Re: dirty balancing deadlock

[Chris Mason <chris.mason@oracle.com>]

On Tue, Feb 20, 2007 at 09:47:11AM +0100, Miklos Szeredi wrote:
> > > How about this?
> > > 
> > > Solves the FUSE deadlock, but not the throttle_vm_writeout() one.
> > > I'll try to tackle that one as well.
> > > 
> > > If the per-bdi dirty counter goes below 16, balance_dirty_pages()
> > > returns.
> > > 
> > > Does the constant need to tunable?  If it's too large, then the global
> > > threshold is more easily exceeded.  If it's too small, then in a tight
> > > situation progress will be slower.
> > 
> > Ok, what is supposed to happen here is that filesystems are supposed to
> > be throttled from making more dirty pages when the system is over the
> > threshold.  Even if filesystem A doesn't have much to contribute, and
> > filesystem B is the cause of 99% of the dirty pages, the goal of the
> > threshold is to prevent more dirty data from happening, and filesystem A
> > should block.
> 
> Which is the cause of the current deadlock.  But if we allow
> filesystem A to go into the red just a little, the deadlock is
> avoided, because it can continue to make progress with cleaning the
> dirtyness produced by B.
> 
> The maximum that filesystems can go over the limit will be
> 
>   (16 + epsilon) * number-of-queues

Right, even for thousands of mounted filesystems ~16 pages per FS
effectively pinned is not horrible.

-chris

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