# 5 lcnt - The Lock Profiler

Internally in the Erlang runtime system locks are used to protect resources from being updated from multiple threads in a fatal way. Locks are necessary to ensure that the runtime system works properly but it also introduces a couple of limitations. Lock contention and locking overhead.

With lock contention we mean when one thread locks a resource and another thread, or threads, tries to acquire the same resource at the same time. The lock will deny the other thread access to the resource and the thread will be blocked from continuing its execution. The second thread has to wait until the first thread has completed its access to the resource and unlocked it. The lcnt tool measures these lock conflicts.

Locks have an inherent cost in execution time and memory space. It takes time initialize, destroy, aquiring or releasing locks. To decrease lock contention it some times necessary to use finer grained locking strategies. This will usually also increase the locking overhead and hence there is a tradeoff between lock contention and overhead. In general, lock contention increases with the number of threads running concurrently. The lcnt tool does not measure locking overhead.

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5.1 Enabling lock-counting

For investigation of locks in the emulator we use an internal tool called lcnt (short for lock-count). The VM needs to be compiled with this option enabled. To compile a lock-counting VM along with a normal VM, use:

    cd $ERL_TOP
    ./configure --enable-lock-counter

Start the lock-counting VM like this:

    $ERL_TOP/bin/erl -emu_type lcnt

To verify that lock counting is enabled check that [lock-counting] appears in the status text when the VM is started.

    Erlang/OTP 20 [erts-9.0] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:10] [hipe]
     [kernel-poll:false] [lock-counting]

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5.2 Getting started

Once you have a lock counting enabled VM the module lcnt can be used. The module is intended to be used from the current running nodes shell. To access remote nodes use lcnt:clear(Node) and lcnt:collect(Node).

All locks are continuously monitored and its statistics updated. Use lcnt:clear/0 to initially clear all counters before running any specific tests. This command will also reset the duration timer internally.

To retrieve lock statistics information, use lcnt:collect/0,1. The collect operation will start a lcnt server if it not already started. All collected data will be built into an Erlang term and uploaded to the server and a duration time will also be uploaded. This duration is the time between lcnt:clear/0,1 and lcnt:collect/0,1.

Once the data is collected to the server it can be filtered, sorted and printed in many different ways.

See the reference manual for a description of each function.

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5.3 Example of usage

From the Erlang shell:

    Erlang R13B03 (erts-5.7.4) [source] [smp:8:8] [rq:8] [async-threads:0] [hipe]
     [kernel-poll:false] [lock-counting]
    1> lcnt:rt_opt({copy_save, true}).
    false
    2> lcnt:clear(), big:bang(1000), lcnt:collect().
    ok
    3> lcnt:conflicts().
                       lock   id  #tries  #collisions  collisions [%]  time [us]  duration [%]
                      -----  --- ------- ------------ --------------- ---------- -------------
             alcu_allocator   50 4113692       158921          3.8632     215464        4.4962
                   pix_lock  256 4007140         4882          0.1218      12221        0.2550
                  run_queue    8 2287246         6949          0.3038       9825        0.2050
                  proc_main 1029 3115778        25755          0.8266       1199        0.0250
                  proc_msgq 1029 2467022         1910          0.0774       1048        0.0219
                proc_status 1029 5708439         2435          0.0427        706        0.0147
     message_pre_alloc_lock    8 2008569          134          0.0067         90        0.0019
                  timeofday    1   54065            8          0.0148         22        0.0005
                    gc_info    1    7071            7          0.0990          5        0.0001
    ok

Another way to to profile a specific function is to use lcnt:apply/3 or lcnt:apply/1 which does lcnt:clear/0 before the function and lcnt:collect/0 after its invocation. This method should only be used in micro-benchmarks since it sets copy_save to true for the duration of the function call, which may cause the emulator to run out of memory if attempted under load.

    Erlang R13B03 (erts-5.7.4) [source] [smp:8:8] [rq:8] [async-threads:0] [hipe]
     [kernel-poll:false] [lock-counting]
    1> lcnt:apply(fun() -> big:bang(1000) end).
    4384.338
    2> lcnt:conflicts().
                       lock   id  #tries  #collisions  collisions [%]  time [us]  duration [%]
                      -----  --- ------- ------------ --------------- ---------- -------------
             alcu_allocator   50 4117913       183091          4.4462     234232        5.1490
                  run_queue    8 2050398         3801          0.1854       6700        0.1473
                   pix_lock  256 4007080         4943          0.1234       2847        0.0626
                  proc_main 1028 3000178        28247          0.9415       1022        0.0225
                  proc_msgq 1028 2293677         1352          0.0589        545        0.0120
                proc_status 1028 5258029         1744          0.0332        442        0.0097
     message_pre_alloc_lock    8 2009322          147          0.0073         82        0.0018
                  timeofday    1   48616            9          0.0185         13        0.0003
                    gc_info    1    7455           12          0.1610          9        0.0002
    ok

The process locks are sorted after its class like all other locks. It is convenient to look at specific processes and ports as classes. We can do this by swapping class and class identifiers with lcnt:swap_pid_keys/0.

    3> lcnt:swap_pid_keys().
    ok
    4> lcnt:conflicts([{print, [name, tries, ratio, time]}]).
                       lock  #tries  collisions [%]  time [us]
                      ----- ------- --------------- ----------
             alcu_allocator 4117913          4.4462     234232
                  run_queue 2050398          0.1854       6700
                   pix_lock 4007080          0.1234       2847
     message_pre_alloc_lock 2009322          0.0073         82
      <nonode@nohost.660.0>   13493          1.4452         41
      <nonode@nohost.724.0>   13504          1.1404         36
      <nonode@nohost.803.0>   13181          1.6235         35
      <nonode@nohost.791.0>   13534          0.8202         22
       <nonode@nohost.37.0>    8744          5.8326         22
      <nonode@nohost.876.0>   13335          1.1174         19
      <nonode@nohost.637.0>   13452          1.3678         19
      <nonode@nohost.799.0>   13497          1.8745         18
      <nonode@nohost.469.0>   11009          2.5343         18
      <nonode@nohost.862.0>   13131          1.2566         16
      <nonode@nohost.642.0>   13216          1.7327         15
      <nonode@nohost.582.0>   13156          1.1098         15
      <nonode@nohost.622.0>   13420          0.7303         14
      <nonode@nohost.596.0>   13141          1.6437         14
      <nonode@nohost.592.0>   13346          1.2064         13
      <nonode@nohost.526.0>   13076          1.1701         13
    ok

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5.4 Example with Mnesia Transaction Benchmark

From the Erlang shell:

    Erlang R13B03 (erts-5.7.4) [source] [smp:8:8] [rq:8] [async-threads:0] [hipe]
     [kernel-poll:false] [lock-counting]

    Eshell V5.7.4  (abort with ^G)
    1> Conf=[{db_nodes, [node()]}, {driver_nodes, [node()]}, {replica_nodes, [node()]},
     {n_drivers_per_node, 10}, {n_branches, 1000}, {n_accounts_per_branch, 10},
     {replica_type, ram_copies}, {stop_after, 60000}, {reuse_history_id, true}].
    [{db_nodes,[nonode@nohost]},
     {driver_nodes,[nonode@nohost]},
     {replica_nodes,[nonode@nohost]},
     {n_drivers_per_node,10},
     {n_branches,1000},
     {n_accounts_per_branch,10},
     {replica_type,ram_copies},
     {stop_after,60000},
     {reuse_history_id,true}]
    2> mnesia_tpcb:init([{use_running_mnesia, false}|Conf]).
    ignore

Initial configuring of the benchmark is done. It is time to profile the actual benchmark and Mnesia

    3> lcnt:apply(fun() -> {ok,{time, Tps,_,_,_,_}} = mnesia_tpcb:run([{use_running_mnesia,
     true}|Conf]), Tps/60 end).
    12037.483333333334
    ok
    4> lcnt:swap_pid_keys().
    ok

The id header represents the number of unique identifiers under a class when the option {combine, true} is used (which is on by default). It will otherwise show the specific identifier. The db_tab listing shows 722287 unique locks, it is one for each ets-table created and Mnesia creates one for each transaction.

    5> lcnt:conflicts().
                       lock     id   #tries  #collisions  collisions [%]  time [us]  duration [%]
                      -----    ---  ------- ------------ --------------- ---------- -------------
             alcu_allocator     50 56355118       732662          1.3001    2934747        4.8862
                     db_tab 722287 94513441        63203          0.0669    1958797        3.2613
                  timeofday      1  2701048       175854          6.5106    1746079        2.9071
                   pix_lock    256 24306168       163214          0.6715     918309        1.5289
                  run_queue      8 11813811       152637          1.2920     357040        0.5945
     message_pre_alloc_lock      8 17671449        57203          0.3237     263043        0.4380
              mnesia_locker      4 17477633      1618548          9.2607      97092        0.1617
                  mnesia_tm      4  9891408       463788          4.6888      86353        0.1438
                    gc_info      1   823460          628          0.0763      24826        0.0413
         meta_main_tab_slot     16 41393400         7193          0.0174      11393        0.0190
     <nonode@nohost.1108.0>      4  4331412          333          0.0077       7148        0.0119
                timer_wheel      1   203185           30          0.0148       3108        0.0052
     <nonode@nohost.1110.0>      4  4291098          210          0.0049        885        0.0015
     <nonode@nohost.1114.0>      4  4294702          288          0.0067        442        0.0007
     <nonode@nohost.1113.0>      4  4346066          235          0.0054        390        0.0006
     <nonode@nohost.1106.0>      4  4348159          287          0.0066        379        0.0006
     <nonode@nohost.1111.0>      4  4279309          290          0.0068        325        0.0005
     <nonode@nohost.1107.0>      4  4292190          302          0.0070        315        0.0005
     <nonode@nohost.1112.0>      4  4208858          265          0.0063        276        0.0005
     <nonode@nohost.1109.0>      4  4377502          267          0.0061        276        0.0005
    ok

The listing shows mnesia_locker, a process, has highly contended locks.

    6> lcnt:inspect(mnesia_locker).
              lock          id  #tries  #collisions  collisions [%]  time [us]  duration [%]
             -----         --- ------- ------------ --------------- ---------- -------------
     mnesia_locker   proc_msgq 5449930        59374          1.0894      69781        0.1162
     mnesia_locker   proc_main 4462782      1487374         33.3284      14398        0.0240
     mnesia_locker proc_status 7564921        71800          0.9491      12913        0.0215
     mnesia_locker   proc_link       0            0          0.0000          0        0.0000
    ok

Listing without class combiner.

    7> lcnt:conflicts([{combine, false}, {print, [name, id, tries, ratio, time]}]).
                       lock                        id   #tries  collisions [%]  time [us]
                      -----                       ---  ------- --------------- ----------
                     db_tab mnesia_transient_decision   722250          3.9463    1856852
                  timeofday                 undefined  2701048          6.5106    1746079
             alcu_allocator                 ets_alloc  7490696          2.2737     692655
             alcu_allocator                 ets_alloc  7081771          2.3294     664522
             alcu_allocator                 ets_alloc  7047750          2.2520     658495
             alcu_allocator                 ets_alloc  5883537          2.3177     610869
                   pix_lock                        58 11011355          1.1924     564808
                   pix_lock                        60  4426484          0.7120     262490
             alcu_allocator                 ets_alloc  1897004          2.4248     219543
     message_pre_alloc_lock                 undefined  4211267          0.3242     128299
                  run_queue                         3  2801555          1.3003     116792
                  run_queue                         2  2799988          1.2700     100091
                  run_queue                         1  2966183          1.2712      78834
              mnesia_locker                 proc_msgq  5449930          1.0894      69781
     message_pre_alloc_lock                 undefined  3495672          0.3262      65773
     message_pre_alloc_lock                 undefined  4189752          0.3174      58607
                  mnesia_tm                 proc_msgq  2094144          1.7184      56361
                  run_queue                         4  2343585          1.3115      44300
                     db_tab                    branch  1446529          0.5229      38244
                    gc_info                 undefined   823460          0.0763      24826
    ok

In this scenario the lock that protects ets-table mnesia_transient_decision has spent most of its waiting for. That is 1.8 seconds in a test that run for 60 seconds. The time is also spread on eight different scheduler threads.

    8> lcnt:inspect(db_tab, [{print, [name, id, tries, colls, ratio, duration]}]).
       lock                        id  #tries  #collisions  collisions [%]  duration [%]
      -----                       --- ------- ------------ --------------- -------------
     db_tab mnesia_transient_decision  722250        28502          3.9463        3.0916
     db_tab                    branch 1446529         7564          0.5229        0.0637
     db_tab                   account 1464500         8203          0.5601        0.0357
     db_tab                    teller 1464529         8110          0.5538        0.0291
     db_tab                   history  722250         3767          0.5216        0.0232
     db_tab              mnesia_stats  750332         7057          0.9405        0.0180
     db_tab        mnesia_trans_store      61            0          0.0000        0.0000
     db_tab        mnesia_trans_store      61            0          0.0000        0.0000
     db_tab        mnesia_trans_store      53            0          0.0000        0.0000
     db_tab        mnesia_trans_store      53            0          0.0000        0.0000
     db_tab        mnesia_trans_store      53            0          0.0000        0.0000
     db_tab        mnesia_trans_store      53            0          0.0000        0.0000
     db_tab        mnesia_trans_store      53            0          0.0000        0.0000
     db_tab        mnesia_trans_store      53            0          0.0000        0.0000
     db_tab        mnesia_trans_store      53            0          0.0000        0.0000
     db_tab        mnesia_trans_store      53            0          0.0000        0.0000
     db_tab        mnesia_trans_store      53            0          0.0000        0.0000
     db_tab        mnesia_trans_store      53            0          0.0000        0.0000
     db_tab        mnesia_trans_store      53            0          0.0000        0.0000
     db_tab        mnesia_trans_store      53            0          0.0000        0.0000
    ok

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5.5 Deciphering the output

Typically high time values are bad and this is often the thing to look for. However, one should also look for high lock acquisition frequencies (#tries) since locks generate overhead and because high frequency could become problematic if they begin to have conflicts even if it is not shown in a particular test.

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5.6 See Also

LCNT Reference Manual