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[software/libev.git] / README

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-libev is modelled after libevent (http://monkey.org/~provos/libevent/), but aims
-to be faster and more correct, and also more featureful. Examples:
-
-- multiple watchers can wait for the same event without deregistering others.
-- fork() is supported and can be handled.
-- timers are handled as a priority queue (faster)
-- watchers use less memory (faster)
-- less calls to epoll_ctl (faster)
+libev is a high-performance event loop/event model with lots of features.
+
+It is modelled (very losely) after libevent
+(http://monkey.org/~provos/libevent/) and the Event perl module, but aims
+to be faster and more correct, and also more featureful.
+
+DIFFERENCES AND COMPARISON TO LIBEVENT:
+
+(comparisons relative to libevent-1.3e and libev-0.00, see also the benchmark
+at http://libev.schmorp.de/bench.html).
+
+- multiple watchers can wait for the same event without deregistering others,
+  both for file descriptors as well as signals.
+  (registering two read events on fd 10 and unregistering one will not
+  break the other).
+
+- fork() is supported and can be handled
+  (there is no way to recover from a fork when libevent is active).
+
+- timers are handled as a priority queue (important operations are O(1))
+  (libevent uses a much less efficient but more complex red-black tree).
+
+- supports absolute (wallclock-based) timers in addition to relative ones,
+  i.e. can schedule timers to occur after n seconds, or at a specific time.
+
+- timers can be repeating (both absolute and relative ones).
+
+- detects time jumps and adjusts timers
+  (works for both forward and backward time jumps and also for absolute timers).
+
+- race-free signal processing
+  (libevent may delay processing signals till after the next event).
+
+- less calls to epoll_ctl
+  (stopping and starting an io watcher between two loop iterations will now
+  result in spuriois epoll_ctl calls).
+
+- usually less calls to gettimeofday and clock_gettime
+  (libevent calls it on every timer event change, libev twice per iteration).
+
+- watchers use less memory
+  (libevent on amd64: 152 bytes, libev: <= 56 bytes).
+
+- library uses less memory
+  (libevent allocates large data structures wether used or not, libev
+  scales all its data structures dynamically).
+
+- no hardcoded arbitrary limits
+  (libevent contains an off-by-one bug and sometimes hardcodes a limit of
+  32000 fds).
+
+- libev separates timer, signal and io watchers from each other
+  (libevent combines them, but with libev you can combine them yourself
+  by reusing the same callback and still save memory).
+
+- simpler design, backends are potentially much simpler
+  (in libevent, backends have to deal with watchers, thus the problems)
+  (epoll backend in libevent: 366 lines, libev: 90 lines, and more features).
+
+- libev handles EBADF gracefully by removing the offending fds.
+
+- doesn't rely on nonportable BSD header files.
+
+- a event.h compatibility header exists, and can be used to run a wide
+  range of libevent programs unchanged (such as evdns.c).
+
+- win32 compatibility for the core parts.
+
+- the event core library (ev and event layer) compiles and works both as
+  C and C++.
+
+whats missing?
+
+- no event-like priority support at the moment (the ev priorities
+  are not yet finished and work differently, but you cna use idle watchers
+  to get a similar effect).
+