Re: [PATCH 0/5] Prevent kswapd dumping excessive amounts of memoryin response to high-order allocations V2

[Simon Kirby]

On Thu, Dec 09, 2010 at 11:45:06AM +0000, Mel Gorman wrote:

> On Wed, Dec 08, 2010 at 05:55:30PM -0800, Simon Kirby wrote:
> > Hmm...
> > 
> > Wouldn't it make more sense for the fast path page allocator to allocate
> > weighted-round-robin (by zone size) from each zone, rather than just
> > starting from the highest and working down?
> > 
> 
> Unfortunately, that would cause other problems. Zones are about
> addressing limitations. The DMA zone is used by callers that cannot
> address above 16M. On the other extreme, the HighMem zone is used for
> addresses that cannot be directly mapped at all times due to a lack of
> virtual address space.
> 
> If we round-robined the zones, callers that could use HighMem or Normal
> may consume memory from DMA32 or DMA causing future allocation requests
> that require those zones to fail.

Yeah, I don't mean in all cases, I mean when no particular zone is
requested; eg, __alloc_pages_nodemask() with a non-picky zone list, or 
when multiple zones are allowed.  This is the case for most allocations.

As soon as my 757 MB Normal fills up, the allocations come from DMA32
anyway.  (Nothing ever comes from DMA because of lowmem_reserve_pages.)

> > This would mean that each zone would get a proportional amount of
> > allocations and reclaiming a bit from each would likely throw out the
> > oldest allocations, rather than some of that and and some more recent
> > stuff that was allocated at the beginning of the lower zone.
> > 
> > For example, with the current approach, a time progression of allocations
> > looks like this (N=Normal, D=DMA32): 1N 2N 3N 4D 5D 6D 7D 8D 9D
> > 
> > ...once the watermark is hit, kswapd reclaims 1 and 4, since they're
> > oldest in each zone, but 2 and 3 were allocated earlier.
> > 
> > Versus a weighted-round-robin approach: 1N 2D 3D 4N 5D 6D 7N 8D 9D
> > 
> > ...kswapd reclaims 1 and 2, and they're oldest in time and maybe LRU.
> > 
> > Things probably eventually mix up enough once the system has reclaimed
> > and allocated more for a while with the current approach, but the
> > allocations are still chunky depending on how many extra things kswapd
> > reclaims to reach higher-order watermarks, and doesn't this always mess
> > with the LRU when the there are multiple usable zones?
> 
> If addressing limitations were not a problem, we'd just have a single
> zone :/

Wouldn't that be nice. ;)

> > Anyway, this approach might be horrible for some other reasons (page
> > allocations hoping to be sequential?  bigger cache footprint?), but it
> > might reduce the requirements for other other workarounds, and it would
> > make the LRU node-wide instead of zone-wide.
> > 
> 
> Node-wide would be preferably from a page aging perspective but as zones
> are about addressing limitations, we need to be able to reclaim zones
> from a specific zone quickly and not have to scan looking for suitable
> pages.

So, I'm not proposing abandoning zones, but simply changing
get_page_from_freelist() to remember where it last walked zonelist, and
try to make a (weighted) round robin out of it.  It can already allocate
from any zone in this case anyway.  (The implementation would be a bit
more complicated than this due to zonelist not being static, of course.)

Even if the checking of other zones happens in a buffered or chunky way
to reduce caching effects, it would still mean that all zones fill up at
roughly the same time, rather than the DMA zone filling up last.  This
way, the oldest pages would all be the ones that want to be reclaimed,
rather than the a bunch of not-oldest pages being reclaimed simply
because the allocator decided to start with a higher zone to avoid
allocating from the DMA zone.

Simon-
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