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@page threading Threading in Tor
Tor is based around a single main thread and one or more worker
threads. We aim (with middling success) to use worker threads for
CPU-intensive activities and the main thread for our networking.
Fortunately (?) we have enough cryptography that moving what we can
of the cryptographic processes to the workers should achieve good
parallelism under most loads. Unfortunately, we only have a small
fraction of our cryptography done in our worker threads right now.
Our threads-and-workers abstraction is defined in workqueue.c, which
combines a work queue with a thread pool, and integrates the
signalling with libevent. Tor's main instance of a work queue is
instantiated in cpuworker.c. It will probably need some refactoring
as more types of work are added.
On a lower level, we provide locks with tor_mutex_t in \refdir{lib/lock}, and
higher-level locking/threading tools in \refdir{lib/thread}, including
conditions (tor_cond_t), thread-local storage (tor_threadlocal_t), and more.
Try to minimize sharing between threads: it is usually best to simply
make the worker "own" all the data it needs while the work is in
progress, and to give up ownership when it's complete.
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