Messages in this thread | | | Date | Mon, 24 Nov 2003 10:00:43 -0800 | From | Andrew Morton <> | Subject | Re: [RFC] Make balance_dirty_pages zone aware (1/2) |
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"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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