%iowait和CPU使用率的正確認知，%iowaitcpu認知

%iowait和CPU使用率的正確認知，%iowaitcpu認知

 resources

• 理解 %IOWAIT (%WIO)
• LINUX系統的CPU使用率和LOAD
• Linux Performance Observability Tools
• How Linux CPU Usage Time and Percentage is calculated
• Linux進程狀態

man (on RHEL 7)

# man mpstat
%usr
       Show the percentage of CPU utilization that occurred while executing at the user level (application).
%nice
       Show the percentage of CPU utilization that occurred while executing at the user level with nice priority.
%sys
       Show the percentage of CPU utilization that occurred while executing at the system level (kernel).Note that this does not include time spent servicing hardware and software interrupts.

%iowait
       Show the percentage of time that the CPU or CPUs were idle during which the system had an outstanding disk I/O request.
%irq
       Show the percentage of time spent by the CPU or CPUs to service hardware interrupts.
%soft
       Show the percentage of time spent by the CPU or CPUs to service software interrupts.
%steal
       Show the percentage of time spent in involuntary wait by the virtual CPU or CPUs while the hypervisor was servicing another virtual  processor.
%guest
       Show the percentage of time spent by the CPU or CPUs to run a virtual processor.
%gnice
       Show the percentage of time spent by the CPU or CPUs to run a niced guest.
%idle
       Show the percentage of time that the CPU or CPUs were idle and the system did not have an outstanding disk I/O request.

# man top
us, user : time running un-niced user processes
sy, system : time running kernel processes
ni, nice : time running niced user processes
id, idle : time spent in the kernel idle handler
wa, IO-wait : time waiting for I/O completion
hi : time spent servicing hardware interrupts
si : time spent servicing software interrupts
st : time stolen from this vm by the hypervisor

TIPS

• CPU Usage Time and Percentage

參考 mpstat 手冊，%usr + %nice + %sys + %iwoait + %irq + %soft + %steal + %guest + %gnice + %idle = 100%

%steal一般是在虛擬機中才能看到數值，比如CPU overcommitment很嚴重的VPS，而%guest和%nice一般都很低,
所以也可以根據/proc/stat或者top可得，user + nice + system + idle + iowait + irq + softirq + steal = 100

To calculate Linux CPU usage time subtract the idle CPU time from the total CPU time as follows:
Total CPU time since boot       = user + nice + system + idle + iowait + irq + softirq + steal
Total CPU Idle time since boot  = idle + iowait
Total CPU usage time since boot = (Total CPU time since boot) - (Total CPU Idle time since boot)
Total CPU percentage            = (Total CPU usage time since boot)/(Total CPU time since boot X 100)

• Linux進程狀態

運行狀態（TASK_RUNNING）：
　　是運行態和就緒態的合並，表示進程正在運行或准備運行，Linux 中使用TASK_RUNNING 宏表示此狀態
可中斷睡眠狀態（淺度睡眠）（TASK_INTERRUPTIBLE）：
　　進程正在睡眠（被阻塞），等待資源到來是喚醒，也可以通過其他進程信號或時鐘中斷喚醒，進入運行隊列。Linux 使用TASK_INTERRUPTIBLE 宏表示此狀態。
不可中斷睡眠狀態（深度睡眠狀態）（TASK_UNINTERRUPTIBLE）：
　　其和淺度睡眠基本類似，但有一點就是不可被其他進程信號或時鐘中斷喚醒。Linux 使用TASK_UNINTERRUPTIBLE 宏表示此狀態。
暫停狀態（TASK_STOPPED）：
　　進程暫停執行接受某種處理。如正在接受調試的進程處於這種狀態，Linux 使用TASK_STOPPED 宏表示此狀態。
僵死狀態（TASK_ZOMBIE）：
　　進程已經結束但未釋放PCB，Linux 使用TASK_ZOMBIE 宏表示此狀態

• %iowait 的正確認知

%iowait 表示在一個采樣周期內有百分之幾的時間屬於以下情況：CPU空閒、並且有仍未完成的I/O請求。

對 %iowait 常見的誤解有兩個：
　　一是誤以為 %iowait 表示CPU不能工作的時間，
　　二是誤以為 %iowait 表示I/O有瓶頸。

首先 %iowait 升高並不能證明等待I/O的進程數量增多了，也不能證明等待I/O的總時間增加了。
　　例如，在CPU繁忙期間發生的I/O，無論IO是多還是少，%iowait都不會變；當CPU繁忙程度下降時，有一部分IO落入CPU空閒時間段內，導致%iowait升高。
　　再比如，IO的並發度低，%iowait就高；IO的並發度高，%iowait可能就比較低。

可見%iowait是一個非常模糊的指標，如果看到 %iowait 升高，還需檢查I/O量有沒有明顯增加，avserv/avwait/avque等指標有沒有明顯增大，應用有沒有感覺變慢，如果都沒有，就沒什麼好擔心的。

• 查看CPU使用率，推薦如下Linux命令：

# top
# sar -u 1 5
# vmstat -n 1 5
# mpstat -P ALL 1 5

• 查看Load的值，推薦如下Linux命令：

# top
# uptime
# sar -q 1 5

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