Wait I/O on SMP systems

[unixfreak]

From performance redbook:

Since AIX Version 4.3.3, the I/O wait time is no longer inflated; all CPUs are no longer attributed wait time when a disk is busy and the CPU is idle. The decision
is based on whether a thread is awaiting an I/O on the CPU being measured.
This method can report accurate wio times when just a few threads are doing I/O and the system is otherwise idle.

My question is, if there is wio% of about 30 on an SMP box does that mean that 30% of the time no processes are waiting for CPU? Would it help putting in more CPUs on the system?


Thanks

 

[p5wizard]

wio% has got nothing to do with waiting for CPU.

If you still see high %idle, then there's no use adding CPUs unless you foresee a significant increase in workload.

30 wio% means that 30% of the processing capacity you have was spent waiting for an IO operation to complete during that measured interval. The rest 70% was either spent as user cycles, system cycles or just plain idle.

What that statement you are referring to explains, is that prior to 433, if you had a 4way SMP and one active process, that consumes about 50% of one processor, and 50% of the time waits for IO to complete, you would have seen 87% IO wait and 12% system/user and no 0% idle, the idle processors simply joined the one waiting processor and ticked their spent time of as IO-wait. From 433 on, you would see 12% active, 12% iowait and 75% idle.

Either get a faster disk subsystem or more cache in that subsystem, get a faster network, or deal with the 30 wio%.

Post back with either a topas screen or vmstat 2 10 output and let's have a closer look if you want.



HTH,

p5wizard

 

[unixfreak]

Ok, maybe I was unclear. Let me give an example.

4 CPUs
2 processes waiting for I/O to complete
5 processes requesting CPU time (in run queue)

How would I/O wait look in this situation? I would expect it to be 0%

Thanks

 

[p5wizard]

The 5 runable processes would lower the iowait, but I don't think you'd see 0%. If you see 30% in this setup, then I'll bet the wio% would soar much higher without the 5 runable processes.

HTH,

p5wizard