Unlike MySQL 5.6, where parallel replication can only be used when replicas have several schemas, MySQL 5.7 replicas can read binlog group commit information coming from the master to replicate transactions in parallel even when a single schema is used. Now, the question is: how many replication threads should you use?
A Simple Benchmark
Let’s assume we have one master and three slaves, all running MySQL 5.7.
One slave is using regular single-threaded replication (the control slave in the graph below), one is using 20 parallel workers (MTS 20 workers) and the last one is using 100 parallel workers (MTS 100 workers).
As a reminder, here are the settings that need to be adjusted for 5.7 parallel replication:
slave_parallel_type = LOGICAL_CLOCK slave_parallel_workers = 20
GTID replication is also highly recommended, if you don’t want to face annoying issues.
Now, let’s run a simple sysbench workload, inserting records in 25 separate tables in the same database with 100 concurrent threads:
sysbench --mysql-user=root --mysql-db=db1 --test=/usr/share/doc/sysbench/tests/db/insert.lua --max-requests=500000 --num-threads=100 --oltp-tables-count=25 run
Because we’re using 100 concurrent threads on the master, we can expect that some parallelization is possible. This means that if we see replication lag with the control slave, we’ll probably see less lag with the 20-worker slave and even less with the 100-worker slave.
This is not exactly what we get:
Parallel replication is indeed useful, but the 100-worker slave doesn’t provide any benefits compared to the 20-worker slave. Replication lag is even slightly worse.
Some Instrumentation With performance_schema
To have a better understanding of how efficiently the parallel replication threads are, let’s enable some instrumentation on slaves (in other words, recording executed transactions):
UPDATE performance_schema.setup_consumers SET ENABLED = 'YES' WHERE NAME LIKE 'events_transactions%'; UPDATE performance_schema.setup_instruments SET ENABLED = 'YES', TIMED = 'YES' WHERE NAME = 'transaction';
For better readability of the results, let’s create a new view (tracking how many transactions are executed by each replication thread):
CREATE VIEW mts_summary_trx AS select performance_schema.events_transactions_summary_by_thread_by_event_name.THREAD_ID AS THREAD_ID, performance_schema.events_transactions_summary_by_thread_by_event_name.COUNT_STAR AS COUNT_STAR from performance_schema.events_transactions_summary_by_thread_by_event_name where performance_schema.events_transactions_summary_by_thread_by_event_name.THREAD_ID in (select performance_schema.replication_applier_status_by_worker.THREAD_ID from performance_schema.replication_applier_status_by_worker);
Now, after running sysbench again, let’s do some math to see how often each replication thread was run:
SELECT SUM(count_star) FROM mts_summary_trx INTO @total; SELECT 100*(COUNT_STAR/@total) AS PCT_USAGE FROM mts_summary_trx; +-----------+ | PCT_USAGE | +-----------+ | 39.5845 | | 31.4046 | | 12.0119 | | 5.9081 | | 3.0375 | | 1.6527 | | 1.0550 | | 0.7576 | | 0.6089 | | 0.5208 | | 0.4642 | | 0.4157 | | 0.3832 | | 0.3682 | | 0.3408 | | 0.3247 | | 0.3076 | | 0.2925 | | 0.2866 | | 0.2749 | +-----------+
We can see that the workload has a limited potential for parallelism–therefore, it’s not worth configuring more than 3-4 replication threads.
The slight performance degradation with 100 replication threads is probably due to the overhead of the coordinator thread.
Estimating the optimal number of replication threads with MySQL 5.7 parallel replication is quite difficult if you're just guessing. The performance_schema provides a simple way to understand how the workload is handled by the replication threads.
It also allows you to see if tuning binlog_group_commit_sync_delay provides more throughput on slaves without too much impact on the master’s performance.