Re: XFS vs Elevators (was Re: [PATCH RFC] nilfs2: continuous snapshotting file system)

[Dave Chinner <david@fromorbit.com>]

On Thu, Aug 21, 2008 at 05:00:39PM +1000, Nick Piggin wrote:
> On Thursday 21 August 2008 16:14, Dave Chinner wrote:
> 
> > I think that we need to issue explicit unplugs to get the log I/O
> > dispatched the way we want on all elevators and stop trying to
> > give elevators implicit hints by abusing the bio types and hoping
> > they do the right thing....
> 
> FWIW, my explicit plugging idea is still hanging around in one of
> Jens' block trees (actually he refreshed it a couple of months ago).
> 
> It provides an API for VM or filesystems to plug and unplug
> requests coming out of the current process, and it can reduce the
> need to idle the queue. Needs more performance analysis and tuning
> though.

We've already got plenty of explicit unplugs in XFS to get stuff
moving quickly - I'll just have to add another....

> But existing plugging is below the level of the elevators, and should
> only kick in for at most tens of ms at queue idle events, so it sounds
> like it may not be your problem. Elevators will need some hint to give
> priority to specific requests -- either via the current threads's io
> priority, or information attached to bios.

It's getting too bloody complex, IMO. What is right for one elevator
is wrong for another, so as a filesystem developer I have to pick
one to target. With the way the elevators have been regressing,
improving and changing behaviour, I am starting to think that I
should be picking the noop scheduler. Any 'advanced' scheduler that
is slower than the same test on the noop scheduler needs fixing...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com