mysql query become very slowly when binlog rotate

[weity ho]

Mysql version is 5.7.13.
Everytime when binlog file rotate,all queries and dml become extremely slow.the server "freezes" for about 40 seconds.
In the slow query log, I just see commits and system/session variable queries (like select @@session.tx_read_only; ) that take about 20 secs.


==============master server my.cnf============
[mysqld]

# Remove leading # and set to the amount of RAM for the most important data
# cache in MySQL. Start at 70% of total RAM for dedicated server, else 10%.
# innodb_buffer_pool_size = 128M
innodb_buffer_pool_size = 2048M
innodb_log_file_size = 360M
innodb_log_buffer_size = 8M
innodb_flush_log_at_trx_commit = 2
innodb_max_dirty_pages_pct = 50


# Remove leading # to turn on a very important data integrity option: logging
# changes to the binary log between backups.
# log_bin

log-bin=/alidata1/binlog/mysql-bin
expire_logs_days=7
binlog-do-db=mydb
sync_binlog=6
server_id=10

slow_query_log=on
slow_query_log_file=/data/slow_query/slow_query.log
log_queries_not_using_indexes=off
long_query_time=1
wait_timeout=10800
max_connections=800

sql_mode=NO_ENGINE_SUBSTITUTION,STRICT_TRANS_TABLES
#character_set_server=utf8
character-set-client-handshake=FALSE
character-set-server=utf8mb4
collation-server=utf8mb4_unicode_ci
init_connect='SET NAMES utf8mb4'

[client]
default-character-set=utf8mb4

[mysql]
default-character-set=utf8mb4


==============binlog files=============
[root@iZ94aem4qr5Z binlog]# ll
total 15873308
-rw-r----- 1 mysql mysql 629162145 Jul 10 16:15 mysql-bin.000253
-rw-r----- 1 mysql mysql 629160790 Jul 10 21:11 mysql-bin.000254
-rw-r----- 1 mysql mysql 629148120 Jul 11 08:35 mysql-bin.000255
-rw-r----- 1 mysql mysql 629167412 Jul 11 13:12 mysql-bin.000256
-rw-r----- 1 mysql mysql 143362983 Jul 11 14:25 mysql-bin.000257
-rw-r----- 1 mysql mysql 782 Jul 11 13:12 mysql-bin.index


==============mysql slow log==============

# User@Host: user[user] @ [10.24.165.8] Id: 130132
# Query_time: 14.186255 Lock_time: 0.000000 Rows_sent: 1 Rows_examined: 0
SET timestamp=1499749959;
select @@session.tx_read_only;

# User@Host: user[user] @ [10.25.208.208] Id: 130317
# Query_time: 13.943930 Lock_time: 0.000000 Rows_sent: 1 Rows_examined: 0
SET timestamp=1499749959;
select @@session.tx_read_only;

# User@Host: user[user] @ [10.24.165.8] Id: 130204
# Query_time: 13.848498 Lock_time: 0.000000 Rows_sent: 1 Rows_examined: 0
SET timestamp=1499749959;
select @@session.tx_read_only;

# User@Host: user[user] @ [10.24.165.8] Id: 130197
# Query_time: 12.988434 Lock_time: 0.000000 Rows_sent: 1 Rows_examined: 0
SET timestamp=1499749959;
select @@session.tx_read_only;