Re: [patch] Converting writeback linked lists to a tree based data structure

[Fengguang Wu <wfg@mail.ustc.edu.cn>]

On Fri, Jan 18, 2008 at 06:41:09AM +0100, Andi Kleen wrote:
> Fengguang Wu <wfg@mail.ustc.edu.cn> writes:
> >
> > Suppose we want to grant longer expiration window for temp files,
> > adding a new list named s_dirty_tmpfile would be a handy solution.
> 
> How would the kernel know that a file is a tmp file?

No idea - but it makes a good example ;-)

But for those making different filesystems for /tmp, /var, /data etc, 
per-superblock expiration parameters may help.

> > So the question is: should we need more than 3 QoS classes?
> 
> [just a random idea; i have not worked out all the implications]
> 
> Would it be possible to derive a writeback apriority from the ionice
> level of the process originating the IO? e.g. we have long standing
> problems that background jobs even when niced and can cause
> significant slow downs to foreground processes by starving IO 
> and pushing out pages. ionice was supposed to help with that
> but in practice it does not seem to have helped too much and I suspect
> it needs more prioritization higher up the VM food chain. Adding
> such priorities to writeback would seem like a step in the right
> direction, although it would of course not solve the problem
> completely.

Good idea. Michael may well be considering similar interfaces :-)