Re: [RFC] using writepage to start io

[Chris Mason <mason@suse.com>]

On Monday, August 06, 2001 11:18:26 PM +0200 Daniel Phillips
<phillips@bonn-fries.net> wrote:

>> Grin, we're talking in circles.  My point is that by having two
>> threads, bdflush is allowed to skip over older buffers in favor of
>> younger ones because somebody else is responsible for writing the
>> older ones out.
> 
> Yes, and you can't imagine an algorithm that could do that with *one* 
> thread?

Imagine one?  Yes.  We're mixing a bunch of issues, so I'll list the 3
different cases again.  memory pressure, write throttling, age limiting.
Pretending that a single thread could get enough context information about
which of the 3 (perhaps more than one) it is currently facing, it can make
the right decisions.

The problem with that right now is that a single thread can't keep up (with
one case, let alone all 3) as the number of devices increases.  We can more
or less just replay the entire l-k discussion(s) on threading models here.

In my mind, in order for a single thread to get the job done, it can't end
up waiting on a device while there are still buffers ready for writeout to
idle devices.

As for a generic mechanism to schedule all FS writeback, I've been trying
to use writepage ;-)  The bad part here it makes the async issues even
bigger, since the flushing thread ends up calling into the FS (who knows
what that might lead to).

-chris