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  1. MariaDB Server
  2. MDEV-29091

Wait events do not distinguish waiting-for-I/O and waiting-for-locks

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      Description

      In MariaDB (as well as in MySQL 8), we have no wait event instrumentation for record locks and for table locks.

      As such, table and row lock waits are reported incorrectly as "wait/io/table/sql/handler".

      This makes performance troubleshooting impossible for customers.

      The expected outcome is to create 2 more instruments: row lock and table lock. These 2 types of lock waits can then be measured inside performance_schema. Being able to separate lock waits from IO waits will aid performance tuning, because reducing them will require difference approaches.

      Steps to reproduce:

      1. Add the following line into my.cnf before instance start

        performance_schema=ON
        

      2. Run the following to set up all consumers and instrumentation

        UPDATE performance_schema.setup_consumers SET ENABLED = 'YES';
        UPDATE performance_schema.setup_instruments SET ENABLED = 'YES', TIMED = 'YES';
        set global innodb_lock_wait_timeout=7200;
        

      3. Set up test table

        create database test;
        use test
        create table t1 (id1 int(11), col1 varchar (200));
        insert into t1 values (1, 'aa');
        insert into t1 values (2, 'bb');
        

      4. Run 3 concurrent sessions to observe the bug
        1. Session 1

          start transaction;
          select id1 from test.t1 where id1=1 for update;
          select sleep(100000);
          

        2. Session 2

          start transaction;
          select id1 from test.t1 where id1=1 for update;
          

        3. Session 3

          MariaDB [test]> select * from performance_schema.threads where type = 'foreground'\G
          *************************** Only relevant row displayed below ***************************
                    THREAD_ID: 12
                         NAME: thread/sql/one_connection
                         TYPE: FOREGROUND
               PROCESSLIST_ID: 4
             PROCESSLIST_USER: haidong
             PROCESSLIST_HOST: NULL
               PROCESSLIST_DB: NULL
          PROCESSLIST_COMMAND: Query
             PROCESSLIST_TIME: 9
            PROCESSLIST_STATE: Sending data
             PROCESSLIST_INFO: select id1 from test.t1 where id1=1 for update
             PARENT_THREAD_ID: 1
                         ROLE: NULL
                 INSTRUMENTED: YES
                      HISTORY: YES
              CONNECTION_TYPE: Socket
                 THREAD_OS_ID: 22508
           
          MariaDB [test]> select * from performance_schema.events_waits_current where thread_id=12\G
          *************************** 1. row ***************************
                      THREAD_ID: 12
                       EVENT_ID: 236
                   END_EVENT_ID: NULL
                     EVENT_NAME: wait/io/table/sql/handler           <<<Incorrect instrument name
                         SOURCE:
                    TIMER_START: 767747433709600
                      TIMER_END: 791300625915200
                     TIMER_WAIT: 23553192205600
                          SPINS: NULL
                  OBJECT_SCHEMA: test
                    OBJECT_NAME: t1
                     INDEX_NAME: NULL
                    OBJECT_TYPE: TABLE
          OBJECT_INSTANCE_BEGIN: 139905003640240
               NESTING_EVENT_ID: 235
             NESTING_EVENT_TYPE: STAGE
                      OPERATION: fetch
                NUMBER_OF_BYTES: 47
                          FLAGS: NULL
          1 row in set (0.001 sec)
          

      I'm working on a fix.

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            Assignee:
            vlad.lesin Vladislav Lesin
            Reporter:
            haidong Haidong Ji
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              Dates

              Created:
              Updated:

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