NetNews Usenet Archive 1992 #20/NN_1992_20.iso/spool/comp/unix/aix/9539

Internet Message Format | 1992-09-10 | 1.8 KB

Path: sparky!uunet!vnet.ibm.com
From: [e-mail] (Patrick Gainer)
Message-ID: <[e-mail]>
Date: Thu, 10 Sep 92 13:58:09 EDT
Newsgroups: comp.unix.aix
Subject: Re: What is wait time  in sar and vmstat
Organization: IBM - Toronto Lab
Disclaimer: This posting represents the poster's views, not those of IBM
News-Software: UReply 3.0
References: <[e-mail]>
Lines: 31

In <[e-mail]> David A. Lee writes:
>The problem is that I cannot find a good description of exactly what
>wait time is. "Normal" unix systems have only user,system, and idle.
>Aix describes wait time (in man vmstat) as cpu cycles checking for
>IO to complete.  Does anyone know exactly what this means?  Are
>wait cycles time when the cpu isnt doing useful work?  Why is the

AIX breaks CPU usage into 4 categories: Idle, Sys, User, and Wait.
One of these 4 categories is updated every 1/100th of a second.
The conditions are as follows:
Idle - incremented when the 'wait' process is executing
Sys  - incremented when a kernel-mode process is executing
User - incremented when a user-mode process is executing
Wait - incremented when the currently executing process is waiting for I/O
       to complete. Therefore, this field counts time when a process is
       not blocked but is waiting for I/O. (If the process is blocked,
       it is not in execution and therefore will not be updating the wait
       field.

Now I don't know exactly under what conditions a process will be waiting
but not blocked but I assume that I/O and CPU are not always concurrent
and that this field indicates the measure of this serialization.
In other words, sometimes there is no useful work to be done while you
wait for I/O. Perhaps someone more familiar with the internals can help
out (or correct me).

>   [e-mail],

Patrick Gainer
[e-mail]