====== High System Load ======

**General Information**

Troubleshooting high system load.

**Checklist**
  * Distro(s): Enterprise Linux 6

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====== Understanding System Load ======

Load average can be seen in both "uptime" and "top". It shows the load average for the last "1 mins, 5 mins, and 15 mins".

===== Traffic/Bridge analogy =====

Reposting takeaway here in case it goes away.
Source: [[http://blog.scoutapp.com/articles/2009/07/31/understanding-load-averages|ScoutBlog Load Average]]

**On a Single Core CPU System**
  * The server is a bridge operator.
  * Cars are processes.
  * Cars on the bridge are using CPU time.
  * Cars waiting to go on the bridge are waiting for CPU time (because the bridge is backed up and they cannot get CPU time immediately.
  * Load of 0.00 means there is no traffic on the bridge.
  * Load of 1.00 means the bridge is at capacity. No more cars(processes) at this very second can get CPU time without waiting.
  * Load over 1.00 means there is a backup.
    * 2.00 => there are "two lanes" worth of cars(processes). One lane is being processed, another lane is waiting for CPU time.

**Multi-CPU/Core Systems**
  * Load is relative to how many CPUs are on the system.
    * 1 CPU/Core = 100% is load 1.00
    * 2 CPU/Cores = 100% is load 2.00
    * 4 CPU/Cores = 100% is load 4.00
  * Example: From the analogy above, each CPU Core can actively process 1 bridge lane.

===== Calculate Overall CPU Load =====
  * Get number of CPUs<code bash>grep -c proc /proc/cpuinfo
OR
nproc</code>
  * Load Average / NumProccessors = decimal % load
    * Example: LoadAvg(1.5) / 2 Processors = 0.75 or 75% system load on a dual core system.

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====== Troubleshooting Tools ======

The following tools are useful when troubleshooting system load.

Typically built in
  * top => live system process view
  * uptime => system uptime and load averages
  * vmstat => virtual memory stats (memory, swap, i/o, cpu)

Need to install (if using a minimal install base)
  * iostat (sysstat package) => print i/o statistics
  * iotop => live disk i/o
  * lsof => list open files

Base Repo
<code bash>
yum -y install iotop lsof sysstat
</code>

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====== Troubleshooting Steps ======

  - **Know how many processors you have**. This is essential to determine if load is high. See "Understanding Load" above for more details.
    - <code bash>grep -c proc /proc/cpuinfo</code>
    - %Load (decimal) = (Load Average / Number Processors)
    - Example: Number of processors = 2, load average seen = 1.50
    - 1.50 / 2 = 0.75 or 75% load on the processors
  - **Check load averages**
    - Uptime shows the load average for the last 1, 5, and 15 minutes. If it is too high or trending up, time to investigate further.
    - <code bash>uptime</code>
  - **What kind of load**
    - Use vmstat to determine what kind of system load. "vmstat 1" prints stats every 1 second.
      - <code bash>vmstat 1</code>
    - Important columns to take note of:
      - CPU: "wa" => Time spent waiting for I/O. If high, something is probably heavily utilizing disk.
      - CPU: "id" => CPU time spent idle. If close to 0, CPU is used heavily.
      - CPU: "sy" => CPU time spent running system/kernel processes. Mail and firewalls are common causes of high system use.
      - CPU: "us" => CPU time spent running user processes. If high, investigate with top.
      - IO: "bi" => blocks received from block device each second. (If high, something is heavily reading a disk)
      - IO: "bo" => blocks sent to a block device each second. (If high, something is heavily writing to disk)
      - SWAP: "si" => Memory swapped in from disk each second.
      - SWAP: "so" => Memory swapped to disk each second.
        - If either are high, memory is most likely also very low.
      - MEMORY: "free" => memory free. If this is low, there is probably swapping going on as well.
    - **Further investigate either high CPU/Memory use or Disk I/O**

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===== High CPU =====

Clues that you should investigate high CPU usage:
  * Low CPU "id" (idle)
  * High CPU "sy" (system processes)
  * High CPU "us" (user processes).

<code bash>
top
</code>

While in top:
  * Turn on highlighting: 'z'
  * Highlight sort column: 'x'

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===== High Memory Use =====

Notes on Linux memory management
  * Linux uses free memory in RAM as a buffer cache to speed up application performance.
  * When memory is needed, the buffer cache shrinks to allow other applications to use it.
  * **Actual free memory = Memory Free + Buffers + Cached**

Clues that you should investigate high memory usage:
  * Memory free: Very low
  * Swap si: High swapping in from disk
  * Swap so: High swapping out to disk

Start top and sort by %mem usage
<code bash>
top -a
</code>

While in top:
  * Turn on highlighting: 'z'
  * Highlight sort column: 'x'

Other memory columns (shift+ '<' or '>' to change sort columns)
  * VIRT = virtual memory size used: code, data, and shared libraries plus swapped pages.
  * RES = resident size: non-swapped physical memory a process is using

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===== Disk I/O =====

  * I/O wait (wa) is the percentage of time a CPU is waiting on disk.
    * If I/O wait % is > (1/# CPU cores), then the CPUs are spending a lot of time waiting on disk.
  * Easiest ways to improve disk I/O
    * Give the system more memory
    * Tune the application to use more in memory caches than disk

Clues that you should investigate high Disk I/O:
  * High CPU "id" (idle)
  * High CPU "wa" (wait)

\\
**iostat** - View I/O stats with extended statistics, every 3 seconds
<code bash>
iostat -x 3
</code>
  * "%util" => If this is close to 100%, the listed "Device" is the one to investigate.

\\
**iotop** - Live disk I/O similar to top
<code bash>
iotop
</code>

\\
**lsof** - If a particular device is discovered, another option for further details is to list open files for that mount point.
  * Device discovered via iostat
  * Mount point discovered
    * If 'dm' device:<code bash>ls -l /dev/mapper</code>
  * Then search lsof for that mount point:<code bash>lsof | grep /var/</code>

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