Debian 7.7 上的 MySQL 5.6.21 — 启动后几秒钟内 nice 会自动设置为 19；不知道为什么

Question

我在装有 Debian 7.7 的 OpenVZ VPS 上运行 MySQL 5.6.21。MySQL 是从 DotDeb 存储库安装的（完整版本号为 5.6.21-1~dotdeb.1）。

每当 MySQL 服务器启动或重新启动时，它的优先度一开始是 0。然而，大约 20 秒后，它会自动重新调整为 19（最低优先级），我不知道为什么。

我搜索了所有配置文件，确认没有设置 cron 作业来执行此操作，并查看了启动脚本 (/etc/init.d/mysql)。那里没有任何东西会导致此问题。

如果我手动将其重新调整回 0，有时它会保持在 0，有时它最终会再次重新调整为 19。

它不会给托管在此 VPS 上的网站造成任何性能问题（我有一些非常高效的缓存，这限制了甚至接触数据库的需要），但这个小秘密开始让我感到不安。我的职业是 MySQL DBA，并拥有 Oracle 颁发的 MySQL 5.6 数据库管理员认证，但我无法弄清楚这个怪癖……这让我非常沮丧。

当然，我可以设置一个每小时一次的 cron 作业来检查 MySQL 服务器进程的优劣，如果它已发生变化，则将其重新设置为 0，但我想知道为什么会发生这种情况，而不仅仅是对其进行修补。

任何想法或帮助都将不胜感激。谢谢！

/etc/mysql/my.cnf

[client]
socket  = /var/run/mysqld/mysqld.sock

[mysqld_safe]
nice      = 0
socket    = /var/run/mysqld/mysqld.sock
log_error = /var/log/mysql/mysql-error.log

[mysqld]
basedir                         = /usr
bind_address                    = 127.0.0.1
connect_timeout                 = 30
datadir                         = /var/lib/mysql
innodb_buffer_pool_size         = 224M
innodb_file_per_table           = 1
innodb_flush_log_at_trx_commit  = 2
innodb_log_buffer_size          = 4M
innodb_log_file_size            = 8M
innodb_open_files               = 2048
innodb_thread_concurrency       = 8
interactive_timeout             = 60
join_buffer_size                = 2M
key_buffer_size                 = 128K
log-queries-not-using-indexes   = 1
long_query_time                 = 2
max_allowed_packet              = 16M
max_connections                 = 125
max_connect_errors              = 100
max_heap_table_size             = 1M
pid_file                        = /var/run/mysqld/mysqld.pid
query_cache_limit               = 8M
query_cache_min_res_unit        = 1K
query_cache_size                = 48M
query_cache_type                = 1
read_rnd_buffer_size            = 1M
skip_external_locking           = 1
slow_query_log                  = 1
slow_query_log_file             = /var/log/mysql/mysql-slow.log
socket                          = /var/run/mysqld/mysqld.sock
sort_buffer_size                = 1M
table_definition_cache          = 2048
table_open_cache                = 2048
table_open_cache_instances      = 4
thread_cache_size               = 16
tmp_table_size                  = 1M
user                            = mysql
wait_timeout                    = 60

[mysqldump]
max_allowed_packet = 16M
quick
quote_names

[isamchk]
key_buffer = 128K

!includedir /etc/mysql/conf.d/

/etc/mysql/conf.d 中的配置文件内容

#
# The MySQL 5.6 database server configuration file.
#
# This custom MySQL 5.6 specific configuration file
# adds on top of the existing default my.cnf file at
# - /etc/mysql/my.cnf.
#
# Please add any extra MySQL 5.6 options in this file
# for sake of clarity.
#
# You may uncomment any existing option to enable it
#
# sql_mode=NO_ENGINE_SUBSTITUTION,STRICT_TRANS_TABLES

[mysqld_safe]
syslog

/etc/init.d/mysql

#!/bin/bash
#
### BEGIN INIT INFO
# Provides:          mysql
# Required-Start:    $remote_fs $syslog
# Required-Stop:     $remote_fs $syslog
# Should-Start:      $network $time
# Should-Stop:       $network $time
# Default-Start:     2 3 4 5
# Default-Stop:      0 1 6
# Short-Description: Start and stop the mysql database server daemon
# Description:       Controls the main MySQL database server daemon "mysqld"
#                    and its wrapper script "mysqld_safe".
### END INIT INFO
#
set -e
set -u
${DEBIAN_SCRIPT_DEBUG:+ set -v -x}

test -x /usr/bin/mysqld_safe || exit 0

. /lib/lsb/init-functions

SELF=$(cd $(dirname $0); pwd -P)/$(basename $0)
CONF=/etc/mysql/my.cnf
MYADMIN="/usr/bin/mysqladmin --defaults-file=/etc/mysql/debian.cnf"

# priority can be overriden and "-s" adds output to stderr
ERR_LOGGER="logger -p daemon.err -t /etc/init.d/mysql -i"

# Safeguard (relative paths, core dumps..)
cd /
umask 077

# mysqladmin likes to read /root/.my.cnf. This is usually not what I want
# as many admins e.g. only store a password without a username there and
# so break my scripts.
export HOME=/etc/mysql/

## Fetch a particular option from mysql's invocation.
#
# Usage: void mysqld_get_param option
mysqld_get_param() {
    /usr/sbin/mysqld --print-defaults \
        | tr " " "\n" \
        | grep -- "--$1" \
        | tail -n 1 \
        | cut -d= -f2
}

## Do some sanity checks before even trying to start mysqld.
sanity_checks() {
  # check for config file
  if [ ! -r /etc/mysql/my.cnf ]; then
    log_warning_msg "$0: WARNING: /etc/mysql/my.cnf cannot be read. See README.Debian.gz"
    echo                "WARNING: /etc/mysql/my.cnf cannot be read. See README.Debian.gz" | $ERR_LOGGER
  fi

  # check for diskspace shortage
  datadir=`mysqld_get_param datadir`
  if LC_ALL=C BLOCKSIZE= df --portability $datadir/. | tail -n 1 | awk '{ exit ($4>4096) }'; then
    log_failure_msg "$0: ERROR: The partition with $datadir is too full!"
    echo                "ERROR: The partition with $datadir is too full!" | $ERR_LOGGER
    exit 1
  fi
}

## Checks if there is a server running and if so if it is accessible.
#
# check_alive insists on a pingable server
# check_dead also fails if there is a lost mysqld in the process list
#
# Usage: boolean mysqld_status [check_alive|check_dead] [warn|nowarn]
mysqld_status () {
    ping_output=`$MYADMIN ping 2>&1`; ping_alive=$(( ! $? ))

    ps_alive=0
    pidfile=`mysqld_get_param pid-file`
    if [ -f "$pidfile" ] && ps `cat $pidfile` >/dev/null 2>&1; then ps_alive=1; fi

    if [ "$1" = "check_alive"  -a  $ping_alive = 1 ] ||
       [ "$1" = "check_dead"   -a  $ping_alive = 0  -a  $ps_alive = 0 ]; then
    return 0 # EXIT_SUCCESS
    else
    if [ "$2" = "warn" ]; then
        echo -e "$ps_alive processes alive and '$MYADMIN ping' resulted in\n$ping_output\n" | $ERR_LOGGER -p daemon.debug
    fi
    return 1 # EXIT_FAILURE
    fi
}

#
# main()
#

case "${1:-''}" in
  'start')
    sanity_checks;
    # Start daemon
    log_daemon_msg "Starting MySQL database server" "mysqld"
    if mysqld_status check_alive nowarn; then
       log_progress_msg "already running"
       log_end_msg 0
    else
        # Could be removed during boot
        test -e /var/run/mysqld || install -m 755 -o mysql -g root -d /var/run/mysqld

        # Start MySQL!
        su - mysql -s /bin/sh -c "/usr/bin/mysqld_safe > /dev/null 2>&1 &"

        # 6s was reported in #352070 to be too few when using ndbcluster
        # 14s was reported in #736452 to be too few with large installs
        for i in $(seq 1 30); do
                sleep 1
            if mysqld_status check_alive nowarn ; then break; fi
        log_progress_msg "."
        done
        if mysqld_status check_alive warn; then
                log_end_msg 0
            # Now start mysqlcheck or whatever the admin wants.
            output=$(/etc/mysql/debian-start)
        [ -n "$output" ] && log_action_msg "$output"
        else
            log_end_msg 1
        log_failure_msg "Please take a look at the syslog"
        fi
    fi
    ;;

  'stop')
    # * As a passwordless mysqladmin (e.g. via ~/.my.cnf) must be possible
    # at least for cron, we can rely on it here, too. (although we have
    # to specify it explicit as e.g. sudo environments points to the normal
    # users home and not /root)
    log_daemon_msg "Stopping MySQL database server" "mysqld"
    if ! mysqld_status check_dead nowarn; then
      set +e
      shutdown_out=`$MYADMIN shutdown 2>&1`; r=$?
      set -e
      if [ "$r" -ne 0 ]; then
        log_end_msg 1
        [ "$VERBOSE" != "no" ] && log_failure_msg "Error: $shutdown_out"
        log_daemon_msg "Killing MySQL database server by signal" "mysqld"
        killall -15 mysqld
            server_down=
        for i in 1 2 3 4 5 6 7 8 9 10; do
              sleep 1
              if mysqld_status check_dead nowarn; then server_down=1; break; fi
            done
          if test -z "$server_down"; then killall -9 mysqld; fi
      fi
        fi

        if ! mysqld_status check_dead warn; then
      log_end_msg 1
      log_failure_msg "Please stop MySQL manually and read /usr/share/doc/mysql-server-5.6/README.Debian.gz!"
      exit -1
    else
      log_end_msg 0
        fi
    ;;

  'restart')
    set +e; $SELF stop; set -e
    $SELF start
    ;;

  'reload'|'force-reload')
    log_daemon_msg "Reloading MySQL database server" "mysqld"
    $MYADMIN reload
    log_end_msg 0
    ;;

  'status')
    if mysqld_status check_alive nowarn; then
      log_action_msg "$($MYADMIN version)"
    else
      log_action_msg "MySQL is stopped."
      exit 3
    fi
    ;;

  *)
    echo "Usage: $SELF start|stop|restart|reload|force-reload|status"
    exit 1
    ;;
esac

Answer 1

听起来好像它可能被设置在mysqld_safe。曾经有一个关于这个的古老错误。

回想一下mysqld_safe调用mysqld来自无限循环内。

转到操作系统并运行以下命令：

grep -n nice `which mysqld_safe`

这将向您显示所有设置、默认和使用 nice 的地方

我刚刚在运行 MySQL 5.5.20 的 CentOS VM 上运行了这个

[~]# cat /etc/redhat-release
CentOS release 5.7 (Final)
[~]# mysql --version
mysql  Ver 14.14 Distrib 5.5.20, for Linux (x86_64) using readline 5.1
[~]# grep -n nice `which mysqld_safe`
17:niceness=0
56:  --nice=NICE                Set the scheduling priority of mysqld
200:      --nice=*) niceness="$val" ;;
629:if test $niceness -eq 0
633:  NOHUP_NICENESS="nohup nice -$niceness"
636:# Using nice with no args to get the niceness level is GNU-specific.
638:# BSD could use "nohup sh -c 'ps -o nice -p $$' | tail -1").
641:if nohup nice > /dev/null 2>&1
643:    normal_niceness=`nice`
644:    nohup_niceness=`nohup nice 2>/dev/null`
646:    numeric_nice_values=1
647:    for val in $normal_niceness $nohup_niceness
654:                numeric_nice_values=0 ;;
658:    if test $numeric_nice_values -eq 1
660:        nice_value_diff=`expr $nohup_niceness - $normal_niceness`
661:        if test $? -eq 0 && test $nice_value_diff -gt 0 && \
662:            nice --$nice_value_diff echo testing > /dev/null 2>&1
667:            niceness=`expr $niceness - $nice_value_diff`
668:            NOHUP_NICENESS="nice -$niceness nohup"

查看第一次出现的 nice（在本例中为 niceness）是否设置为 19。

你也可以直接 grep 查找数字 19mysqld_safe 脚本。

我以前从未听说过随机设置友善度。不过，我首先要说的是，mysqld_safe

请记住，mysqld 可能已作为 nohup 顺利启动。

根据维基百科上关于尼斯

nice 是 Unix 和类 Unix 操作系统（如 Linux）上的一个程序。它直接映射到同名的内核调用。nice 用于调用具有特定优先级的实用程序或 shell 脚本，从而为该进程提供比其他进程更多或更少的 CPU 时间。nice 值为 -20 表示优先级最高，19 或 20 表示优先级最低。进程的默认 nice 值继承自其父进程，通常为 0。

注意最后一句话

进程的默认优良性是从其父进程继承的，通常为 0。

这听起来令人不安，因为 mysqld 可能会重新继承其原始的 nice 值。谁是mysqld就像我在一开始说的：mysqld_safe。

您还应确保您的操作系统没有以 19 为 nice 运行。只需运行 nice 即可发现。

另外，请注意 nice 的使用信息

[~]# nice --help
Usage: nice [OPTION] [COMMAND [ARG]...]
Run COMMAND with an adjusted niceness, which affects process scheduling.
With no COMMAND, print the current niceness.  Nicenesses range from
-20 (most favorable scheduling) to 19 (least favorable).

  -n, --adjustment=N   add integer N to the niceness (default 10)
      --help     display this help and exit
      --version  output version information and exit

NOTE: your shell may have its own version of nice, which usually supersedes
the version described here.  Please refer to your shell's documentation
for details about the options it supports.**

Report bugs to <[email protected]>.

根据上述情况NOTE:，你的 shell 的 nice 可能有问题。如果mysqld_safe从上而下继承了它的美好，mysqld将继承了祖父母的善良。