Persistent term

[NelsonVides]

Couldn't resist the temptation 😱

TL;DR;

The PT implementation spends A LOT less time finding the pool details. At some point perfs seems to complain we now spend more time in getting the process info than anything else (grabbing the main_lock for the process and seeing the message queue length and so on, can't get any faster than that by now).

How it is all done: the wpool record contains now one more field with all the worker names as a tuple, so that not only the names are pre-generated, but also getting them is a O(1) element(N #wpool.workers) op. Also, #wpool.next is now an atomics_ref(), so that the rotation for the next worker doesn't take any pt modifications – the idea is from https://www.erlang.org/blog/persistent_term/ and already successfully applied by me in https://github.com/esl/segmented_cache 😄

Then all strategies just get the #wpool record from the persistent_term storage, and operate on fully shared and immutable data, so many other helper functions are removed in favour of just find_wpool(Name).

Code is quite opinionated, and we'd need to take very serious the fact that this would make dynamically adding or removing workers to a pool very very hard, as it would require modifying a persistent_term record.

Time for opinions 😄

---

reports

fprof

Note a general improvement of 25% over finding the worker and applying all the strategy, and also, some 25% less garbage collections.

In main:

 {[{{wpool,call,4},                              100,  204.591,    0.317}],
 { {wpool_pool,best_worker,1},                  100,  204.591,    0.317},     %
 [{{wpool_pool,min_message_queue,1},            100,  203.504,    0.419},
  {{wpool_pool,find_wpool,1},                   100,    0.770,    0.439}]}.

{[{{wpool_pool,best_worker,1},                  100,  203.504,    0.419}],
 { {wpool_pool,min_message_queue,1},            100,  203.504,    0.419},     %
 [{{wpool_pool,min_message_queue,3},            100,  201.402,    0.675},
  {{rand,uniform,1},                            100,    1.551,    0.565},
  {{erlang,setelement,3},                       100,    0.132,    0.132}]}.

{[{{wpool_pool,min_message_queue,1},            100,  201.402,    0.675},
  {{wpool_pool,min_message_queue,3},           10000,    0.000,   62.036}],
 { {wpool_pool,min_message_queue,3},           10100,  201.402,   62.711},     %
 [{{wpool_pool,worker_name,2},                 10000,   38.456,   25.041},
  {{wpool_pool,queue_length,1},                10000,   37.420,   24.245},
  {{wpool_pool,next_wpool,1},                  10000,   36.806,   24.847},
  {{lists,min,1},                               100,   13.245,    0.152},
  {{erlang,whereis,1},                         10000,   12.573,   12.311},
  {garbage_collect,                              33,    0.191,    0.191},
  {{wpool_pool,min_message_queue,3},           10000,    0.000,   62.036}]}.

{[{{wpool_pool,min_message_queue,3},           10000,   38.456,   25.041}],
 { {wpool_pool,worker_name,2},                 10000,   38.456,   25.041},     %
 [{{ets,lookup,2},                             10000,   13.344,   13.158},
  {garbage_collect,                              21,    0.071,    0.071}]}.

{[{{wpool_pool,min_message_queue,3},           10000,   37.420,   24.245}],
 { {wpool_pool,queue_length,1},                10000,   37.420,   24.245},     %
 [{{erlang,process_info,2},                    10000,   13.175,   12.575}]}.

{[{{wpool_pool,min_message_queue,3},           10000,   36.806,   24.847}],
 { {wpool_pool,next_wpool,1},                  10000,   36.806,   24.847},     %
 [{{erlang,setelement,3},                      10000,   11.923,   11.692},
  {garbage_collect,                               9,    0.036,    0.036}]}.

{[{{wpool_pool,best_worker,1},                  100,    0.770,    0.439}],
 { {wpool_pool,find_wpool,1},                   100,    0.770,    0.439},     %
 [{{ets,lookup,2},                              100,    0.183,    0.183},
  {{erlang,whereis,1},                          100,    0.148,    0.141}]}.

{[{{erlang,whereis,1},                           67,    0.269,    0.269},
  {{erlang,process_info,2},                      46,    0.248,    0.248},
  {{erlang,setelement,3},                        48,    0.231,    0.231},
  {{wpool_pool,min_message_queue,3},             33,    0.191,    0.191},
  {{ets,lookup,2},                               49,    0.186,    0.186},
  {{gen_server,call,3},                          14,    0.073,    0.073},
  {{wpool_pool,worker_name,2},                   21,    0.071,    0.071},
  {{wpool_pool,next_wpool,1},                     9,    0.036,    0.036},
  {{erlang,integer_to_list,1},                    4,    0.021,    0.021},
  {{io_lib_format,general_case_10,5},             4,    0.018,    0.018},
  {{io_lib_pretty,pp_tail,10},                    2,    0.010,    0.010},
  {{io_lib_format_ryu_table,value,1},             1,    0.009,    0.009},
  {{erlang,'++',2},                               2,    0.007,    0.007},
  {{string,uppercase_list,2},                     1,    0.006,    0.006},
  {{lists,reverse,2},                             1,    0.006,    0.006},
  {{io_lib_format,compute_shortest,6},            1,    0.006,    0.006},
  {{io_lib_format,convert_to_decimal,3},          1,    0.004,    0.004},
  {{io_lib_format,collect_cc,2},                  1,    0.004,    0.004},
  {{gen,do_call,4},                               1,    0.004,    0.004},
  {{io_lib_format,mulShiftAll,4},                 1,    0.003,    0.003},
  {{io_lib_format,mulShift64,3},                  1,    0.003,    0.003},
  {{gen,call,4},                                  1,    0.003,    0.003},
  {{lists,do_flatten,2},                          1,    0.001,    0.001},
  {{io_lib_format,count_small,2},                 1,    0.001,    0.001}],
 { garbage_collect,                             311,    1.411,    1.411},     %
 [ ]}.

in the persistent_term branch:

{[{{wpool,call,4},                              100,  138.649,    0.243}],     
 { {wpool_pool,best_worker,1},                  100,  138.649,    0.243},     %
 [{{wpool_pool,min_message_queue,1},            100,  137.763,    0.257},      
  {{wpool_pool,find_wpool,1},                   100,    0.643,    0.364}]}.    

{[{{wpool_pool,best_worker,1},                  100,  137.763,    0.257}],     
 { {wpool_pool,min_message_queue,1},            100,  137.763,    0.257},     %
 [{{wpool_pool,min_message_queue,5},            100,  136.132,    0.899},      
  {{rand,uniform,1},                            100,    1.374,    0.495}]}.    

{[{{wpool_pool,min_message_queue,1},            100,  136.132,    0.899},      
  {{wpool_pool,min_message_queue,5},           10000,    0.000,   55.225}],     
 { {wpool_pool,min_message_queue,5},           10100,  136.132,   56.124},     %
 [{{wpool_pool,queue_length,1},                10000,   35.359,   23.258},      
  {{lists,min,1},                               100,   12.112,    0.115},      
  {{erlang,whereis,1},                         10000,   11.238,   10.997},      
  {{wpool_pool,next_to_check,2},               10000,   10.575,   10.571},      
  {{wpool_pool,worker_name,2},                 10000,   10.532,   10.468},      
  {garbage_collect,                              42,    0.192,    0.192},      
  {{wpool_pool,min_message_queue,5},           10000,    0.000,   55.225}]}.    

{[{{wpool_pool,min_message_queue,5},           10000,   35.359,   23.258}],     
 { {wpool_pool,queue_length,1},                10000,   35.359,   23.258},     %
 [{{erlang,process_info,2},                    10000,   12.101,   11.584}]}.    

{[{{wpool_pool,min_message_queue,5},            100,   12.112,    0.115}],     
 { {lists,min,1},                               100,   12.112,    0.115},     %
 [{{lists,min,2},                               100,   11.997,    0.118}]}.    

{[{{wpool_pool,best_worker,1},                  100,    0.643,    0.364}],     
 { {wpool_pool,find_wpool,1},                   100,    0.643,    0.364},     %
 [{{erlang,whereis,1},                          100,    0.148,    0.144},      
  {{persistent_term,get,1},                     100,    0.131,    0.131}]}.

{[{{erlang,process_info,2},                      40,    0.370,    0.370},      
  {{erlang,whereis,1},                           75,    0.245,    0.245},      
  {{wpool_pool,min_message_queue,5},             42,    0.192,    0.192},      
  {{gen_server,call,3},                          14,    0.091,    0.091},      
  {{wpool_pool,worker_name,2},                   17,    0.064,    0.064},      
  {{erlang,monotonic_time,0},                     2,    0.014,    0.014},      
  {{io_lib_format,general_case_10,5},             2,    0.011,    0.011},      
  {{erlang,integer_to_list,1},                    4,    0.011,    0.011},      
  {{wpool_bench,run_task,3},                      2,    0.008,    0.008},      
  {{erlang,'++',2},                               2,    0.008,    0.008},      
  {{io_lib_format,collect_cc,2},                  1,    0.007,    0.007},      
  {{lists,foldl,3},                               1,    0.006,    0.006},      
  {{lists,duplicate,3},                           1,    0.006,    0.006},      
  {{wpool_pool,next_to_check,2},                  2,    0.004,    0.004},      
  {{lists,reverse,2},                             1,    0.004,    0.004},      
  {{io_lib_format,bounds,6},                      1,    0.004,    0.004},      
  {{erlang,iolist_size,1},                        1,    0.004,    0.004},      
  {{erts_internal,time_unit,0},                   2,    0.002,    0.002},      
  {{io_lib_pretty,pp_tail,10},                    1,    0.001,    0.001},      
  {{io_lib_pretty,cind_tail,8},                   1,    0.001,    0.001}],     
 { garbage_collect,                             212,    1.053,    1.053},     %
 [ ]}.

perf

Note how there's way more time now on timer:tc or in io:format, proportionally.

main

persistent_term

[codecov[bot]]

Codecov Report

Merging #183 (41fd7c0) into main (dcae5ca) will decrease coverage by 0.06%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #183      +/-   ##
==========================================
- Coverage   92.85%   92.79%   -0.07%     
==========================================
  Files          10       10              
  Lines         448      430      -18     
==========================================
- Hits          416      399      -17     
+ Misses         32       31       -1     

Impacted Files	Coverage Δ
src/wpool_sup.erl	100.00% <ø> (ø)
src/wpool_pool.erl	96.35% <100.00%> (+0.27%)	:arrow_up:
src/wpool_queue_manager.erl	88.46% <100.00%> (-0.15%)	:arrow_down:

---

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[elbrujohalcon]

I don't understand what CodeCov is complaining about… So, I'll just ignore that thing and merge anyway…

[elbrujohalcon]

I don't understand what CodeCov is complaining about… So, I'll just ignore that thing and merge anyway…

I analyzed the situation manually and I figured out the issue is just that the module has now fewer lines of code. 🤦🏻‍♂️

[elbrujohalcon]

Code is quite opinionated, and we'd need to take very serious the fact that this would make dynamically adding or removing workers to a pool very very hard, as it would require modifying a persistent_term record.

It's not terrible and that is a slightly utopic feature anyway…