Re: throttling dirtiers

[Andrew Morton <akpm@zip.com.au>]

Benjamin LaHaise wrote:
> 
> On Wed, Jul 31, 2002 at 01:06:12PM -0700, William Lee Irwin III wrote:
> > I'm not a fan of this kind of global decision. For example, I/O devices
> > may be fast enough and memory small enough to dump all memory in < 1s,
> > in which case dirtying most or all of memory is okay from a latency
> > standpoint, or it may take hours to finish dumping out 40% of memory,
> > in which case it should be far more eager about writeback.
> 
> Why?  Filling the entire ram with dirty pages is okay, and in fact you
> want to support that behaviour for apps that "just fit" (think big
> scientific apps).  The only interesting point is that when you hit the
> limit of available memory, the system needs to block on *any* io
> completing and resulting in clean memory (which is reasonably low
> latency), not a specific io which may have very high latency.
> 

I hear what you say.  Sometimes we want to allow a lot of
writeback buffering.  But sometimes we don't.

But let's back off a bit.   The problem is that a process
doing a large write() can penalise innocent processes which
want to allocate memory.

How to fix that?
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