Re: [PATCH 2/9]: Reduce Log I/O latency

[Matt Mackall <mpm@selenic.com>]

On Fri, Nov 23, 2007 at 09:29:09AM +1100, David Chinner wrote:
> On Thu, Nov 22, 2007 at 12:10:29PM -0600, Matt Mackall wrote:
> > On Thu, Nov 22, 2007 at 09:31:59PM +1100, David Chinner wrote:
> > [...]
> > > > In other words, I/O priority is per-spindle and not per-filesystem and
> > > > thus this change has consequences that leak outside the filesystem in
> > > > question. That's bad.
> > > 
> > > This has nothing to do with this patch - it's a problem with sharing
> > > a single resource in a RT system between two non-deterministic
> > > constructs. e.g. I can put two ext3 filesystems on the one spindle,
> > > run two completely independent RT workloads on the different
> > > filesystems and have one workload DOS the other due to differences
> > > in priority at the spindle.
> > 
> > Sure. And it's up to the RT system designer not to do something stupid
> > like that. The problem is that your patch potentially promotes a
> > non-RT I/O activity to an RT one without regard to the rest of the
> > system.
> 
> So this:
> 
> http://marc.info/?l=linux-kernel&m=119247074517414&w=2
> 
> shouldn't be allowed, either? (rt kjournald for ext3)

No, I think not. If a user wants to manually promote kjournald, that's fine.

> > Perfectly understood. And that's fine. A system designer is allowed to
> > shoot himself in the foot.
> 
> Ok. I'll point anyone that complains at you, Matt ;)
> 
> > I don't think there's any fundamental reason the I/O subsystem or
> > filesystems can't be taught to handle priority inversion, which is
> > much more acceptable and general fix.
> 
> See my reply to Andi.

I did. And I'll admit it's pretty thorny and I certainly don't know
enough about XFS internals to comment further.

> > If I've got XFS on filesystems A and B on the same spindle (or volume
> > group?) and my real RT I/O takes place only on B, then I want log
> > flushing to happen in RT on B. But -never on A-. If I can do this with
> > a tunable, I'm perfectly happy.
> 
> No, not another mount option. I'm just going to drop this one for
> now...

I was actually just suggesting allowing a user to do ioprio_set on the
appropriate kernel threads.

-- 
Mathematics is the supreme nostalgia of our time.