Re: [PATCH 7/8] writeback: sync old inodes first in background writeback

[Wu Fengguang <fengguang.wu@intel.com>]

On Mon, Jul 26, 2010 at 11:11:37AM +0800, Rik van Riel wrote:
> On 07/25/2010 11:08 PM, Wu Fengguang wrote:
> 
> > We do need some throttling under memory pressure. However stall time
> > more than 1s is not acceptable. A simple congestion_wait() may be
> > better, since it waits on _any_ IO completion (which will likely
> > release a set of PG_reclaim pages) rather than one specific IO
> > completion. This makes much smoother stall time.
> > wait_on_page_writeback() shall really be the last resort.
> > DEF_PRIORITY/3 means 1/16=6.25%, which is closer.
> 
> I agree with the max 1 second stall time, but 6.25% of
> memory could be an awful lot of pages to scan on a system
> with 1TB of memory :)

I totally ignored the 1TB systems out of this topic, because in such
systems, <PAGE_ALLOC_COSTLY_ORDER pages are easily available? :)

> Not sure what the best approach is, just pointing out
> that DEF_PRIORITY/3 may be too much for large systems...

What if DEF_PRIORITY/3 is used under PAGE_ALLOC_COSTLY_ORDER?

Thanks,
Fengguang