Why MySQL Group Replication slave nodes have high delay with write node? - mysql

Question

MySQL 5.7.17 MGR deploy in single-primary mode, 3 node all one one machine, the same configuration.
And then we test insert on the primary node and observe that the slave nodes have high delay with the write node, even primary node finish the insert test, slave node data is incresing!
Why MySQL Group Replication slave nodes have high delay with write node?
here is the my.cnf:
[mysqld]
datadir=/dba/mysql/data/s1
basedir=/dba/mysql/mysql-5.7/
port=24801
socket=/dba/mysql/data/s1/s1.sock
server_id=1
gtid_mode=ON
enforce_gtid_consistency=ON
master_info_repository=TABLE
relay_log_info_repository=TABLE
binlog_checksum=NONE
log_slave_updates=ON
log_bin=binlog
binlog_format=ROW
transaction_write_set_extraction=XXHASH64
loose-group_replication_group_name="aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa"
loose-group_replication_start_on_boot=off
loose-group_replication_local_address= "127.0.0.1:24901"
loose-group_replication_group_seeds= "127.0.0.1:24901,127.0.0.1:24902,127.0.0.1:24903"
loose-group_replication_bootstrap_group= off
loose-group_replication_single_primary_mode=true
loose-group_replication_enforce_update_everywhere_checks=false
slave_parallel_type=LOGICAL_CLOCK
slave_preserve_commit_order=1
slave_parallel_workers=4
and the MGR config :
mysql> show variables like '%group_replication%';
+----------------------------------------------------+-------------------------------------------------+
| Variable_name | Value |
+----------------------------------------------------+-------------------------------------------------+
| group_replication_allow_local_disjoint_gtids_join | OFF |
| group_replication_allow_local_lower_version_join | OFF |
| group_replication_auto_increment_increment | 7 |
| group_replication_bootstrap_group | OFF |
| group_replication_components_stop_timeout | 31536000 |
| group_replication_compression_threshold | 1000000 |
| group_replication_enforce_update_everywhere_checks | OFF |
| group_replication_flow_control_applier_threshold | 25000 |
| group_replication_flow_control_certifier_threshold | 25000 |
| group_replication_flow_control_mode | QUOTA |
| group_replication_force_members | |
| group_replication_group_name | aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa |
| group_replication_group_seeds | 127.0.0.1:24901,127.0.0.1:24902,127.0.0.1:24903 |
| group_replication_gtid_assignment_block_size | 1000000 |
| group_replication_ip_whitelist | AUTOMATIC |
| group_replication_local_address | 127.0.0.1:24901 |
| group_replication_poll_spin_loops | 0 |
| group_replication_recovery_complete_at | TRANSACTIONS_APPLIED |
| group_replication_recovery_reconnect_interval | 60 |
| group_replication_recovery_retry_count | 10 |
| group_replication_recovery_ssl_ca | |
| group_replication_recovery_ssl_capath | |
| group_replication_recovery_ssl_cert | |
| group_replication_recovery_ssl_cipher | |
| group_replication_recovery_ssl_crl | |
| group_replication_recovery_ssl_crlpath | |
| group_replication_recovery_ssl_key | |
| group_replication_recovery_ssl_verify_server_cert | OFF |
| group_replication_recovery_use_ssl | OFF |
| group_replication_single_primary_mode | ON |
| group_replication_ssl_mode | DISABLED |
| group_replication_start_on_boot | OFF |
+----------------------------------------------------+-------------------------------------------------+
32 rows in set (0.01 sec)