ev_io - is this file descriptor readable or writable?ev_io - is this file descriptor readable or writable?
+
+ev_timer - relative and optionally repeating timeoutsev_periodic - to cron or not to cron?ev_signal - signal me when a signal gets signalled!The newest version of this document is also available as a html-formatted +web page you might find easier to navigate when reading it for the first +time: http://cvs.schmorp.de/libev/ev.html.
Libev is an event loop: you register interest in certain events (such as a file descriptor being readable or a timeout occuring), and it will manage these event sources and provide your program with events.
@@ -184,13 +190,15 @@ you actually want to know.You can find out the major and minor version numbers of the library +
You can find out the major and minor ABI version numbers of the library
you linked against by calling the functions ev_version_major and
ev_version_minor. If you want, you can compare against the global
symbols EV_VERSION_MAJOR and EV_VERSION_MINOR, which specify the
version of the library your program was compiled against.
These version numbers refer to the ABI version of the library, not the +release version.
Usually, it's a good idea to terminate if the major versions mismatch, -as this indicates an incompatible change. Minor versions are usually +as this indicates an incompatible change. Minor versions are usually compatible to older versions, so a larger minor version alone is usually not a problem.
Example: Make sure we haven't accidentally been linked against the wrong @@ -232,13 +240,14 @@ might be supported on the current system, you would need to look at recommended ones.
See the description of ev_embed watchers for more info.
Sets the allocation function to use (the prototype and semantics are -identical to the realloc C function). It is used to allocate and free -memory (no surprises here). If it returns zero when memory needs to be -allocated, the library might abort or take some potentially destructive -action. The default is your system realloc function.
+Sets the allocation function to use (the prototype is similar - the +semantics is identical - to the realloc C function). It is used to +allocate and free memory (no surprises here). If it returns zero when +memory needs to be allocated, the library might abort or take some +potentially destructive action. The default is your system realloc +function.
You could override this function in high-availability programs to, say, free some memory if it cannot allocate memory, to use a special allocator, or even to sleep a while and retry until some memory is available.
@@ -326,6 +335,23 @@ or setgid) then libev will not look at the environment variable override the flags completely if it is found in the environment. This is useful to try out specific backends to test their performance, or to work around bugs. +EVFLAG_FORKCHECKInstead of calling ev_default_fork or ev_loop_fork manually after
+a fork, you can also make libev check for a fork in each iteration by
+enabling this flag.
This works by calling getpid () on every iteration of the loop,
+and thus this might slow down your event loop if you do a lot of loop
+iterations and little real work, but is usually not noticeable (on my
+Linux system for example, getpid is actually a simple 5-insn sequence
+without a syscall and thus very fast, but my Linux system also has
+pthread_atfork which is even faster).
The big advantage of this flag is that you can forget about fork (and +forget about forgetting to tell libev about forking) when you use this +flag.
+This flag setting cannot be overriden or specified in the LIBEV_FLAGS
+environment variable.
EVBACKEND_SELECT (value 1, portable select backend)Like ev_default_fork, but acts on an event loop created by
ev_loop_new. Yes, you have to call this on every allocated event loop
after fork, and how you do this is entirely your own problem.
Returns the count of loop iterations for the loop, which is identical to
+the number of times libev did poll for new events. It starts at 0 and
+happily wraps around with enough iterations.
This value can sometimes be useful as a generation counter of sorts (it
+"ticks" the number of loop iterations), as it roughly corresponds with
+ev_prepare and ev_check calls.
ev_prepare/ev_check watchers is
usually a better approach for this kind of thing.
Here are the gory details of what ev_loop does:
* If there are no active watchers (reference count is zero), return.
- - Queue prepare watchers and then call all outstanding watchers.
+ - Before the first iteration, call any pending watchers.
+ * If there are no active watchers (reference count is zero), return.
+ - Queue all prepare watchers and then call all outstanding watchers.
- If we have been forked, recreate the kernel state.
- Update the kernel state with all outstanding changes.
- Update the "event loop time".
@@ -741,8 +777,9 @@ it.
Returns a true value iff the watcher is pending, (i.e. it has outstanding
events but its callback has not yet been invoked). As long as a watcher
is pending (but not active) you must not call an init function on it (but
-ev_TYPE_set is safe) and you must make sure the watcher is available to
-libev (e.g. you cnanot free () it).
+ev_TYPE_set is safe), you must not change its priority, and you must
+make sure the watcher is available to libev (e.g. you cannot free ()
+it).
free () it).
Change the callback. You can change the callback at virtually any time (modulo threads).
Set and query the priority of the watcher. The priority is a small
+integer between EV_MAXPRI (default: 2) and EV_MINPRI
+(default: -2). Pending watchers with higher priority will be invoked
+before watchers with lower priority, but priority will not keep watchers
+from being executed (except for ev_idle watchers).
This means that priorities are only used for ordering callback +invocation after new events have been received. This is useful, for +example, to reduce latency after idling, or more often, to bind two +watchers on the same event and make sure one is called first.
+If you need to suppress invocation when higher priority events are pending
+you need to look at ev_idle watchers, which provide this functionality.
You must not change the priority of a watcher as long as it is active or +pending.
+The default priority used by watchers when no priority has been set is
+always 0, which is supposed to not be too high and not be too low :).
Setting a priority outside the range of EV_MINPRI to EV_MAXPRI is
+fine, as long as you do not mind that the priority value you query might
+or might not have been adjusted to be within valid range.
Invoke the watcher with the given loop and revents. Neither
+loop nor revents need to be valid as long as the watcher callback
+can deal with that fact.
If the watcher is pending, this function returns clears its pending status
+and returns its revents bitset (as if its callback was invoked). If the
+watcher isn't pending it does nothing and returns 0.
read(2) returni
EAGAIN is far preferable to a program hanging until some data arrives.
If you cannot run the fd in non-blocking mode (for example you should not play around with an Xlib connection), then you have to seperately re-test -wether a file descriptor is really ready with a known-to-be good interface +whether a file descriptor is really ready with a known-to-be good interface such as poll (fortunately in our Xlib example, Xlib already does this on its own, so its quite safe to use).
+ +Some backends (e.g kqueue, epoll) need to be told about closing a file
+descriptor (either by calling close explicitly or by any other means,
+such as dup). The reason is that you register interest in some file
+descriptor, but when it goes away, the operating system will silently drop
+this interest. If another file descriptor with the same number then is
+registered with libev, there is no efficient way to see that this is, in
+fact, a different file descriptor.
To avoid having to explicitly tell libev about such cases, libev follows
+the following policy: Each time ev_io_set is being called, libev
+will assume that this is potentially a new file descriptor, otherwise
+it is assumed that the file descriptor stays the same. That means that
+you have to call ev_io_set (or ev_io_init) when you change the
+descriptor even if the file descriptor number itself did not change.
This is how one would do it normally anyway, the important point is that +the libev application should not optimise around libev but should leave +optimisations to libev.
+ + + +This will act as if the timer timed out and restart it again if it is repeating. The exact semantics are:
-If the timer is started but nonrepeating, stop it.
-If the timer is repeating, either start it if necessary (with the repeat -value), or reset the running timer to the repeat value.
+If the timer is pending, its pending status is cleared.
+If the timer is started but nonrepeating, stop it (as if it timed out).
+If the timer is repeating, either start it if necessary (with the
+repeat value), or reset the running timer to the repeat value.
This sounds a bit complicated, but here is a useful and typical
-example: Imagine you have a tcp connection and you want a so-called
-idle timeout, that is, you want to be called when there have been,
-say, 60 seconds of inactivity on the socket. The easiest way to do
-this is to configure an ev_timer with after=repeat=60 and calling
+example: Imagine you have a tcp connection and you want a so-called idle
+timeout, that is, you want to be called when there have been, say, 60
+seconds of inactivity on the socket. The easiest way to do this is to
+configure an ev_timer with a repeat value of 60 and then call
ev_timer_again each time you successfully read or write some data. If
you go into an idle state where you do not expect data to travel on the
-socket, you can stop the timer, and again will automatically restart it if
-need be.
You can also ignore the after value and ev_timer_start altogether
-and only ever use the repeat value:
ev_timer_stop the timer, and ev_timer_again will
+automatically restart it if need be.
+ That means you can ignore the after value and ev_timer_start
+altogether and only ever use the repeat value and ev_timer_again:
ev_timer_init (timer, callback, 0., 5.);
ev_timer_again (loop, timer);
...
@@ -977,8 +1073,8 @@ and only ever use the repeat value:
ev_timer_again (loop, timer);
- This is more efficient then stopping/starting the timer eahc time you want -to modify its timeout value.
+This is more slightly efficient then stopping/starting the timer each time +you want to modify its timeout value.
ev_now ()
+ 10.) and then reset your system clock to the last year, then it will
take a year to trigger the event (unlike an ev_timer, which would trigger
-roughly 10 seconds later and of course not if you reset your system time
-again).
+roughly 10 seconds later).
They can also be used to implement vastly more complex timers, such as -triggering an event on eahc midnight, local time.
+triggering an event on each midnight, local time or other, complicated, +rules.As with timers, the callback is guarenteed to be invoked only when the
time (at) has been passed, but if multiple periodic timers become ready
during the same loop iteration then order of execution is undefined.
In this configuration the watcher triggers an event at the wallclock time
at and doesn't repeat. It will not adjust when a time jump occurs,
that is, if it is to be run at January 1st 2011 then it will run when the
system time reaches or surpasses this time.
In this mode the watcher will always be scheduled to time out at the next
-at + N * interval time (for some integer N) and then repeat, regardless
-of any time jumps.
at + N * interval time (for some integer N, which can also be negative)
+and then repeat, regardless of any time jumps.
This can be used to create timers that do not drift with respect to system time:
ev_periodic_set (&periodic, 0., 3600., 0); @@ -1071,8 +1167,11 @@ by 3600.Another way to think about it (for the mathematically inclined) is that
+ev_periodicwill try to run the callback in this mode at the next possible time wheretime = at (mod interval), regardless of any time jumps.For numerical stability it is preferable that the
atvalue is near +ev_now ()(the current time), but there is no range requirement for +this value.
In this mode the values for interval and at are both being
ignored. Instead, each time the periodic watcher gets scheduled, the
@@ -1081,7 +1180,7 @@ current time as second argument.
NOTE: This callback MUST NOT stop or destroy any periodic watcher,
ever, or make any event loop modifications. If you need to stop it,
return now + 1e30 (or so, fudge fudge) and stop it afterwards (e.g. by
-starting a prepare watcher).
ev_prepare watcher, which is legal).
Its prototype is ev_tstamp (*reschedule_cb)(struct ev_periodic *w,
ev_tstamp now), e.g.:
static ev_tstamp my_rescheduler (struct ev_periodic *w, ev_tstamp now) @@ -1111,6 +1210,13 @@ reason I omitted it as an example). when you changed some parameters or the reschedule callback would return a different time than the last time it was called (e.g. in a crond like program when the crontabs have changed). +
When repeating, this contains the offset value, otherwise this is the
+absolute point in time (the at value passed to ev_periodic_set).
Can be modified any time, but changes only take effect when the periodic
+timer fires or ev_periodic_again is being called.
st_nlink field being zero (which is
otherwise always forced to be at least one) and all the other fields of
the stat buffer having unspecified contents.
+The path should be absolute and must not end in a slash. If it is +relative and your working directory changes, the behaviour is undefined.
Since there is no standard to do this, the portable implementation simply
calls stat (2) regularly on the path to see if it changed somehow. You
can specify a recommended polling interval for this case. If you specify
@@ -1339,13 +1447,15 @@ was some error while stating the file.
ev_idle - when you've got nothing better to do...Idle watchers trigger events when there are no other events are pending -(prepare, check and other idle watchers do not count). That is, as long -as your process is busy handling sockets or timeouts (or even signals, -imagine) it will not be triggered. But when your process is idle all idle -watchers are being called again and again, once per event loop iteration - -until stopped, that is, or your process receives more events and becomes -busy.
+Idle watchers trigger events when no other events of the same or higher +priority are pending (prepare, check and other idle watchers do not +count).
+That is, as long as your process is busy handling sockets or timeouts +(or even signals, imagine) of the same or higher priority it will not be +triggered. But when your process is idle (or only lower-priority watchers +are pending), the idle watchers are being called once per event loop +iteration - until stopped, that is, or your process receives more events +and becomes busy again with higher priority stuff.
The most noteworthy effect is that as long as any idle watchers are active, the process will not block when waiting for new events.
Apart from keeping your process non-blocking (which is a useful @@ -1415,6 +1525,15 @@ with priority higher than or equal to the event loop and one coroutine of lower priority, but only once, using idle watchers to keep the event loop from blocking if lower-priority coroutines are active, thus mapping low-priority coroutines to idle/background tasks).
+It is recommended to give ev_check watchers highest (EV_MAXPRI)
+priority, to ensure that they are being run before any other watchers
+after the poll. Also, ev_check watchers (and ev_prepare watchers,
+too) should not activate ("feed") events into libev. While libev fully
+supports this, they will be called before other ev_check watchers did
+their job. As ev_check watchers are often used to embed other event
+loops those other event loops might be in an unusable state until their
+ev_check watcher ran (always remind yourself to coexist peacefully with
+others).
ev_prepare_set and ev_check
macros, but using them is utterly, utterly and completely pointless.
Example: To include a library such as adns, you would add IO watchers -and a timeout watcher in a prepare handler, as required by libadns, and -in a check watcher, destroy them and call into libadns. What follows is -pseudo-code only of course:
+There are a number of principal ways to embed other event loops or modules
+into libev. Here are some ideas on how to include libadns into libev
+(there is a Perl module named EV::ADNS that does this, which you could
+use for an actually working example. Another Perl module named EV::Glib
+embeds a Glib main context into libev, and finally, Glib::EV embeds EV
+into the Glib event loop).
Method 1: Add IO watchers and a timeout watcher in a prepare handler,
+and in a check watcher, destroy them and call into libadns. What follows
+is pseudo-code only of course. This requires you to either use a low
+priority for the check watcher or use ev_clear_pending explicitly, as
+the callbacks for the IO/timeout watchers might not have been called yet.
static ev_io iow [nfd];
static ev_timer tw;
static void
io_cb (ev_loop *loop, ev_io *w, int revents)
{
- // set the relevant poll flags
- // could also call adns_processreadable etc. here
- struct pollfd *fd = (struct pollfd *)w->data;
- if (revents & EV_READ ) fd->revents |= fd->events & POLLIN;
- if (revents & EV_WRITE) fd->revents |= fd->events & POLLOUT;
}
// create io watchers for each fd and a timer before blocking
static void
adns_prepare_cb (ev_loop *loop, ev_prepare *w, int revents)
{
- int timeout = 3600000;truct pollfd fds [nfd];
+ int timeout = 3600000;
+ struct pollfd fds [nfd];
// actual code will need to loop here and realloc etc.
adns_beforepoll (ads, fds, &nfd, &timeout, timeval_from (ev_time ()));
@@ -1453,7 +1575,7 @@ pseudo-code only of course:
ev_timer_init (&tw, 0, timeout * 1e-3);
ev_timer_start (loop, &tw);
- // create on ev_io per pollfd
+ // create one ev_io per pollfd
for (int i = 0; i < nfd; ++i)
{
ev_io_init (iow + i, io_cb, fds [i].fd,
@@ -1461,7 +1583,6 @@ pseudo-code only of course:
| (fds [i].events & POLLOUT ? EV_WRITE : 0)));
fds [i].revents = 0;
- iow [i].data = fds + i;
ev_io_start (loop, iow + i);
}
}
@@ -1473,11 +1594,79 @@ pseudo-code only of course:
ev_timer_stop (loop, &tw);
for (int i = 0; i < nfd; ++i)
- ev_io_stop (loop, iow + i);
+ {
+ // set the relevant poll flags
+ // could also call adns_processreadable etc. here
+ struct pollfd *fd = fds + i;
+ int revents = ev_clear_pending (iow + i);
+ if (revents & EV_READ ) fd->revents |= fd->events & POLLIN;
+ if (revents & EV_WRITE) fd->revents |= fd->events & POLLOUT;
+
+ // now stop the watcher
+ ev_io_stop (loop, iow + i);
+ }
adns_afterpoll (adns, fds, nfd, timeval_from (ev_now (loop));
}
+
+Method 2: This would be just like method 1, but you run adns_afterpoll
+in the prepare watcher and would dispose of the check watcher.
Method 3: If the module to be embedded supports explicit event +notification (adns does), you can also make use of the actual watcher +callbacks, and only destroy/create the watchers in the prepare watcher.
+ static void
+ timer_cb (EV_P_ ev_timer *w, int revents)
+ {
+ adns_state ads = (adns_state)w->data;
+ update_now (EV_A);
+
+ adns_processtimeouts (ads, &tv_now);
+ }
+
+ static void
+ io_cb (EV_P_ ev_io *w, int revents)
+ {
+ adns_state ads = (adns_state)w->data;
+ update_now (EV_A);
+
+ if (revents & EV_READ ) adns_processreadable (ads, w->fd, &tv_now);
+ if (revents & EV_WRITE) adns_processwriteable (ads, w->fd, &tv_now);
+ }
+
+ // do not ever call adns_afterpoll
+
+
+Method 4: Do not use a prepare or check watcher because the module you
+want to embed is too inflexible to support it. Instead, youc na override
+their poll function. The drawback with this solution is that the main
+loop is now no longer controllable by EV. The Glib::EV module does
+this.
static gint
+ event_poll_func (GPollFD *fds, guint nfds, gint timeout)
+ {
+ int got_events = 0;
+
+ for (n = 0; n < nfds; ++n)
+ // create/start io watcher that sets the relevant bits in fds[n] and increment got_events
+
+ if (timeout >= 0)
+ // create/start timer
+
+ // poll
+ ev_loop (EV_A_ 0);
+
+ // stop timer again
+ if (timeout >= 0)
+ ev_timer_stop (EV_A_ &to);
+
+ // stop io watchers again - their callbacks should have set
+ for (n = 0; n < nfds; ++n)
+ ev_io_stop (EV_A_ iow [n]);
+
+ return got_events;
+ }
+
@@ -1682,11 +1871,19 @@ the callback model to a model using method callbacks on objects.
#include <ev++.h>
-(it is not installed by default). This automatically includes ev.h
-and puts all of its definitions (many of them macros) into the global
-namespace. All C++ specific things are put into the ev namespace.
-It should support all the same embedding options as ev.h, most notably
-EV_MULTIPLICITY.
+This automatically includes ev.h and puts all of its definitions (many
+of them macros) into the global namespace. All C++ specific things are
+put into the ev namespace. It should support all the same embedding
+options as ev.h, most notably EV_MULTIPLICITY.
+Care has been taken to keep the overhead low. The only data member the C++
+classes add (compared to plain C-style watchers) is the event loop pointer
+that the watcher is associated with (or no additional members at all if
+you disable EV_MULTIPLICITY when embedding libev).
+Currently, functions, and static and non-static member functions can be
+used as callbacks. Other types should be easy to add as long as they only
+need one additional pointer for context. If you need support for other
+types of functors please contact the author (preferably after implementing
+it).
Here is a list of things available in the ev namespace:
ev::READ, ev::WRITE etc.All of those classes have these methods:
The constructor takes a pointer to an object and a method pointer to
-the event handler callback to call in this class. The constructor calls
-ev_init for you, which means you have to call the set method
-before starting it. If you do not specify a loop then the constructor
-automatically associates the default loop with this watcher.
The constructor (optionally) takes an event loop to associate the watcher
+with. If it is omitted, it will use EV_DEFAULT.
The constructor calls ev_init for you, which means you have to call the
+set method before starting it.
It will not set a callback, however: You have to call the templated set
+method to set a callback before you can start the watcher.
(The reason why you have to use a method is a limitation in C++ which does +not allow explicit template arguments for constructors).
The destructor automatically stops the watcher if it is active.
This method sets the callback method to call. The method has to have a
+signature of void (*)(ev_TYPE &, int), it receives the watcher as
+first argument and the revents as second. The object must be given as
+parameter and is stored in the data member of the watcher.
This method synthesizes efficient thunking code to call your method from
+the C callback that libev requires. If your compiler can inline your
+callback (i.e. it is visible to it at the place of the set call and
+your compiler is good :), then the method will be fully inlined into the
+thunking function, making it as fast as a direct C callback.
Example: simple class declaration and watcher initialisation
+ struct myclass
+ {
+ void io_cb (ev::io &w, int revents) { }
+ }
+
+ myclass obj;
+ ev::io iow;
+ iow.set <myclass, &myclass::io_cb> (&obj);
+
+
+ Also sets a callback, but uses a static method or plain function as
+callback. The optional data argument will be stored in the watcher's
+data member and is free for you to use.
The prototype of the function must be void (*)(ev::TYPE &w, int).
See the method-set above for more details.
Example:
+ static void io_cb (ev::io &w, int revents) { }
+ iow.set <io_cb> ();
+
+
+ Associates a different struct ev_loop with this watcher. You can only
@@ -1726,13 +1962,14 @@ do this when the watcher is inactive (and not pending either).
Basically the same as ev_TYPE_set, with the same args. Must be
-called at least once. Unlike the C counterpart, an active watcher gets
-automatically stopped and restarted.
Starts the watcher. Note that there is no loop argument as the
-constructor already takes the loop.
Starts the watcher. Note that there is no loop argument, as the
+constructor already stores the event loop.
Libev can be compiled with a variety of options, the most fundemantal is
-EV_MULTIPLICITY. This option determines wether (most) functions and
+EV_MULTIPLICITY. This option determines whether (most) functions and
callbacks have an initial struct ev_loop * argument.
To make it easier to write programs that cope with either variant, the following macros are defined:
@@ -1820,8 +2058,9 @@ suitable for use withEV_A.
loop, if multiple loops are supported ("ev loop default").
-Example: Declare and initialise a check watcher, working regardless of -wether multiple loops are supported or not.
+Example: Declare and initialise a check watcher, utilising the above +macros so it will work regardless of whether multiple loops are supported +or not.
static void
check_cb (EV_P_ ev_timer *w, int revents)
{
@@ -1833,9 +2072,6 @@ wether multiple loops are supported or not.
ev_check_start (EV_DEFAULT_ &check);
ev_loop (EV_DEFAULT_ 0);
-
-
-
struct ev_loop * as first argument, and you can creat
additional independent event loops. Otherwise there will be no support
for multiple event loops and there is no first event loop pointer
argument. Instead, all functions act on the single default loop.
+ The range of allowed priorities. EV_MINPRI must be smaller or equal to
+EV_MAXPRI, but otherwise there are no non-obvious limitations. You can
+provide for more priorities by overriding those symbols (usually defined
+to be -2 and 2, respectively).
When doing priority-based operations, libev usually has to linearly search +all the priorities, so having many of them (hundreds) uses a lot of space +and time, so using the defaults of five priorities (-2 .. +2) is usually +fine.
+If your embedding app does not need any priorities, defining these both to
+0 will save some memory and cpu.
If undefined or defined to be 1, then periodic timers are supported. If
defined to be 0, then they are not. Disabling them saves a few kB of
+code.
If undefined or defined to be 1, then idle watchers are supported. If
+defined to be 0, then they are not. Disabling them saves a few kB of
code.
The usage in rxvt-unicode is simpler. It has a ev_cpp.h header file -that everybody includes and which overrides some autoconf choices:
-#define EV_USE_POLL 0 +that everybody includes and which overrides some configure choices: +#define EV_MINIMAL 1 + #define EV_USE_POLL 0 #define EV_MULTIPLICITY 0 - #define EV_PERIODICS 0 + #define EV_PERIODIC_ENABLE 0 + #define EV_STAT_ENABLE 0 + #define EV_FORK_ENABLE 0 #define EV_CONFIG_H <config.h> + #define EV_MINPRI 0 + #define EV_MAXPRI 0 #include "ev++.h" @@ -2162,16 +2423,48 @@ that everybody includes and which overrides some autoconf choices:In this section the complexities of (many of) the algorithms used inside libev will be explained. For complexity discussions about backends see the documentation for
+ev_default_init.All of the following are about amortised time: If an array needs to be +extended, libev needs to realloc and move the whole array, but this +happens asymptotically never with higher number of elements, so O(1) might +mean it might do a lengthy realloc operation in rare cases, but on average +it is much faster and asymptotically approaches constant time.
This means that, when you have a watcher that triggers in one hour and +there are 100 watchers that would trigger before that then inserting will +have to skip those 100 watchers.
+That means that for changing a timer costs less than removing/adding them +as only the relative motion in the event queue has to be paid for.
+These just add the watcher into an array or at the head of a list. +=item Stopping check/prepare/idle watchers: O(1)
+These watchers are stored in lists then need to be walked to find the +correct watcher to remove. The lists are usually short (you don't usually +have many watchers waiting for the same fd or signal).
+A change means an I/O watcher gets started or stopped, which requires +libev to recalculate its status (and possibly tell the kernel).
+Priorities are implemented by allocating some space for each +priority. When doing priority-based operations, libev usually has to +linearly search all the priorities.
+