|
|
|
- Each 'struct qemud_client' object has a lock. The server lock
|
|
must be held before acquiring it. Once the client lock is acquired
|
|
the server lock can (optionally) be dropped.
|
|
|
|
- The event loop has its own self-contained lock. You can ignore
|
|
this as a caller of virEvent APIs.
|
|
|
|
|
|
The server lock is used in conjunction with a condition variable
|
|
to pass jobs from the event loop thread to the workers. The main
|
|
event loop thread handles I/O from the client socket, and once a
|
|
complete RPC message has been read off the wire (and optionally
|
|
decrypted), it will be placed onto the 'dx' job queue for the
|
|
associated client object. The job condition will be signalled and
|
|
a worker will wakup and process it.
|
|
|
|
The worker thread must quickly drop its locks on the server and
|
|
client to allow the main event loop thread to continue running
|
|
with its other work. Critically important, is that now libvirt
|
|
API call will ever be made with the server or client locks held.
|
|
|
|
-- End
|