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Persistent terms are useful for storing Erlang terms that are never
or infrequently updated. They have the following advantages:
* Constant time access. A persistent term is not copied when it is
looked up. The constant factor is lower than for ETS, and no locks
are taken when looking up a term.
* Persistent terms are not copied in garbage collections.
* There is only ever one copy of a persistent term (until it is
deleted). That makes them useful for storing configuration data
that needs to be easily accessible by all processes.
Persistent terms have the following drawbacks:
* Updates are expensive. The hash table holding the keys for the
persistent terms are updated whenever a persistent term is added,
updated or deleted.
* Updating or deleting a persistent term triggers a "global GC", which
will schedule a heap scan of all processes to search the heap of all
processes for the deleted term. If a process still holds a reference
to the deleted term, the process will be garbage collected and the
term copied to the heap of the process. This global GC can make the
system less responsive for some time.
Three BIFs (implemented in C in the emulator) is the entire
interface to the persistent term functionality:
* put(Key, Value) to store a persistent term.
* get(Key) to look up a persistent term.
* erase(Key) to delete a persistent term.
There are also two additional BIFs to obtain information about
persistent terms:
* info() to return a map with information about persistent terms.
* get() to return a list of a {Key,Value} tuples for all persistent
terms. (The values are not copied.)
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Communication between Erlang processes has conceptually always been
performed through asynchronous signaling. The runtime system
implementation has however previously preformed most operation
synchronously. In a system with only one true thread of execution, this
is not problematic (often the opposite). In a system with multiple threads
of execution (as current runtime system implementation with SMP support)
it becomes problematic. This since it often involves locking of structures
when updating them which in turn cause resource contention. Utilizing
true asynchronous communication often avoids these resource contention
issues.
The case that triggered this change was contention on the link lock due
to frequent updates of the monitor trees during communication with a
frequently used server. The signal order delivery guarantees of the
language makes it hard to change the implementation of only some signals
to use true asynchronous signaling. Therefore the implementations
of (almost) all signals have been changed.
Currently the following signals have been implemented as true
asynchronous signals:
- Message signals
- Exit signals
- Monitor signals
- Demonitor signals
- Monitor triggered signals (DOWN, CHANGE, etc)
- Link signals
- Unlink signals
- Group leader signals
All of the above already defined as asynchronous signals in the
language. The implementation of messages signals was quite
asynchronous to begin with, but had quite strict delivery constraints
due to the ordering guarantees of signals between a pair of processes.
The previously used message queue partitioned into two halves has been
replaced by a more general signal queue partitioned into three parts
that service all kinds of signals. More details regarding the signal
queue can be found in comments in the erl_proc_sig_queue.h file.
The monitor and link implementations have also been completely replaced
in order to fit the new asynchronous signaling implementation as good
as possible. More details regarding the new monitor and link
implementations can be found in the erl_monitor_link.h file.
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This improves the latency of file operations as dirty schedulers
are a bit more eager to run jobs than async threads, and use a
single global queue rather than per-thread queues, eliminating the
risk of a job stalling behind a long-running job on the same thread
while other async threads sit idle.
There's no such thing as a free lunch though; the lowered latency
comes at the cost of increased busy-waiting which may have an
adverse effect on some applications. This behavior can be tweaked
with the +sbwt flag, but unfortunately it affects all types of
schedulers and not just dirty ones. We plan to add type-specific
flags at a later stage.
sendfile has been moved to inet_drv to lessen the effect of a nasty
race; the cooperation between inet_drv and efile has never been
airtight and the socket dying at the wrong time (Regardless of
reason) could result in fd aliasing. Moving it to the inet driver
makes it impossible to trigger this by closing the socket in the
middle of a sendfile operation, while still allowing it to be
aborted -- something that can't be done if it stays in the file
driver.
The race still occurs if the controlling process dies in the short
window between dispatching the sendfile operation and the dup(2)
call in the driver, but it's much less likely to happen now.
A proper fix is in the works.
--
Notable functional differences:
* The use_threads option for file:sendfile/5 no longer has any
effect.
* The file-specific DTrace probes have been removed. The same
effect can be achieved with normal tracing together with the
nif__entry/nif__return probes to track scheduling.
--
OTP-14256
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Update compiler documentation and remove superfluous erlc flags.
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Add the possibility to use modules as trace data receivers. The functions
in the module have to be nifs as otherwise complex trace probes will be
very hard to handle (complex means trace probes for ports for example).
This commit changes the way that the ptab->tracer field works from always
being an immediate, to now be NIL if no tracer is present or else be
the tuple {TracerModule, TracerState} where TracerModule is an atom that
is later used to lookup the appropriate tracer callbacks to call and
TracerState is just passed to the tracer callback. The default process and
port tracers have been rewritten to use the new API.
This commit also changes the order which trace messages are delivered to the
potential tracer process. Any enif_send done in a tracer module may be delayed
indefinitely because of lock order issues. If a message is delayed any other
trace message send from that process is also delayed so that order is preserved
for each traced entity. This means that for some trace events (i.e. send/receive)
the events may come in an unintuitive order (receive before send) to the
trace receiver. Timestamps are taken when the trace message is generated so
trace messages from differented processes may arrive with the timestamp
out of order.
Both the erlang:trace and seq_trace:set_system_tracer accept the new tracer
module tracers and also the backwards compatible arguments.
OTP-10267
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as a system process with preloaded code.
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Needed by Dialyzer.
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Using the low-level BEAM instructions, we can loop over each message in
the process queue and removes the first message that matches, without
receiving them all to later send them back to itself.
The function prim_eval:'receive'/2 is equivalent to the
following pseudo-code:
'receive'(F, T) ->
RESET MESSAGE QUEUE POINTER,
LOOP:
case PEEK CURRENT MESSAGE WITH TIMEOUT T of
{ok,Msg} ->
case F(Msg) of
nomatch ->
DECREMENT TIMEOUT T,
ADVANCE MESSAGE QUEUE POINTER,
GOTO LOOP;
Result ->
RESET MESSAGE QUEUE POINTER,
Result
end;
timeout ->
RESET MESSAGE QUEUE POINTER,
timeout
end.
To not break Dialyzer and other tools, we use a stub Erlang module which
abstract code is forcefully inserted into prim_inet.erl afterwards
compilation.
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rickard/r16/port-optimizations/OTP-10336
* rickard/port-optimizations/OTP-10336:
Change annotate level for emacs-22 in cerl
Update etp-commands
Add documentation on communication in Erlang
Add support for busy port message queue
Add driver callback epilogue
Implement true asynchronous signaling between processes and ports
Add erl_drv_[send|output]_term
Move busy port flag
Use rwlock for driver list
Optimize management of port tasks
Improve configuration of process and port tables
Remove R9 compatibility features
Use ptab functionality also for ports
Prepare for use of ptab functionality also for ports
Atomic port state
Generalize process table implementation
Implement functionality for delaying thread progress from unmanaged threads
Conflicts:
erts/doc/src/erl_driver.xml
erts/doc/src/erlang.xml
erts/emulator/beam/beam_bif_load.c
erts/emulator/beam/beam_bp.c
erts/emulator/beam/beam_emu.c
erts/emulator/beam/bif.c
erts/emulator/beam/copy.c
erts/emulator/beam/erl_alloc.c
erts/emulator/beam/erl_alloc.types
erts/emulator/beam/erl_bif_info.c
erts/emulator/beam/erl_bif_port.c
erts/emulator/beam/erl_bif_trace.c
erts/emulator/beam/erl_init.c
erts/emulator/beam/erl_message.c
erts/emulator/beam/erl_port_task.c
erts/emulator/beam/erl_process.c
erts/emulator/beam/erl_process.h
erts/emulator/beam/erl_process_lock.c
erts/emulator/beam/erl_trace.c
erts/emulator/beam/export.h
erts/emulator/beam/global.h
erts/emulator/beam/io.c
erts/emulator/sys/unix/sys.c
erts/emulator/sys/vxworks/sys.c
erts/emulator/test/port_SUITE.erl
erts/etc/unix/cerl.src
erts/preloaded/ebin/erlang.beam
erts/preloaded/ebin/prim_inet.beam
erts/preloaded/src/prim_inet.erl
lib/hipe/cerl/erl_bif_types.erl
lib/kernel/doc/src/inet.xml
lib/kernel/src/inet.erl
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OTP-10106
OTP-10107
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Remove cleartool invocation and adapt docs to reflect git transition.
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