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Diffstat (limited to 'erts/doc/src/erl_nif.xml')
| -rw-r--r-- | erts/doc/src/erl_nif.xml | 245 |
1 files changed, 180 insertions, 65 deletions
diff --git a/erts/doc/src/erl_nif.xml b/erts/doc/src/erl_nif.xml index ef64ac66dc..fefec3eeb6 100644 --- a/erts/doc/src/erl_nif.xml +++ b/erts/doc/src/erl_nif.xml @@ -138,29 +138,6 @@ ok automatically unloaded when the module code that it belongs to is purged by the code server.</p> - <p><marker id="lengthy_work"/> - As mentioned in the <seealso marker="#WARNING">warning</seealso> text at - the beginning of this document it is of vital importance that a native function - return relatively quickly. It is hard to give an exact maximum amount - of time that a native function is allowed to work, but as a rule of thumb - a well-behaving native function should return to its caller before a - millisecond has passed. This can be achieved using different approaches. - If you have full control over the code to execute in the native - function, the best approach is to divide the work into multiple chunks of - work and call the native function multiple times, either directly from Erlang code - or by having a native function schedule a future NIF call via the - <seealso marker="#enif_schedule_nif"> enif_schedule_nif</seealso> function. Function - <seealso marker="#enif_consume_timeslice">enif_consume_timeslice</seealso> can be - used to help with such work division. In some cases, however, this might not - be possible, e.g. when calling third-party libraries. Then you typically want - to dispatch the work to another thread, return - from the native function, and wait for the result. The thread can send - the result back to the calling thread using message passing. Information - about thread primitives can be found below. If you have built your system - with <em>the currently experimental</em> support for dirty schedulers, - you may want to try out this functionality by dispatching the work to a - <seealso marker="#dirty_nifs">dirty NIF</seealso>, - which does not have the same duration restriction as a normal NIF.</p> </description> <section> <title>FUNCTIONALITY</title> @@ -328,37 +305,161 @@ ok </list></p> </item> - <tag>Long-running NIFs</tag> - <item><p><marker id="dirty_nifs"/>Native functions - <seealso marker="#lengthy_work"> - must normally run quickly</seealso>, as explained earlier in this document. They - generally should execute for no more than a millisecond. But not all native functions - can execute so quickly; for example, functions that encrypt large blocks of data or - perform lengthy file system operations can often run for tens of seconds or more.</p> - <p>If the functionality of a long-running NIF can be split so that its work can be - achieved through a series of shorter NIF calls, the application can either make that series - of NIF calls from the Erlang level, or it can call a NIF that first performs a chunk of the - work, then invokes the <seealso marker="#enif_schedule_nif">enif_schedule_nif</seealso> - function to schedule another NIF call to perform the next chunk. The final call scheduled - in this manner can then return the overall result. Breaking up a long-running function in - this manner enables the VM to regain control between calls to the NIFs, thereby avoiding - degraded responsiveness, scheduler load balancing problems, and other strange behaviours.</p> - <p>A NIF that cannot be split and cannot execute in a millisecond or less is called a "dirty NIF" - because it performs work that the Erlang runtime cannot handle cleanly. - <em>Note that the dirty NIF functionality described here is experimental</em> and that you have to - enable support for dirty schedulers when building OTP in order to try the functionality out. - Applications that make use of such functions must indicate to the runtime that the functions are - dirty so they can be handled specially. To schedule a dirty NIF for execution, the - appropriate flags value can be set for the NIF in its <seealso marker="#ErlNifFunc">ErlNifFunc</seealso> - entry, or the application can call <seealso marker="#enif_schedule_nif">enif_schedule_nif</seealso>, - passing to it a pointer to the dirty NIF to be executed and indicating with the <c>flags</c> - argument whether it expects the operation to be CPU-bound or I/O-bound.</p> - <note><p>Dirty NIF support is available only when the emulator is configured with dirty - schedulers enabled. This feature is currently disabled by default. The Erlang runtime - without SMP support currently do not support dirty schedulers even when the dirty - scheduler support has been enabled. To check at runtime for the presence - of dirty scheduler threads, code can use the <seealso marker="#enif_system_info"><c> - enif_system_info()</c></seealso> API function.</p></note> + <tag><marker id="lengthy_work"/>Long-running NIFs</tag> + + <item><p> + As mentioned in the <seealso marker="#WARNING">warning</seealso> text at + the beginning of this document it is of <em>vital importance</em> that a + native function return relatively quickly. It is hard to give an exact + maximum amount of time that a native function is allowed to work, but as a + rule of thumb a well-behaving native function should return to its caller + before a millisecond has passed. This can be achieved using different + approaches. If you have full control over the code to execute in the + native function, the best approach is to divide the work into multiple + chunks of work and call the native function multiple times. In some + cases this might however not always be possible, e.g. when calling + third-party libraries.</p> + + <p>The + <seealso marker="#enif_consume_timeslice">enif_consume_timeslice()</seealso> + function can be used to inform the runtime system about the lenght of the + NIF call. It should typically always be used unless the NIF executes very + quickly.</p> + + <p>If the NIF call is too lenghty one needs to handle this in one of the + following ways in order to avoid degraded responsiveness, scheduler load + balancing problems, and other strange behaviours:</p> + + <taglist> + <tag>Yielding NIF</tag> + <item> + <p> + If the functionality of a long-running NIF can be split so that + its work can be achieved through a series of shorter NIF calls, + the application can either make that series of NIF calls from the + Erlang level, or it can call a NIF that first performs a chunk of + the work, then invokes the + <seealso marker="#enif_schedule_nif">enif_schedule_nif</seealso> + function to schedule another NIF call to perform the next chunk. + The final call scheduled in this manner can then return the + overall result. Breaking up a long-running function in + this manner enables the VM to regain control between calls to the + NIFs. + </p> + <p> + This approach is always preferred over the other alternatives + described below. This both from a performance perspective and + a system characteristics perspective. + </p> + </item> + + <tag>Threaded NIF</tag> + <item> + <p> + This is accomplished by dispatching the work to another thread + managed by the NIF library, return from the NIF, and wait for the + result. The thread can send the result back to the Erlang + process using <seealso marker="#enif_send">enif_send</seealso>. + Information about thread primitives can be found below. + </p> + </item> + + <tag><marker id="dirty_nifs"/>Dirty NIF</tag> + <item> + + <note> + <p> + <em>The dirty NIF functionality described here + is experimental</em>. Dirty NIF support is available only when + the emulator is configured with dirty schedulers enabled. This + feature is currently disabled by default. The Erlang runtime + without SMP support do not support dirty schedulers even when + the dirty scheduler support has been enabled. To check at + runtime for the presence of dirty scheduler threads, code can + use the + <seealso marker="#enif_system_info"><c>enif_system_info()</c></seealso> + API function. + </p> + </note> + + <p> + A NIF that cannot be split and cannot execute in a millisecond or + less is called a "dirty NIF" because it performs work that the + Erlang runtime cannot handle cleanly. Applications that make use + of such functions must indicate to the runtime that the functions + are dirty so they can be handled specially. To schedule a dirty + NIF for execution, the appropriate flags value can be set for the + NIF in its <seealso marker="#ErlNifFunc"><c>ErlNifFunc</c></seealso> + entry, or the application can call + <seealso marker="#enif_schedule_nif"><c>enif_schedule_nif</c></seealso>, + passing to it a pointer to the dirty NIF to be executed and + indicating with the <c>flags</c> argument whether it expects the + operation to be CPU-bound or I/O-bound. A dirty NIF executing + on a dirty scheduler does not have the same duration restriction + as a normal NIF. + </p> + + <p> + While a process is executing a dirty NIF some operations that + communicate with it may take a very long time to complete. + Suspend, or garbage collection of a process executing a dirty + NIF cannot be done until the dirty NIF has returned, so other + processes waiting for such operations to complete might have to + wait for a very long time. Blocking multi scheduling, i.e., + calling + <seealso marker="erlang#system_flag_multi_scheduling"><c>erlang:system_flag(multi_scheduling, + block)</c></seealso>, might also take a very long time to + complete. This since all ongoing dirty operations on all + dirty schedulers need to complete before the the block + operation can complete. + </p> + + <p> + A lot of operations communicating with a process executing a + dirty NIF can, however, complete while it is executing the + dirty NIF. For example, retreiving information about it via + <c>process_info()</c>, setting its group leader, + register/unregister its name, etc. + </p> + + <p> + Termination of a process executing a dirty NIF can only be + completed up to a certain point while it is executing the + dirty NIF. All Erlang resources such as registered names, + ETS tables, etc will be released. All links and monitors + will be triggered. The actual execution of the NIF will + however <em>not</em> be stopped. The NIF can safely contiue + execution, allocate heap memory, etc, but it is of course better + to stop executing as soon as possible. The NIF can check + whether current process is alive or not using + <seealso marker="#enif_is_current_process_alive"><c>enif_is_current_process_alive</c></seealso>. + Communication using + <seealso marker="#enif_send"><c>enif_send</c></seealso>, + and <seealso marker="#enif_port_command"><c>enif_port_command</c></seealso> + will also be dropped when the sending process is not alive. + Deallocation of certain internal resources such as process + heap, and process control block will be delayed until the + dirty NIF has completed. + </p> + + <p>Currently known issues that are planned to be fixed:</p> + <list> + <item> + <p> + Since purging of a module currently might need to garbage + collect a process in order to determine if it has + references to the module, a process executing a dirty + NIF might delay purging for a very long time. Delaying + a purge operatin implies delaying <em>all</em> code + loding operations which might cause severe problems for + the system as a whole. + </p> + </item> + </list> + + </item> + </taglist> + </item> </taglist> </section> @@ -507,6 +608,10 @@ typedef struct { CPU-bound, its <c>flags</c> field should be set to <c>ERL_NIF_DIRTY_JOB_CPU_BOUND</c>, or for I/O-bound jobs, <c>ERL_NIF_DIRTY_JOB_IO_BOUND</c>.</p> + <note><p>If one of the + <c>ERL_NIF_DIRTY_JOB_*_BOUND</c> flags is set, and the runtime + system has no support for dirty schedulers, the runtime system + will refuse to load the NIF library.</p></note> </item> <tag><marker id="ErlNifBinary"/>ErlNifBinary</tag> <item> @@ -962,6 +1067,13 @@ typedef enum { <fsummary>Determine if a term is a binary</fsummary> <desc><p>Return true if <c>term</c> is a binary</p></desc> </func> + <func><name><ret>int</ret><nametext>enif_is_current_process_alive(ErlNifEnv* env)</nametext></name> + <fsummary>Determine if currently executing process is alive or not.</fsummary> + <desc><p>Return true if currently executing process is currently alive; otherwise + false.</p> + <p>This function can only be used from a NIF-calling thread, and with an + environment corresponding to currently executing processes.</p></desc> + </func> <func><name><ret>int</ret><nametext>enif_is_empty_list(ErlNifEnv* env, ERL_NIF_TERM term)</nametext></name> <fsummary>Determine if a term is an empty list</fsummary> <desc><p>Return true if <c>term</c> is an empty list.</p></desc> @@ -992,11 +1104,10 @@ typedef enum { <func><name><ret>int</ret><nametext>enif_is_on_dirty_scheduler(ErlNifEnv* env)</nametext></name> <fsummary>Check to see if executing on a dirty scheduler thread</fsummary> <desc> - <p>Check to see if the current NIF is executing on a dirty scheduler thread. If the - emulator is built with threading support, calling <c>enif_is_on_dirty_scheduler</c> - from within a dirty NIF returns true. It returns false when the calling NIF is a regular - NIF running on a normal scheduler thread, or when the emulator is built without threading - support.</p> + <p>Check to see if the current NIF is executing on a dirty scheduler thread. If + executing on a dirty scheduler thread true returned; otherwise false.</p> + <p>This function can only be used from a NIF-calling thread, and with an + environment corresponding to currently executing processes.</p> </desc> </func> <func><name><ret>int</ret><nametext>enif_is_pid(ErlNifEnv* env, ERL_NIF_TERM term)</nametext></name> @@ -1010,7 +1121,8 @@ typedef enum { <func><name><ret>int</ret><nametext>enif_is_port_alive(ErlNifEnv* env, ErlNifPort *port_id)</nametext></name> <fsummary>Determine if a local port is alive or not.</fsummary> <desc><p>Return true if <c>port_id</c> is currently alive.</p> - <p>This function can only be used in a from a NIF-calling thread.</p></desc> + <p>This function is only thread-safe when the emulator with SMP support is used. + It can only be used in a non-SMP emulator from a NIF-calling thread.</p></desc> </func> <func><name><ret>int</ret><nametext>enif_is_process_alive(ErlNifEnv* env, ErlNifPid *pid)</nametext></name> <fsummary>Determine if a local process is alive or not.</fsummary> @@ -1478,9 +1590,7 @@ enif_map_iterator_destroy(env, &iter); <fsummary>Send a port_command to to_port</fsummary> <desc> <p>This function works the same as <seealso marker="erlang#port_command-2">erlang:port_command/2</seealso> - except that it is always completely asynchronous. This call may return false - if it detects that the port is already dead, otherwise it will return true. - </p> + except that it is always completely asynchronous.</p> <taglist> <tag><c>env</c></tag> <item>The environment of the calling process. May not be NULL.</item> @@ -1499,7 +1609,10 @@ enif_map_iterator_destroy(env, &iter); calls to <c>enif_alloc_env</c>, <c>enif_make_copy</c>, <c>enif_port_command</c> and <c>enif_free_env</c> into one call. This optimization is only usefull when a majority of the terms are to be copied from <c>env</c> to the <c>msg_env</c>.</p> - <p>The call may return false if it detects that the command failed for some reason. Otherwise true is returned.</p> + <p>This function return true if the command was successfully sent; otherwise, + false. The call may return false if it detects that the command failed for some + reason. For example, <c>*to_port</c> does not refer to a local port, if currently + executing process, i.e. the sender, is not alive, or if <c>msg</c> is invalid.</p> <p>See also: <seealso marker="#enif_get_local_port"><c>enif_get_local_port</c></seealso>.</p> </desc> </func> @@ -1630,7 +1743,9 @@ enif_map_iterator_destroy(env, &iter); <tag><c>msg</c></tag> <item>The message term to send.</item> </taglist> - <p>Return true on success, or false if <c>*to_pid</c> does not refer to an alive local process.</p> + <p>Return true if the message was successfully sent; otherwise, false. The send + operation will fail if <c>*to_pid</c> does not refer to an alive local process, + or if currently executing process, i.e. the sender, is not alive.</p> <p>The message environment <c>msg_env</c> with all its terms (including <c>msg</c>) will be invalidated by a successful call to <c>enif_send</c>. The environment should either be freed with <seealso marker="#enif_free_env">enif_free_env</seealso> |