RHCE

RHCE.markdown

Useful knowledge

Command Line

• \CMD: run CMD in a sub-shell? (useful for not using .bashrc or .zshrc which might have alias)

System Performance Tools

top: general performance monitoring

• press 1: toggle list cpu usage by cores (useful for understanding load average and io wait)
• < and >: sort with prev or next colume (starts from cpu)
• f (field): add or remove column, in top view, press W to save view to ~/.toprc
• if see high cpu wait, time to debug disk
• press ?: list of available options

Mem

• press E/e to zoom out the memory scale: KB -> MB -> GB
• VIRT: virtual memory reserved, this should be large, but not actually all used (32TB in total for 64-bit OS)
• RES: resident memory is the real allocated memory

iostat: disk performance monitoring

• iostat 2 (sec interval) 5 (loop)
• first output gives you a summary of usage since startup
• the rest gives summary of interval

iotop: disk usage per process

• which process is hogging my disk

free: memory performance only

• free -m: in MB
• total = used + free + cache/buffer
• available: free + can be clear cached.
• swap

swapon: about swap

• swapon -s: which partition used for swap

vmstat: virtual memory usage

• support polling in interval

sar: performance history

• cron job in /etc/cron.d/sysstat
• logs in /var/log/sa
• configuration in /etc/sysconfig/sysstat
• LANG=C sar: set LANG=C to use 24H instead of AM/PM

arguments

• -b: IO (tps: transcation per sec, bread/s: block read per sec)
• -P CPU_NUM: CPU information
• -n dev: network (IFACE: interface, rxpck/s: received packet per sec, txpck/s: transmitted packet per sec)