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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 modelled (very losely) after libevent
+(http://monkey.org/~provos/libevent/), but aims to be faster and more
+correct, and also more featureful. Examples:
+
+(comparisons relative to libevent-1.3e and libev-0.00)
+
+- 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)
+
+- can correctly remove timers while executing callbacks
+  (libevent doesn't handle this reliably and can crash)
+
+- 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.
+
+whats missing?
+
+- evdns, evhttp, bufferevent are missing, libev is only an even library at
+  the moment.
+
+- no priority support at the moment
+
+- kqueue, poll (libev currently implements epoll and select)
+
+- windows support (whats windows?)