Re: High load average on disk I/O on 2.6.17-rc3

["Martin J. Bligh" <mbligh@mbligh.org>]

Erik Mouw wrote:
> On Mon, May 08, 2006 at 05:31:29PM +0200, Arjan van de Ven wrote:
> 
>>On Mon, 2006-05-08 at 17:22 +0200, Erik Mouw wrote:
>>
>>>... except that any kernel < 2.6 didn't account tasks waiting for disk
>>>IO.
>>
>>they did. It was "D" state, which counted into load average.
> 
> 
> They did not or at least to a much lesser extent. That's the reason why
> ZenIV.linux.org.uk had a mail DoS during the last FC release and why we
> see load average questions on lkml.
> 
> I've seen it on our servers as well: when using 2.4 and doing 50 MB/s
> to disk (through NFS), the load just was slightly above 0. When we
> switched the servers to 2.6 it went to ~16 for the same disk usage.

Looks like both count it, or something stranger is going on.

2.6.16:

static unsigned long count_active_tasks(void)
{
         return (nr_running() + nr_uninterruptible()) * FIXED_1;
}

2.4.0:

static unsigned long count_active_tasks(void)
{
         struct task_struct *p;
         unsigned long nr = 0;

         read_lock(&tasklist_lock);
         for_each_task(p) {
                 if ((p->state == TASK_RUNNING ||
                      (p->state & TASK_UNINTERRUPTIBLE)))
                         nr += FIXED_1;
         }
         read_unlock(&tasklist_lock);
         return nr;
}