Re: [RFC] Make balance_dirty_pages zone aware (1/2)

[Andrew Morton <akpm@osdl.org>]

"Martin J. Bligh" <mbligh@aracnet.com> wrote:
>
> >> Currently the VM decides to start doing background writeback of pages if 
> >>  10% of the systems pages are dirty, and starts doing synchronous 
> >>  writeback of pages if 40% are dirty.  This is great for smaller memory 
> >>  systems, but in larger memory systems (>2GB or so), a process can dirty 
> >>  ALL of lowmem (ZONE_NORMAL, 896MB) without hitting the 40% dirty page 
> >>  ratio needed to force the process to do writeback. 
> > 
> > Yes, it has been that way for a year or so.  I was wondering if anyone
> > would hit any problems in practice.  Have you hit any problem in practice?
> > 
> > I agree that the per-zonification of this part of the VM/VFS makes some
> > sense, although not _complete_ sense, because as you've seen, we need to
> > perform writeout against all zones' pages if _any_ zone exceeds dirty
> > limits.  This could do nasty things on a 1G highmem machine, due to the
> > tiny highmem zone.  So maybe that zone should not trigger writeback.
> > 
> > However the simplest fix is of course to decrease the default value of the
> > dirty thresholds - put them back to the 2.4 levels.  It all depends upon
> > the nature of the problems which you have been observing?
> 
> I'm not sure that'll fix the problem for NUMA boxes, which is where we 
> started.

What problems?

> When any node fills up completely with dirty pages (which would
> only require one process doing a streaming write (eg an ftp download),
> it seems we'll get into trouble.

What trouble?

> If we change the thresholds from 40% to
> 20%, that just means you need a slightly larger system to trigger it,
> it never fixes the problem ;-(

What problem?


If we make the dirty threshold a proportion of the initial amount of free
memory in ZONE_NORMAL, as is done in 2.4 it will not be possible to fill
any node with dirty pages.

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