[ViewVC] Diff of: cvs/libev/ev.pod

Comparing libev/ev.pod (file contents):
Revision 1.281 by root, Tue Mar 9 08:46:17 2010 UTC vs.
Revision 1.285 by root, Tue Mar 16 00:20:17 2010 UTC

 So when you encounter spurious, unexplained daemon exits, make sure you
 ignore SIGPIPE (and maybe make sure you log the exit status of your daemon
 somewhere, as that would have given you a big clue).
+=head3 The special problem of accept()ing when you can't
+Many implementations of the POSIX C<accept> function (for example,
+found in port-2004 Linux) have the peculiar behaviour of not removing a
+connection from the pending queue in all error cases.
+For example, larger servers often run out of file descriptors (because
+of resource limits), causing C<accept> to fail with C<ENFILE> but not
+rejecting the connection, leading to libev signalling readiness on
+the next iteration again (the connection still exists after all), and
+typically causing the program to loop at 100% CPU usage.
+Unfortunately, the set of errors that cause this issue differs between
+operating systems, there is usually little the app can do to remedy the
+situation, and no known thread-safe method of removing the connection to
+cope with overload is known (to me).
+One of the easiest ways to handle this situation is to just ignore it
+- when the program encounters an overload, it will just loop until the
+situation is over. While this is a form of busy waiting, no OS offers an
+event-based way to handle this situation, so it's the best one can do.
+A better way to handle the situation is to log any errors other than
+C<EAGAIN> and C<EWOULDBLOCK>, making sure not to flood the log with such
+messages, and continue as usual, which at least gives the user an idea of
+what could be wrong ("raise the ulimit!"). For extra points one could stop
+the C<ev_io> watcher on the listening fd "for a while", which reduces CPU
+usage.
+If your program is single-threaded, then you could also keep a dummy file
+descriptor for overload situations (e.g. by opening F</dev/null>), and
+when you run into C<ENFILE> or C<EMFILE>, close it, run C<accept>,
+close that fd, and create a new dummy fd. This will gracefully refuse
+clients under typical overload conditions.
+The last way to handle it is to simply log the error and C<exit>, as
+is often done with C<malloc> failures, but this results in an easy
+opportunity for a DoS attack.
 =head3 Watcher-Specific Functions
 =over 4
 fine.
 If your embedding application does not need any priorities, defining these
 both to C<0> will save some memory and CPU.
-=item EV_PERIODIC_ENABLE
+=item EV_PERIODIC_ENABLE, EV_IDLE_ENABLE, EV_EMBED_ENABLE, EV_STAT_ENABLE,
+EV_PREPARE_ENABLE, EV_CHECK_ENABLE, EV_FORK_ENABLE, EV_SIGNAL_ENABLE,
+EV_ASYNC_ENABLE, EV_CHILD_ENABLE.
-If undefined or defined to be C<1>, then periodic timers are supported. If
+If undefined or defined to be C<1> (and the platform supports it), then
-defined to be C<0>, then they are not. Disabling them saves a few kB of
+the respective watcher type is supported. If defined to be C<0>, then it
-code.
+is not. Disabling watcher types mainly saves codesize.
-=item EV_IDLE_ENABLE
+=item EV_FEATURES
-If undefined or defined to be C<1>, then idle watchers are supported. If
-defined to be C<0>, then they are not. Disabling them saves a few kB of
-code.
-=item EV_EMBED_ENABLE
-If undefined or defined to be C<1>, then embed watchers are supported. If
-defined to be C<0>, then they are not. Embed watchers rely on most other
-watcher types, which therefore must not be disabled.
-=item EV_STAT_ENABLE
-If undefined or defined to be C<1>, then stat watchers are supported. If
-defined to be C<0>, then they are not.
-=item EV_FORK_ENABLE
-If undefined or defined to be C<1>, then fork watchers are supported. If
-defined to be C<0>, then they are not.
-=item EV_ASYNC_ENABLE
-If undefined or defined to be C<1>, then async watchers are supported. If
-defined to be C<0>, then they are not.
-=item EV_MINIMAL
 If you need to shave off some kilobytes of code at the expense of some
-speed (but with the full API), define this symbol to C<1>. Currently this
+speed (but with the full API), you can define this symbol to request
-is used to override some inlining decisions, saves roughly 30% code size
+certain subsets of functionality. The default is to enable all features
-on amd64. It also selects a much smaller 2-heap for timer management over
+that can be enabled on the platform.
-the default 4-heap.
-You can save even more by disabling watcher types you do not need
+Note that using autoconf will usually override most of the features, so
-and setting C<EV_MAXPRI> == C<EV_MINPRI>. Also, disabling C<assert>
+using this symbol makes sense mostly when embedding libev.
-(C<-DNDEBUG>) will usually reduce code size a lot.
-Defining C<EV_MINIMAL> to C<2> will additionally reduce the core API to
+A typical way to use this symbol is to define it to C<0> (or to a bitset
-provide a bare-bones event library. See C<ev.h> for details on what parts
+with some broad features you want) and then selectively re-enable
-of the API are still available, and do not complain if this subset changes
+additional parts you want, for example if you want everything minimal,
-over time.
+but multiple event loop support, async and child watchers and the poll
+backend, use this:
+   #define EV_FEATURES 0
+   #define EV_MULTIPLICITY 1
+   #define EV_USE_POLL 1
+   #define EV_CHILD_ENABLE 1
+   #define EV_ASYNC_ENABLE 1
+The actual value is a bitset, it can be a combination of the following
+values:
+=over 4
+=item C<1> - faster/larger code
+Use larger code to speed up some operations.
+Currently this is used to override some inlining decisions (enlarging the  roughly
+30% code size on amd64.
+Also disables C<assert>'s in the code, unless you define C<NDEBUG>
+explicitly to C<0>.
+Use of compiler flags such as C<-Os> with gcc that optimise for size are
+recommended when disabling this feature.
+=item C<2> - faster/larger data structures
+Replaces the small 2-heap for timer management by a faster 4-heap, larger
+hash table sizes and so on. This will usually further increase codesize
+and can additionally have an effect on the size of data structures at
+runtime.
+=item C<4> - full API configuration
+This enables priorities (sets C<EV_MAXPRI>=2 and C<EV_MINPRI>=-2), and
+enables multiplicity (C<EV_MULTIPLICITY>=1).
+It also enables a lot of the "lesser used" core API functions. See C<ev.h>
+for details on which parts of the API are still available without this
+feature, and do not complain if this subset changes over time.
+=item C<8> - enable all optional watcher types
+Enables all optional watcher types.  If you want to selectively enable
+only some watcher types other than I/O and timers (e.g. prepare,
+embed, async, child...) you can enable them manually by defining
+C<EV_watchertype_ENABLE> to C<1> instead.
+=item C<16> - enable all backends
+This enables all backends - without this feature, you need to enable at
+least one backend manually (C<EV_USE_SELECT> is a good choice).
+=item C<32> - enable OS-specific "helper" APIs
+Enable inotify, eventfd, signalfd and similar OS-specific helper APIs by
+default.
+=back
+Compiling with C<gcc -Os -DEV_STANDALONE -DEV_USE_EPOLL=1 -DEV_FEATURES=0>
+reduces the compiled size of libev from 24.7Kb to 6.5Kb on my GNU/Linux
+amd64 system, while still giving you I/O watchers, timers and monotonic
+clock support.
+With an intelligent-enough linker (gcc+binutils are intelligent enough
+when you use C<-Wl,--gc-sections -ffunction-sections>) functions unused by
+your program might be left out as well - a binary starting a timer and an
+I/O watcher then might come out at only 5Kb.
 =item EV_AVOID_STDIO
 If this is set to C<1> at compiletime, then libev will avoid using stdio
 functions (printf, scanf, perror etc.). This will increase the codesize
 statically allocates some 12-24 bytes per signal number.
 =item EV_PID_HASHSIZE
 C<ev_child> watchers use a small hash table to distribute workload by
-pid. The default size is C<16> (or C<1> with C<EV_MINIMAL>), usually more
+pid. The default size is C<16> (or C<1> with C<EV_FEATURES> disabled),
-than enough. If you need to manage thousands of children you might want to
+usually more than enough. If you need to manage thousands of children you
-increase this value (I<must> be a power of two).
+might want to increase this value (I<must> be a power of two).
 =item EV_INOTIFY_HASHSIZE
 C<ev_stat> watchers use a small hash table to distribute workload by
-inotify watch id. The default size is C<16> (or C<1> with C<EV_MINIMAL>),
+inotify watch id. The default size is C<16> (or C<1> with C<EV_FEATURES>
-usually more than enough. If you need to manage thousands of C<ev_stat>
+disabled), usually more than enough. If you need to manage thousands of
-watchers you might want to increase this value (I<must> be a power of
+C<ev_stat> watchers you might want to increase this value (I<must> be a
-two).
+power of two).
 =item EV_USE_4HEAP
 Heaps are not very cache-efficient. To improve the cache-efficiency of the
 timer and periodics heaps, libev uses a 4-heap when this symbol is defined
 to C<1>. The 4-heap uses more complicated (longer) code but has noticeably
 faster performance with many (thousands) of watchers.
-The default is C<1> unless C<EV_MINIMAL> is set in which case it is C<0>
+The default is C<1>, unless C<EV_FEATURES> overrides it, in which case it
-(disabled).
+will be C<0>.
 =item EV_HEAP_CACHE_AT
 Heaps are not very cache-efficient. To improve the cache-efficiency of the
 timer and periodics heaps, libev can cache the timestamp (I<at>) within
 the heap structure (selected by defining C<EV_HEAP_CACHE_AT> to C<1>),
 which uses 8-12 bytes more per watcher and a few hundred bytes more code,
 but avoids random read accesses on heap changes. This improves performance
 noticeably with many (hundreds) of watchers.
-The default is C<1> unless C<EV_MINIMAL> is set in which case it is C<0>
+The default is C<1>, unless C<EV_FEATURES> overrides it, in which case it
-(disabled).
+will be C<0>.
 =item EV_VERIFY
 Controls how much internal verification (see C<ev_loop_verify ()>) will
 be done: If set to C<0>, no internal verification code will be compiled
 called. If set to C<2>, then the internal verification code will be
 called once per loop, which can slow down libev. If set to C<3>, then the
 verification code will be called very frequently, which will slow down
 libev considerably.
-The default is C<1>, unless C<EV_MINIMAL> is set, in which case it will be
+The default is C<1>, unless C<EV_FEATURES> overrides it, in which case it
-C<0>.
+will be C<0>.
 =item EV_COMMON
 By default, all watchers have a C<void *data> member. By redefining
 this macro to a something else you can include more and other types of
 file.
 The usage in rxvt-unicode is simpler. It has a F<ev_cpp.h> header file
 that everybody includes and which overrides some configure choices:
-   #define EV_MINIMAL 1
+   #define EV_FEATURES 0
-   #define EV_USE_POLL 0
+   #define EV_USE_SELECT 1
-   #define EV_MULTIPLICITY 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"
 And a F<ev_cpp.C> implementation file that contains libev proper and is compiled:

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Comparing libev/ev.pod (file contents): Revision 1.281 by root, Tue Mar 9 08:46:17 2010 UTC vs. Revision 1.285 by root, Tue Mar 16 00:20:17 2010 UTC

Diff Legend

Comparing libev/ev.pod (file contents):
Revision 1.281 by root, Tue Mar 9 08:46:17 2010 UTC vs.
Revision 1.285 by root, Tue Mar 16 00:20:17 2010 UTC