Re: [git patches] IDE update

André Tomt
2005-07-04T23:25:50Z

Al Boldi wrote:
> Bartlomiej Zolnierkiewicz wrote: {
>
>>>>On 7/4/05, Al Boldi <a1426z@gawab.com> wrote:
>>>>Hdparm -tT gives 38mb/s in 2.4.31
>>>>Cat /dev/hda > /dev/null gives 2% user 33% sys 65% idle
>>>>
>>>>Hdparm -tT gives 28mb/s in 2.6.12
>>>>Cat /dev/hda > /dev/null gives 2% user 25% sys 0% idle 73% IOWAIT

The "hdparm doesn't get as high scores as in 2.4" is a old discussed to
death "problem" on LKML. So far nobody has been able to show it affects
anything but that pretty useless quasi-benchmark.

>>>>It feels like DMA is not being applied properly in 2.6.12.
>>>
>>>Same on 2.6.10,11,12.
>>>No errors though, only sluggish system.

Really sluggish or just "benchmark-sluggish"? If the former, try
selecting a different IO elevator/sheduler. If the latter it doesn't
matter much, at least not with the very simple hdparm test :-)

>>
>> What about earlier kernels?
>> Please try to narrow down the problem to a specific kernel version.
>> }
>
> Don't know about 2.6.0-2.6.9, but 2.4.31 is ok.
>
> Bartlomiej,
> When you compare 2.4.31 with 2.6.12 don't you see this problem on your
> machine?
> If you have a fast system the slowdown won't show, but your IOWAIT will be
> higher anyway!

Nothing wrong with 73% iowait, I'd even consider it very low while
putting load on a harddrive. Its just time spent waiting for data to be
returned from disk, and thus I usually expect no lower than ~98-99%
while stressing any disk. Harddisks are _slow as snails_ compared to cpu
cycles ;-)

Beware 2.4 didn't export that statistic at all to userspace, so 0%
iowait gets reported from most 2.6-ready reporting tools on 2.4.