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SubjectRe: Subtle MM bug
On 7 Jan 2001, Linus Torvalds wrote:

> That doesn't resolve the "2.4.x behaves badly" thing, though.
> I've seen that one myself, and it seems to be simply due to the
> fact that we're usually so good at gettign memory from
> page_launder() that we never bother to try to swap stuff out.
> And when we _do_ start swapping stuff out it just moves to the
> dirty list, and page_launder() will take care of it.
> So far so good. The problem appears to be that we don't swap
> stuff out smoothly: we start doing the VM scanning, but when we
> get enough dirty pages, we'll let it be, and go back to
> page_launder() again. Which means that we don't walk theough the
> whole VM space, we just do some "spot cleaning".

You are right in that we need to refill the inactive list
before calling page_launder(), but we'll also need a few
other modifications:

1. adopt the latest FreeBSD tactic in page_launder()
- mark dirty pages we see but don't flush
- in the first loop, flush up to maxlaunder of the
already seen dirty pages
- in the second loop, flush as many pages as we
need to refill the free&inactive_clean list

2. go back to having a _static_ free target, at
max(freepages.high, SUM(zone->pages_high) ... this
means free_shortage() will never be very big

3. keep track of how many pages we need to free in
page_launder() and substract one from the target
when we submit a page for IO ... no need to flush
20MB of dirty pages when we only need 1MB pages

I have these things in my local tree and it seems to smooth
out the load quite well for a very large haskell run and for
the fillmem program from Juan Quintela's memtest suite.

When combined with your idea of refilling the freelist _first_,
we should be able to get the VM quite a bit smoother under loads
with lots of dirty pages.

I will work on this while travelling to and being in Australia.
Expect a clean patch to fix this problem once the 2.4 bugfix-only
period is over.

Other people on this list are invited to apply the VM patches from
my home page and give them a good beating. I want to be able to
submit a well-tested, known-good patch to Linus once 2.4 is out of
the bugfix-only period...


Virtual memory is like a game you can't win;
However, without VM there's truly nothing to lose...

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