memcached is using a number of threads:
The dispatcher thread listens on all of the server sockets, and accepts a client connection which is then dispatched to the worker threads on a round robin model.
The TAP thread is responsible for serving all TAP communication.
The number of worker threads is specified with the
threadsparameter in the configuration. The worker threads are responsible for serving the clients requests, and the number of worker threads cannot be increased at runtime.
Some commands is implemented by dispatching separate threads to execute the user command. There is currently no checks trying to protect ourself from clients trying to allocate too many threads (but the commands themselves is not available to the average user. See RBAC.json)
- Bucket creation/deletion
- Reload of the SASL password database
The Connection class represents a Socket (it is used by both clients and servers objects). We should refactor the code so that it represents client connections and have a separate object (and state machines) for a server object.
The Connection object is bound to a thread object, and never change (except for clients running TAP, which is moved over to the TAP thread after the initial packet...)
A client is bound to its worker thread when the client is created (let's ignore the TAP threads, they're special and should hopefully die ;-)). When the clients event loop is triggered, it acquires the threads lock and runs the entire state machine. This means that the thread can safely access any resource available in the LIBEVENT thread object. Given that it already holds a lock to a LIBEVENT thread object it is NOT allowed to try to acquire ANY other thread locks. That would lead to a deadlock.
Given that all of the clients share the same set of worker threads, the
clients cannot block while waiting for a resource to become available. The
model in use is that we're expecting the engines to return EWOULDBLOCK and
we're expecting the client to run the task in a different thread and call
notify_io_complete when the resource is available. The reason it has to
be a different thread is that
notify_io_complete WILL try to lock the
thread object where the connection is bound to (which would cause a deadlock
if called while holding a thread lock).