Re: A unresponsive file system can hang all I/O in the system on linux-2.6.23-rc6 (dirty_thresh problem?)

From: Daniel Phillips
Date: Fri Sep 28 2007 - 20:47:13 EST

On Thursday 27 September 2007 23:50, Andrew Morton wrote:
> Actually we perhaps could address this at the VFS level in another
> way. Processes which are writing to the dead NFS server will
> eventually block in balance_dirty_pages() once they've exceeded the
> memory limits and will remain blocked until the server wakes up -
> that's the behaviour we want.

It is not necessary to restrict total dirty pages at all. Instead it is
necessary to restrict total writeout in flight. This is evident from
the fact that making progress is the one and only reason our kernel
exists, and writeout is how we make progress clearing memory. In other
words, if we guarantee the progress of writeout, we will live happily
ever after and not have to sell the farm.

The current situation has an eerily similar feeling to the VM
instability in early 2.4, which was never solved until we convinced
ourselves that the only way to deal with Moore's law as applied to
number of memory pages was to implement positive control of swapout in
the form of reverse mapping[1]. This time round, we need to add
positive control of writeout in the form of rate limiting.

I _think_ Peter is with me on this, and not only that, but between the
too of us we already have patches for most of the subsystems that need
it, and we have both been busy testing (different subsets of) these
patches to destruction for the better part of a year.

Anyway, to fix the immediate bug before the one true dirty_limit removal
patch lands (promise) I think you are on the right track by noticing
that balance_dirty_pages has to become aware of how congested the
involved block device is, since blocking a writeout process on an
underused block device is clearly a bad idea. Note how much this idea
looks like rate limiting.

[1] We lost the scent for a number of reasons, not least because the
experimental implementation of reverse mapping at the time was buggy
for reasons entirely unrelated to the reverse mapping itself.


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