From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Thu, 9 May 2002 10:30:23 -0400 (EDT) From: Bill Davidsen Subject: Re: [RFC][PATCH] IO wait accounting In-Reply-To: Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org Return-Path: To: Rik van Riel Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org List-ID: On Wed, 8 May 2002, Rik van Riel wrote: > the following patch implements simple IO wait accounting, with the > following two oddities: > > 1) only page fault IO is currently counted > 2) while idle, a tick can be counted as both system time > and iowait time, hence IO wait time is not substracted > from idle time (also to ensure backwards compatability > with procps) > > I'm doubting whether or not to change these two issues and if > they should be changed, how should they behave instead ? I'm delighted that someone else is looking at this as well. I've been trying to do a similar thing to determine how well tuning disks, using the vm of the moment, and various RAID configs perform. I have been simply counting WaitIO ticks when there is (a) no runable process in the system, and (b) at least one process blocked for disk i/o, either page or program. And instead of presenting it properly I just stuffed it in a variable and read it from kmem. While I don't defend my data presentation (I didn't want to break any /proc-reading tools) what I was trying to measure is how often would the system run faster if it had faster disk. And unfortunately the answer was that with my typical load disk is not a problem if the system has enough memory. There is always something which wants the CPU. However, disk speed does make a big change in responsiveness, even though the CPU stays busy. I think what is useful is both what I measured, idle time due to disk, and also some responsiveness value, which would be the sum of wait time for all processes waiting i/o (ticks times processes waiting). You can consider if two processes waiting for 50ms is more or less desirable than one waiting 100ms, of course. -- bill davidsen CTO, TMR Associates, Inc Doing interesting things with little computers since 1979. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/