> On Tue, 16 Jan 2007 17:30:26 -0800 (PST) Christoph Lameter <[email protected]> wrote:
> > Nope. You've completely omitted the little fact that we'll do writeback in
> > the offending zone off the LRU. Slower, maybe. But it should work and the
> > system should recover. If it's not doing that (it isn't) then we should
> > fix it rather than avoiding it (by punting writeback over to pdflush).
>
> pdflush is not running *at* all nor is dirty throttling working. That is
> correct behavior? We could do background writeback but we choose not to do
> so? Instead we wait until we hit reclaim and then block (well it seems
> that we do not block the blocking there also fails since we again check
> global ratios)?
I agree that it is a worthy objective to be able to constrain a cpuset's
dirty memory levels. But as a performance optimisation and NOT as a
correctness fix.
Consider: non-exclusive cpuset A consists of mems 0-15, non-exclusive
cpuset B consists of mems 0-3. A task running in cpuset A can freely dirty
all of cpuset B's memory. A task running in cpuset B gets oomkilled.
Consider: a 32-node machine has nodes 0-3 full of dirty memory. I create a
cpuset containing nodes 0-2 and start using it. I get oomkilled.
There may be other scenarios.
IOW, we have a correctness problem, and we have a probable,
not-yet-demonstrated-and-quantified performance problem. Fixing the latter
(in the proposed fashion) will *not* fix the former.
So what I suggest we do is to fix the NFS bug, then move on to considering
the performance problems.
On reflection, I agree that your proposed changes are sensible-looking for
addressing the probable, not-yet-demonstrated-and-quantified performance
problem. The per-inode (should be per-address_space, maybe it is?) node
map is unfortunate. Need to think about that a bit more. For a start, it
should be dynamically allocated (from a new, purpose-created slab cache):
most in-core inodes don't have any dirty pages and don't need this
additional storage.
Also, I worry about the worst-case performance of that linear search across
the inodes.
But this is unrelated to the NFS bug ;)
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