Re: [PATCH] cfq + io priorities

[Albert Cahalan <albert@users.sf.net>]

Jens Axboe writes:
> On Sun, Nov 09 2003, Guillaume Chazarain wrote:

>> A process has an assigned io nice level, anywhere
>> from 0 to 20. Both of
>>
>> OK, I ask THE question : why not using the normal
>> nice level, via current->static_prio ? This way,
>> cdrecord would be RT even in IO, and nice -19
>> updatedb would have a minimal impact on the system.
>
> I don't want to tie io prioritites to cpu
> priorities, that's a design decision.

Sure, but do it in a way that's friendly to
all the apps and admins that only know "nice".

nice_cpu   sets CPU niceness
nice_net   sets net niceness
nice_disk  sets disk niceness
...
nice       sets all niceness values at once

>>> these end values are "special" - 0 means the process
>>> is only allowed to do io if the disk is idle, and 20
>>> means the process io is considered
>>
>> So a process with ioprio == 0 can be forever
>> starved. As it's not
>
> Yes
>
>> done this way for nice -19 tasks (unlike FreeBSD),
>> wouldn't it be safer to give a very long deadline
>> to ioprio == 0 requests ?
>
> ioprio == 0 means idle IO. It follows from that that
> you can risk infinite starvation if other io is happening.
> Otherwise it would not be idle io :-)
>
> CFQ doesn't assign request deadlines. That would
> be another way of handling starvation.

Keeping IO niceness as similar to CPU niceness as
you can would be very good for admins.