Re: [ck] Re: -mm merge plans for 2.6.23

["Michael Chang" <thenewme91@gmail.com>]

On 7/26/07, Andrew Morton <akpm@linux-foundation.org> wrote:
> On Wed, 25 Jul 2007 09:09:01 -0700
> "Ray Lee" <ray-lk@madrabbit.org> wrote:
>
> > No, there's a third case which I find the most annoying. I have
> > multiple working sets, the sum of which won't fit into RAM. When I
> > finish one, the kernel had time to preemptively swap back in the
> > other, and yet it didn't. So, I sit around, twiddling my thumbs,
> > waiting for my music player to come back to life, or thunderbird,
> > or...
>
> In fact I'd restate the problem as "system is in steady state A, then there
> is a workload shift causing transition to state B, then the system goes
> idle.  We now wish to reinstate state A in anticipation of a resumption of
> the original workload".
>
> swap-prefetch solves a part of that.
>
> A complete solution for anon and file-backed memory could be implemented
> (ta-da) in userspace using the kernel inspection tools in -mm's maps2-*
> patches.  We would need to add a means by which userspace can repopulate
> swapcache, but that doesn't sound too hard (especially when you haven't
> thought about it).
>
> And userspace can right now work out which pages from which files are in
> pagecache so this application can handle pagecache, swap and file-backed
> memory.  (file-backed memory might not even need special treatment, given
> that it's pagecache anyway).
>
> And userspace can do a much better implementation of this
> how-to-handle-large-load-shifts problem, because it is really quite
> complex.  The system needs to be monitored to determine what is the "usual"
> state (ie: the thing we wish to reestablish when the transient workload
> subsides).  The system then needs to be monitored to determine when the
> exceptional workload has started, and when it has subsided, and userspace
> then needs to decide when to start reestablishing the old working set, at
> what rate, when to abort doing that, etc.
>
> All this would end up needing runtime configurability and tweakability and
> customisability.  All standard fare for userspace stuff - much easier than
> patching the kernel.

Maybe I'm missing something here, but if the problem is resource
allocation when switching from state A to state B, and from B to C,
etc.; wouldn't it be a bad thing if state B happened to be (in the
future) this state-shifting userspace daemon of which you speak? (Or
is that likely to be impossible/unlikely for some other reason which
alludes me at the moment?)

--
Michael Chang

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