Re: [00/17] Large Blocksize Support V3

[Nick Piggin]

Mel Gorman wrote:
> On (25/04/07 23:46), Christoph Lameter didst pronounce:
>
> > On Thu, 26 Apr 2007, Nick Piggin wrote:
> >
> >
> > > Yeah. IMO anti-fragmentation and defragmentation is the hack, and we
> > > should stay away from higher order allocations whenever possible.
> >
> > Right and we need to create series of other approaches that we then label 
> > "non-hack" to replace it. 
>
>
> To date, there hasn't been a credible alternative to dealing with
> fragmentation. Breaking the 1:1 virtual:physical mapping and defragmenting
> would incur a serious performance hit.

Depends what you mean by "dealing with", I guess.

I would say there has been no credible alternative to virtually mapping
the kernel.


> > > Hardware is built to handle many small pages efficintly, and I don't
> > > understand how it could be an SGI-only issue. Sure, you may have an
> > > order of magnitude or more memory than anyone else, but even my lowly
> > > desktop _already_ has orders of magnitude more pages than it has TLB
> > > entries or cache -- if a workload is cache-nice for me, it probably
> > > will be on a 1TB machine as well, and if it is bad for the 1TB machine,
> > > it is also bad on mine.
> >
> > There have been numbers of people that have argued the same point. Just 
> > because we have developed a way of thinking to defend our traditional 4k 
> > values does not make them right.
> >
> >
> > > If this is instead an issue of io path or reclaim efficiency, then it
> > > would be really nice to see numbers... but I don't think making these
> > > fundamental paths more complex and slower is a nice way to fix it
> > > (larger PAGE_SIZE would be, though).
> >
> > The code paths can stay the same. You can switch CONFIG_LARGE pages off
> > if you do not want it and it is as it was.
>
>
> It may not even need that that much effort. The most stressful use of the
> high order allocation paths here require the creation of a filesystem and
> is a deliberate action by the user.

Saying "oh this stuff may not always work quite right for everyone, but
it is OK because it is a special purpose solution for now" IMO is a big
sign saying that it is a bad design, and including it means we're lumped
with it forever.

--
SUSE Labs, Novell Inc.
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