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authorRickard Green <[email protected]>2016-05-02 16:02:02 +0200
committerRickard Green <[email protected]>2016-05-11 16:28:27 +0200
commit8cbf3bd3b9d552423812df4acc7a40d9a23fdeae (patch)
tree9bde60d5088188c656367d3f3d11b19b15e3b42c /erts/doc
parent0aa5f873fdfb391a455bc134256b7c464ffa161b (diff)
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Add better support for communication with a process executing dirty NIF
- Termination of a process... - Modify trace flags of process... - Process info on process... - Register/unregister of name on process... - Set group leader on process... ... while it is executing a dirty NIF.
Diffstat (limited to 'erts/doc')
-rw-r--r--erts/doc/src/erl_nif.xml245
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, &amp;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, &amp;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, &amp;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>