PMM-7958 Fix connection issue.

[askomorokhov]

https://jira.percona.com/browse/PMM-7958
FB: Percona-Lab/pmm-submodules#1735

Ref: prometheus-community#529

---

Before the fix db.SetMaxIdleConns(1), you can see:

• the relatively slow performance;
• errors in connection;
• idle connections in all DBs;
• errors on drop DB.

ab report:

[root@371cb8fe24c2 /]# ab -n 100 -c 2  http://localhost:42006/metrics
This is ApacheBench, Version 2.3 <$Revision: 1843412 $>
Copyright 1996 Adam Twiss, Zeus Technology Ltd, http://www.zeustech.net/
Licensed to The Apache Software Foundation, http://www.apache.org/

Benchmarking localhost (be patient)...

..done


Server Software:        
Server Hostname:        localhost
Server Port:            42006

Document Path:          /metrics
Document Length:        483030 bytes

Concurrency Level:      2
Time taken for tests:   1213.730 seconds
Complete requests:      100
Failed requests:        97
   (Connect: 0, Receive: 0, Length: 97, Exceptions: 0)
Total transferred:      48320076 bytes
HTML transferred:       48308876 bytes
Requests per second:    0.08 [#/sec] (mean)
Time per request:       24274.606 [ms] (mean)
Time per request:       12137.303 [ms] (mean, across all concurrent requests)
Transfer rate:          38.88 [Kbytes/sec] received

Connection Times (ms)
              min  mean[+/-sd] median   max
Connect:        0    0   0.2      0       2
Processing: 12166 24031 4348.8  24260   36231
Waiting:    12159 24022 4349.4  24250   36225
Total:      12167 24031 4348.8  24260   36231

Percentage of the requests served within a certain time (ms)
  50%  24260
  66%  24288
  75%  24327
  80%  24405
  90%  30291
  95%  30313
  98%  30464
  99%  36231
 100%  36231 (longest request)
[root@371cb8fe24c2 /]# 

Error on drop db

postgres=# drop database my_db;                                                                                                                                                                                                                               
ERROR:  database "my_db" is being accessed by other users                                                                                                                                                                                                     
DETAIL:  There is 1 other session using the database.    

---

After the fix db.SetMaxIdleConns(-1), you can see:

• an increase in performance;
• no error in connection
• no idle connections in all DBs;
• no error on drop db.

ab report:

[root@371cb8fe24c2 /]# ab -n 100 -c 2  http://localhost:42006/metrics
This is ApacheBench, Version 2.3 <$Revision: 1843412 $>
Copyright 1996 Adam Twiss, Zeus Technology Ltd, http://www.zeustech.net/
Licensed to The Apache Software Foundation, http://www.apache.org/

Benchmarking localhost (be patient).....done


Server Software:        
Server Hostname:        localhost
Server Port:            42006

Document Path:          /metrics
Document Length:        483102 bytes

Concurrency Level:      2
Time taken for tests:   80.211 seconds
Complete requests:      100
Failed requests:        99
   (Connect: 0, Receive: 0, Length: 99, Exceptions: 0)
Total transferred:      48324786 bytes
HTML transferred:       48313586 bytes
Requests per second:    1.25 [#/sec] (mean)
Time per request:       1604.226 [ms] (mean)
Time per request:       802.113 [ms] (mean, across all concurrent requests)
Transfer rate:          588.35 [Kbytes/sec] received

Connection Times (ms)
              min  mean[+/-sd] median   max
Connect:        0    0   0.1      0       0
Processing:  1062 1583 393.6   1466    3203
Waiting:     1054 1573 392.6   1455    3188
Total:       1062 1583 393.6   1466    3203

Percentage of the requests served within a certain time (ms)
  50%   1466
  66%   1621
  75%   1749
  80%   1852
  90%   2182
  95%   2309
  98%   3171
  99%   3203
 100%   3203 (longest request)

Idle connection to db

postgres=# SELECT * FROM pg_stat_activity WHERE datname='my_db';                                                                                                                                                                                              
 datid | datname | pid | leader_pid | usesysid | usename  | application_name | client_addr | client_hostname | client_port |         backend_start         | xact_start |          query_start          |         state_change          | wait_event_type | wa
it_event | state | backend_xid | backend_xmin |       query       |  backend_type                                                                                                                                                                             
-------+---------+-----+------------+----------+----------+------------------+-------------+-----------------+-------------+-------------------------------+------------+-------------------------------+-------------------------------+-----------------+---
---------+-------+-------------+--------------+-------------------+----------------                                                                                                                                                                           
 16384 | my_db   |  80 |            |       10 | postgres |                  | 172.25.0.4  |                 |       37364 | 2021-05-03 08:51:20.152445+00 |            | 2021-05-03 08:51:23.561045+00 | 2021-05-03 08:51:23.561157+00 | Client          | Cl
ientRead | idle  |             |              | SELECT version(); | client backend                                                                                                                                                                            
(1 row)        

No idle connection to DBs:

postgres=# SELECT COUNT(*) FROM pg_stat_activity WHERE state='idle';
-[ RECORD 1 ]
count | 0

No error on drop db.

postgres=# drop database my_db;                                                                                                                                                                                                                               
DROP DATABASE