WL#7364: RBR: Enhanced Applier Thread Progress Details
Affects: Server-8.0 — Status: Complete
EXECUTIVE SUMMARY ================= This worklog aims at improving/fixing the stats that are shown through SHOW PROCESSLIST, INFORMATION_SCHEMA.PROCESSLIST, PERFORMANCE_SCHEMA.THREADS and PERFORMANCE_SCHEMA.EVENTS_STAGES_* for the SQL thread when it is applying row events. This is motivated by the fact that users do not have a clue of what is happening when the SQL thread is applying a ROW event. In fact, currently wrong information can be reported through PROCESSLIST. Therefore, users should at least be able to see through PERFORMANCE_SCHEMA tables what is the correct state of the SQL thread and what exactly it is doing at the moment. This minimal information is of great help to troubleshoot Row-based Replication problems. The work on this worklog should be later extended with further PERFORMANCE_SCHEMA additions that export many more stats for RBR. SOLUTION ======== - Fix the command field: set it to NULL (or QUERY(?)) - Put in the "Info" field the contents of Rows_query_log_event. - Fix a proper THD_STAGE_INFO for when the thread is applying row events. - Add RBR stages for reporting how far the SQL thread has processed each ROW event (lets call it bundle of RBR changes). This is reported through performance_schema stages tables.
FUNCTIONAL REQUIREMENTS ----------------------- F1. SHOW PROCESSLIST SHALL have the following fields updated with accurate information when processing a Delete, Write, or Update_rows_log_event: - command: NULL / (QUERY?) - db --> the database the row event is changing - Info --> NULL or the contents of Rows_query_log_event if available or the statement itself (for BINLOG '' statements) F2. SELECT command, db, Info FROM INFORMATION_SCHEMA.PROCESSLIST SHALL have the following fields updated with accurate information when processing a Delete, Write, or Update_rows_log_event: - command --> same as SHOW PROCESSLIST - db --> same as SHOW PROCESSLIST - Info --> same as SHOW PROCESSLIST F3. SELECT PROCESSLIST_DB, PROCESSLIST_STATE, PROCESSLIST_INFO FROM PERFORMANCE_SCHEMA.THREADS SHALL have the following fields updated with accurate information when processing a Delete, Write or Update_rows_log_event: - processlist_db --> same as SHOW PROCESSLIST - processlist_command --> same as SHOW PROCESSLIST - processlist_info --> same as SHOW PROCESSLIST F4. There SHALL be three new Performance schema stages: - Applying batch of row changes (write) - Applying batch of row changes (update) - Applying batch of row changes (delete) F5. These new stages SHALL report progress (WORK_COMPLETED and WORK_ESTIMATED on the PERFORMANCE_SCHEMA.EVENTS_STAGES_CURRENT table). F6. The data presented SHALL be discarded once the worker thread stops. F7. BINLOG statements SHALL comply with F1, F2, F3 and F4. NON-FUNCTIONAL REQUIREMENTS --------------------------- NFR1. There SHALL NOT be a significant performance degradation on the binlog applier. (No more than 3%)
- Three new performance schema stages are implemented: - "Applying batch of row changes (write)" - "Applying batch of row changes (update)" - "Applying batch of row changes (delete)" - These stages are set while the applier thread applies the rows. As such the regular performance schema stages operations apply. - The following commands will report more accurate information than that that is exposed today (see requirements section): - SHOW PROCESSLIST - SELECT command, db, Info FROM INFORMATION_SCHEMA.PROCESSLIST - SELECT PROCESSLIST_DB, PROCESSLIST_STATE, PROCESSLIST_INFO FROM PERFORMANCE_SCHEMA.THREADS - The following command will report accurate information when instrumentation for the three stages mentioned above is enabled: - SELECT WORK_ESTIMATED, WORK_COMPLETED FROM performance_schema.events_stages_current WHERE EVENT_NAME LIKE 'stage/sql/Applying%'
Stage Instrumentation --------------------- - Progress for each stage is to be accounted for in the member function Rows_log_event::do_post_row_operations . - Estimation of work to do is done after processing the first row change. This is needed since estimation is performed based on pointers to the data in the buffer: m_rows_end, m_rows_buf and m_curr_row . Therefore, we know the size of the buffer and the approximate size of each row change after we process the first row and set the m_curr_row correctly. - Context lifecycle is set as part of the Rows_log_event.
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