>I am inferring this using iostat which shows that average device >utilization fluctuates between 83 and 99 percent and the average >request size is around 650 sectors (going to the device) without >writepages. > >With writepages, device utilization never drops below 95 percent and >is usually about 98 percent utilized, and the average request size to >the device is around 1000 sectors.
Well that blows away the only two ways I know that this effect can happen. The first has to do with certain code being more efficient than other code at assembling I/Os, but the fact that the CPU utilization is the same in both cases pretty much eliminates that. The other is where the interactivity of the I/O generator doesn't match the buffering in the device so that the device ends up 100% busy processing small I/Os that were sent to it because it said all the while that it needed more work. But in the small-I/O case, we don't see a 100% busy device. So why would the device be up to 17% idle, since the writepages case makes it apparent that the I/O generator is capable of generating much more work? Is there some queue plugging (I/O scheduler delays sending I/O to the device even though the device is idle) going on? -- Bryan Henderson IBM Almaden Research Center San Jose CA Filesystems - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
