Review CPU and memory on your Linux serverResources like CPU and memory (RAM) are an integral part of your site's functionality. If your code uses too much, the site can become slow or stop working altogether. All other processes (like email) share the same resources with the sites on your server. Any process taking more than their (expected) fair share, can take the server down completely.
Why would I need to check resource usage?
- Your database driven site (like WordPress) is slow.
- Email is taking a long time to arrive.
- Your site displays an error or just spins
- Tasks performed on your server are taking longer than normal (like backups).
How do I check resource usage?
Linux offers several tools to investigate CPU and memory usage like top, sar, and watch.
The top command allow you to view system tasks running in real-time. You can see overall info like uptime, load average, CPU and memory usage. The -c switch allows you to see program name under COMMAND. For a full list of shortcut keys, use man top in SSH.
[root@server ~]$ top -c top - 15:08:23 up 115 days, 6:37, 2 users, load average: 1.76, 1.35, 1.13 Tasks: 330 total, 1 running, 326 sleeping, 0 stopped, 3 zombie Cpu(s): 0.7%us, 2.9%sy, 0.0%ni, 81.5%id, 14.9%wa, 0.0%hi, 0.1%si, 0.0%st Mem: 32754672k total, 28808088k used, 3946584k free, 321260k buffers Swap: 8388604k total, 25992k used, 8362612k free, 8140560k cached PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 6926 mysql 20 0 130g 16g 22m S 12.0 52.2 27065:56 /usr/sbin/mysqld --basedir=/usr --datadir=/var/lib/mysql --plugin-dir=/usr/lib64/mysql/plugin 2454 td-agent 20 0 1210m 160m 2440 S 0.7 0.5 347:19.48 /opt/td-agent/embedded/bin/ruby -Eascii-8bit:ascii-8bit /
The System Activity Reporter (sar) is a tool to help you monitor your server's usage statistics. Sar is part of the sysstat package. Using the command sar alone will give you statistics for the last 24 hours.
sar -f allows you to view historic log files starting at 12:00 a.m. for the day.
[root@server ~]$ sar -f /var/log/sa/sa10 Linux 2.6.32-042stab141.3 (server-hostname.net) 01/10/2022 _x86_64_ (24 CPU) 12:00:01 AM CPU %user %nice %system %iowait %steal %idle 12:10:01 AM all 1.92 0.00 0.39 0.00 0.00 97.69 12:20:01 AM all 1.63 0.00 0.34 0.00 0.00 98.02 12:30:01 AM all 1.64 0.00 0.41 0.01 0.00 97.94 12:40:01 AM all 2.85 0.00 0.61 0.00 0.00 96.54 12:50:01 AM all 2.71 0.00 0.57 0.01 0.00 96.72 01:00:01 AM all 2.02 0.00 0.38 0.00 0.00 97.60 01:10:01 AM all 1.43 0.00 0.30 0.00 0.00 98.27
sar -r will display free and used memory stats
[root@server ~]$ sar -r Linux 2.6.32-042stab141.3 (server-hostname.net) 02/02/2022 _x86_64_ (24 CPU) 12:00:01 AM kbmemfree kbmemused %memused kbbuffers kbcached kbcommit %commit 12:10:01 AM 99454800 32354304 24.55 1429856 22624260 8285744 5.91 12:20:01 AM 99760972 32048132 24.31 1429884 22631184 7960428 5.68 12:30:01 AM 99751920 32057184 24.32 1429920 22638392 7974804 5.69 12:40:01 AM 99638056 32171048 24.41 1429960 22643192 8035000 5.73 12:50:01 AM 99752696 32056408 24.32 1430012 22644804 7891100 5.63
The Linux watch allows you to watch commands or tasks as they complete or execute. watch "mysql -e'show full processlist;'" will show all running mysql queries, updating every 2 seconds.
Every 2.0s: mysql -e'show full processlist;' Sat Feb 12 11:09:56 2022 Id User Host db Command Time State Info 26574057 mysql_db1 localhost mysql_db1 Query 2 Sending data SELECT * from db_data_main where DOT_NUMBER = 1381207 26574063 mysql_db1 localhost mysql_db1 Query 1 Sending data SELECT * from db_data_main where DOT_NUMBER = 3173380 26574064 mysql_wp3 localhost mysql_wp3 Sleep 0 NULL 26574068 root localhost NULL Query 0 init show full processlist
High resource usage can be a one-off caused by malicious traffic or a hung process (like a large backup). Frequent high resource usage not caused by traffic or hung process can only be resolved through optimization or upgrade. Keep in mind if you do not optimize your server and sites, an upgrade is only a temporary fix.
Signs it may be time to optimize (server and site(s))
- Not caused by malicious traffic
- Site code is outdated/generating deprecated errors
- CPU usage is high but memory usage is low.
- MySQL queries lag when traffic increases
Signs it may be time to upgrade
- Traffic to your site(s) is steadily increasing
- The server has been optimized for your site needs
- You have optimized your site(s) by eliminating long mysql queries and update the code to support newer versions of PHP.
- You are using caching (like Memcache) but the load remains high