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