lcnt(3erl) Erlang Module Definition lcnt(3erl)
NAME
lcnt - A runtime system Lock Profiling tool.
DESCRIPTION
The lcnt module is used to profile the internal ethread locks in the
Erlang Runtime System. With lcnt enabled, internal counters in the run-
time system are updated each time a lock is taken. The counters stores
information about the number of acquisition tries and the number of
collisions that has occurred during the acquisition tries. The counters
also record the waiting time a lock has caused for a blocked thread
when a collision has occurred.
The data produced by the lock counters will give an estimate on how
well the runtime system will behave from a parallelizable view point
for the scenarios tested. This tool was mainly developed to help Erlang
runtime developers iron out potential and generic bottlenecks.
Locks in the emulator are named after what type of resource they pro-
tect and where in the emulator they are initialized, those are lock
'classes'. Most of those locks are also instantiated several times, and
given unique identifiers, to increase locking granularity. Typically an
instantiated lock protects a disjunct set of the resource, for example
ets tables, processes or ports. In other cases it protects a specific
range of a resource, for example pix_lock which protects index to
process mappings, and is given a unique number within the class. A
unique lock in lcnt is referenced by a name (class) and an identifier:
{Name, Id}.
Some locks in the system are static and protects global resources, for
example bif_timers and the run_queue locks. Other locks are dynamic and
not necessarily long lived, for example process locks and ets-table
locks. The statistics data from short lived locks can be stored sepa-
rately when the locks are deleted. This behavior is by default turned
off to save memory but can be turned on via lcnt:rt_opt({copy_save,
true}). The lcnt:apply/1,2,3 functions enables this behavior during
profiling.
EXPORTS
start() -> {ok, Pid} | {error, {already_started, Pid}}
Types:
Pid = pid()
Starts the lock profiler server. The server only act as a medium
for the user and performs filtering and printing of data col-
lected by lcnt:collect/1.
stop() -> ok
Stops the lock profiler server.
collect() -> ok
Same as collect(node()).
collect(Node) -> ok
Types:
Node = node()
Collects lock statistics from the runtime system. The function
starts a server if it is not already started. It then populates
the server with lock statistics. If the server held any lock
statistics data before the collect then that data is lost.
clear() -> ok
Same as clear(node()).
clear(Node) -> ok
Types:
Node = node()
Clears the internal lock statistics from the runtime system.
This does not clear the data on the server only on runtime sys-
tem. All counters for static locks are zeroed, all dynamic locks
currently alive are zeroed and all saved locks now destroyed are
removed. It also resets the duration timer.
conflicts() -> ok
Same as conflicts([]).
conflicts([Option]) -> ok
Types:
Option = {sort, Sort} | {reverse, bool()} | {thresholds,
[Thresholds]} | {print, [Print | {Print, integer()}]} |
{max_locks, MaxLocks} | {combine, bool()}
Sort = name | id | type | tries | colls | ratio | time | en-
try
Thresholds = {tries, integer()} | {colls, integer()} | {time,
integer()}
Print = name | id | type | entry | tries | colls | ratio |
time | duration
MaxLocks = integer() | none
Prints a list of internal locks and its statistics.
For option description, see lcnt:inspect/2.
locations() -> ok
Same as locations([]).
locations([Option]) -> ok
Types:
Option = {sort, Sort} | {thresholds, [Thresholds]} | {print,
[Print | {Print, integer()}]} | {max_locks, MaxLocks} | {com-
bine, bool()}
Sort = name | id | type | tries | colls | ratio | time | en-
try
Thresholds = {tries, integer()} | {colls, integer()} | {time,
integer()}
Print = name | id | type | entry | tries | colls | ratio |
time | duration
MaxLocks = integer() | none
Prints a list of internal lock counters by source code loca-
tions.
For option description, see lcnt:inspect/2.
inspect(Lock) -> ok
Same as inspect(Lock, []).
inspect(Lock, [Option]) -> ok
Types:
Lock = Name | {Name, Id | [Id]}
Name = atom() | pid() | port()
Id = atom() | integer() | pid() | port()
Option = {sort, Sort} | {thresholds, [Thresholds]} | {print,
[Print | {Print, integer()}]} | {max_locks, MaxLocks} | {com-
bine, bool()} | {locations, bool()}
Sort = name | id | type | tries | colls | ratio | time
Thresholds = {tries, integer()} | {colls, integer()} | {time,
integer()}
Print = name | id | type | entry | tries | colls | ratio |
time | duration
MaxLocks = integer() | none
Prints a list of internal lock counters for a specific lock.
Lock Name and Id for ports and processes are interchangeable
with the use of lcnt:swap_pid_keys/0 and is the reason why pid()
and port() options can be used in both Name and Id space. Both
pids and ports are special identifiers with stripped creation
and can be recreated with lcnt:pid/2,3 and lcnt:port/1,2.
Option description:
{combine, bool()}:
Combine the statistics from different instances of a lock
class.
Default: true
{locations, bool()}:
Print the statistics by source file and line numbers.
Default: false
{max_locks, MaxLocks}:
Maximum number of locks printed or no limit with none.
Default: 20
{print, PrintOptions}:
Printing options:
name:
Named lock or named set of locks (classes). The same name
used for initializing the lock in the VM.
id:
Internal id for set of locks, not always unique. This
could be table name for ets tables (db_tab), port id for
ports, integer identifiers for allocators, etc.
type:
Type of lock: rw_mutex, mutex, spinlock, rw_spinlock or
proclock.
entry:
In combination with {locations, true} this option prints
the lock operations source file and line number entry-
points along with statistics for each entry.
tries:
Number of acquisitions of this lock.
colls:
Number of collisions when a thread tried to acquire this
lock. This is when a trylock is EBUSY, a write try on read
held rw_lock, a try read on write held rw_lock, a thread
tries to lock an already locked lock. (Internal states su-
pervises this).
ratio:
The ratio between the number of collisions and the number
of tries (acquisitions) in percentage.
time:
Accumulated waiting time for this lock. This could be
greater than actual wall clock time, it is accumulated for
all threads. Trylock conflicts does not accumulate time.
duration:
Percentage of accumulated waiting time of wall clock time.
This percentage can be higher than 100% since accumulated
time is from all threads.
Default: [name,id,tries,colls,ratio,time,duration]
{reverse, bool()}:
Reverses the order of sorting.
Default: false
{sort, Sort}:
Column sorting orders.
Default: time
{thresholds, Thresholds}:
Filtering thresholds. Anything values above the threshold
value are passed through.
Default: [{tries, 0}, {colls, 0}, {time, 0}]
information() -> ok
Prints lcnt server state and generic information about collected
lock statistics.
swap_pid_keys() -> ok
Swaps places on Name and Id space for ports and processes.
load(Filename) -> ok
Types:
Filename = filename()
Restores previously saved data to the server.
save(Filename) -> ok
Types:
Filename = filename()
Saves the collected data to file.
CONVENIENCE FUNCTIONS
The following functions are used for convenience.
EXPORTS
apply(Fun) -> term()
Types:
Fun = fun()
Same as apply(Fun, []).
apply(Fun, Args) -> term()
Types:
Fun = fun()
Args = [term()]
Same as apply(Module, Function, Args).
apply(Module, Function, Args) -> term()
Types:
Module = atom()
Function = atom()
Args = [term()]
Clears the lock counters and then setups the instrumentation to
save all destroyed locks. After setup the function is called,
passing the elements in Args as arguments. When the function re-
turns the statistics are immediately collected to the server.
After the collection the instrumentation is returned to its pre-
vious behavior. The result of the applied function is returned.
Warning:
This function should only be used for micro-benchmarks; it sets
copy_save to true for the duration of the call, which can
quickly lead to running out of memory.
pid(Id, Serial) -> pid()
Same as pid(node(), Id, Serial).
pid(Node, Id, Serial) -> pid()
Types:
Node = node()
Id = integer()
Serial = integer()
Creates a process id with creation 0.
port(Id) -> port()
Same as port(node(), Id).
port(Node, Id) -> port()
Types:
Node = node()
Id = integer()
Creates a port id with creation 0.
INTERNAL RUNTIME LOCK COUNTER CONTROLLERS
The following functions control the behavior of the internal counters.
EXPORTS
rt_collect() -> [lock_counter_data()]
Same as rt_collect(node()).
rt_collect(Node) -> [lock_counter_data()]
Types:
Node = node()
Returns a list of raw lock counter data.
rt_clear() -> ok
Same as rt_clear(node()).
rt_clear(Node) -> ok
Types:
Node = node()
Clear the internal counters. Same as lcnt:clear(Node).
rt_mask() -> [category_atom()]
Same as rt_mask(node()).
rt_mask(Node) -> [category_atom()]
Types:
Node = node()
Refer to rt_mask/2 for a list of valid categories. All cate-
gories are enabled by default.
rt_mask(Categories) -> ok | {error, copy_save_enabled}
Types:
Categories = [atom()]
Same as rt_mask(node(), Categories).
rt_mask(Node, Categories) -> ok | {error, copy_save_enabled}
Types:
Node = node()
Categories = [atom()]
Sets the lock category mask to the given categories.
This will fail if the copy_save option is enabled; see
lcnt:rt_opt/2.
Valid categories are:
* allocator
* db (ETS tables)
* debug
* distribution
* generic
* io
* process
* scheduler
This list is subject to change at any time, as is the category
any given lock may belong to.
rt_opt({Type, bool()}) -> bool()
Same as rt_opt(node(), {Type, Opt}).
rt_opt(Node, {Type, bool()}) -> bool()
Types:
Node = node()
Type = copy_save | process_locks
Option description:
{copy_save, bool()}:
Retains the statistics of destroyed locks.
Default: false
Warning:
This option will use a lot of memory when enabled, which must
be reclaimed with lcnt:rt_clear. Note that it makes no dis-
tinction between locks that were destroyed and locks for which
counting was disabled, so enabling this option will disable
changes to the lock category mask.
{process_locks, bool()}:
Profile process locks, equal to adding process to the lock
category mask; see lcnt:rt_mask/2
Default: true
SEE ALSO
LCNT User's Guide
Ericsson AB tools 3.4 lcnt(3erl)