Re: OT: high WIO on Linux

On 03/10/2006 02:05:11 AM, Mark Brinsmead wrote:
> Sorry if this is a bit off topic, but I imagine that a few of you may
> have encountered this in the past...
>
> Recently, I have observed a couple different Linux boxes (I'm reasonably
> sure both were RHEL3) displaying very unusual-looking WIO statistics.
> On one, I was seeing about 15% USER, and 80%+ WIO. (Okay, that's bad,
> but not preposterous, except that 'iostat' showed all disks were
> essentially idle!) The other was even weirder -- 50% IDLE and 50% WIO.
> There was nothing there to generate I/O requests.

First of all, Linux sucks. Let me clarify things a bit: the whole concept of %WIO makes me cringe. CPU works in user mode, kernel mode, is on the interrupt stack or is idle. Period. When process doesn't use CPU, it isn't in "%WIO mode", it's idle. Like Eric Idle, only with lowercase "i". Second, Linux cannot do process accounting. Linux cannot tell you how many I/O requests did each process generate in the last 10 seconds.

>
> Both machines had (at least) one thing in common -- aside from running
> Oracle. They each had *much* more RAM than SWAP. (RAM/SWAP was 6GB/2GB
> for the first case, and 4GB/1GB for the second.)
>
> Is it possible that all of these I/O waits are being generated inside
> the kernel -- perhaps by the filesystem buffer cache trying to steal
> pages from itself, or something equally unproductive.

So, what do you get when you say "iostat -x"?

>
> By the way, in neither case was there noticeable evidence of paging or
> swapping. The swap space itself was esentially unused...
>
> Can anybody offer a plausible (even if it is hypothetical) explanation
> for this behaviour? Or maybe direct me to some resources that will help
> me better understand *how* or *why* such things can happen?
>
> I mean, really! 50% IDLE + 50% WIO is awfully strange! My Linux skills
> are maybe a bit "lightweight", but I've been working with UNIX for
> decades and never seen (or at least never *noticed*) anything remotely
> like this...
>
> Cheers,
> -- Mark.

What is with the network? Is anyone pinging you? How many packets/sec do you have? Does uptime show any significant run queue? Always look on the bright side of life, it's Friday.  

>
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-- 
Mladen Gogala
http://www.mgogala.com

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