Re: [PATCH 0/3] Fix malloc() stall in zone_reclaim() and bring behaviour more in line with expectations V3

[Mel Gorman <mel@csn.ul.ie>]

On Thu, Jun 11, 2009 at 04:30:06PM -0700, Andrew Morton wrote:
> On Thu, 11 Jun 2009 11:47:50 +0100
> Mel Gorman <mel@csn.ul.ie> wrote:
> 
> > The big change with this release is that the patch reintroducing
> > zone_reclaim_interval has been dropped as Ram reports the malloc() stalls
> > have been resolved. If this bug occurs again, the counter will be there to
> > help us identify the situation.
> 
> What is the exact relationship between this work and the somewhat
> mangled "[PATCH for mmotm 0/5] introduce swap-backed-file-mapped count
> and fix
> vmscan-change-the-number-of-the-unmapped-files-in-zone-reclaim.patch"
> series?
> 

The patch series "Fix malloc() stall in zone_reclaim() and bring
behaviour more in line with expectations V3" replaces
vmscan-change-the-number-of-the-unmapped-files-in-zone-reclaim.patch.

Portions of the patch series "Introduce swap-backed-file-mapped count" are
potentially follow-on work if a failure case can be identified. The series
brings the kernel behaviour more in line with documentation, but it's easier
to fix the documentation.

> That five-patch series had me thinking that it was time to drop 
> 
> vmscan-change-the-number-of-the-unmapped-files-in-zone-reclaim.patch

This patch gets replaced. All the lessons in the new patch are included.
They could be merged together.

> vmscan-drop-pf_swapwrite-from-zone_reclaim.patch

This patch is wrong, but only sortof. It should be dropped or replaced with
another version. Kosaki, could you resubmit this patch except that you check
if RECLAIM_SWAP is set in zone_reclaim_mode when deciding whether to set
PF_SWAPWRITE or not please?

Your patch is correct if zone_reclaim_mode 1, but incorrect if it's 7 for
example.

> vmscan-zone_reclaim-use-may_swap.patch
> 

This is a tricky one. Kosaki, I think this patch is a little dangerous. With
this applied, pages get unmapped whether RECLAIM_SWAP is set or not. This
means that zone_reclaim() now has more work to do when it's enabled and it
incurs a number of minor faults for no reason as a result of trying to avoid
going off-node. I don't believe that is desirable because it would manifest
as high minor fault counts on NUMA and would be difficult to pin down why
that was happening.

I think the code makes more sense than the documentation and it's the
documentation that should be fixed. Our current behaviour is to discard
clean, swap-backed, unmapped pages that require no further IO. This is
reasonable behaviour for zone_reclaim_mode == 1 so maybe the patch
should change the documentation to

        1       = Zone reclaim discards clean unmapped disk-backed pages
        2       = Zone reclaim writes dirty pages out
        4       = Zone reclaim unmaps and swaps pages

If you really wanted to strict about the meaning of RECLAIM_SWAP, then
something like the following would be reasonable;

	.may_unmap = !!(zone_reclaim_mode & RECLAIM_SWAP),
	.may_swap = !!(zone_reclaim_mode & RECLAIM_SWAP),

because a system administrator is not going to distinguish between
unmapping and swap. I would assume at least that RECLAIM_SWAP implies
unmapping pages for swapping but an updated documentation wouldn't hurt
with

	4       = Zone reclaim unmaps and swaps pages

> (they can be removed cleanly, but I haven't tried compiling the result)
> 
> but your series is based on those.
> 

The patchset only depends on
vmscan-change-the-number-of-the-unmapped-files-in-zone-reclaim.patch
and then only because of merge conflicts. All the lessons in
vmscan-change-the-number-of-the-unmapped-files-in-zone-reclaim.patch are
incorporated.

> We have 142 MM patches queued, and we need to merge next week.
> 

I'm sorry my timing for coming out with the zone_reclaim() patches sucks
and that I failed to spot these patches earlier. Despite the abundance
of evidence, I'm not trying to be deliberatly awkward :/

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab