Re: [Lhms-devel] [PATCH 0/7] Fragmentation Avoidance V19

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

On Wed, 2 Nov 2005, Nick Piggin wrote:

> Martin J. Bligh wrote:
>
> > > But let's move this to another thread if it is going to continue. I
> > > would be happy to discuss scheduler problems.
> >
> >
> > My point was that most things we do add complexity to the codebase,
> > including the things you do yourself ... I'm not saying the we're worse
> > off for the changes you've made, by any means - I think they've been
> > mostly beneficial.
>
> Heh - I like the "mostly" ;)
>
> > I'm just pointing out that we ALL do it, so let us
> > not be too quick to judge when others propose adding something that does ;-)
> >
>
> What I'm getting worried about is the marked increase in the
> rate of features and complexity going in.
>
> I am almost certainly never going to use memory hotplug or
> demand paging of hugepages. I am pretty likely going to have
> to wade through this code at some point in the future if it
> is merged.
>

Plenty of features in the kernel I don't use either :) .

> It is also going to slow down my kernel by maybe 1% when
> doing kbuilds, but hey let's not worry about that until we've
> merged 10 more such slowdowns (ok that wasn't aimed at you or
> Mel, but my perception of the status quo).
>

Ok, my patches show performance gains and losses on different parts of
Aim9. page_test is slightly down but fork_test was considerably up. Both
would have an effect on kbuild so more figures are needed on mode
machines. That will only be found from testing from a variety of machines.

> >
> > > You can't what? What doesn't work? If you have no hard limits set,
> > > then the frag patches can't guarantee anything either.
> > >
> > > You can't have it both ways. Either you have limits for things or
> > > you don't need any guarantees. Zones handle the former case nicely,
> > > and we currently do the latter case just fine (along with the frag
> > > patches).
> >
> >
> > I'll go look through Mel's current patchset again. I was under the
> > impression it didn't suffer from this problem, at least not as much
> > as zones did.
> >
>
> Over time, I don't think it can offer any stronger a guarantee
> than what we currently have. I'm not even sure that it would be
> any better at all for problematic workloads as time -> infinity.
>

Not as they currently stand no. As I've said elsewhere, to really
guarantee things, kswapd would need to know how to clear out UesrRclm
pages from the other reserve types.

> > Nothing is guaranteed. You can shag the whole machine and/or VM in
> > any number of ways ... if we can significantly improve the probability of
> > existing higher order allocs working, and new functionality has
> > an excellent probability of success, that's as good as you're going to get.
> > Have a free "perfect is the enemy of good" Linus quote, on me ;-)
> >
>
> I think it falls down if these higher order allocations actually
> get *used* for anything. You'll simply be going through the process
> of replacing your contiguous, easy-to-reclaim memory with pinned
> kernel memory.
>

And a misconfigured zone-based approach just falls apart. Going to finish
that summary mail to avoid repetition.

> However, for the purpose of memory hot unplug, a new zone *will*
> guarantee memory can be reclaimed and unplugged.
>

>

-- 
Mel Gorman
Part-time Phd Student                          Java Applications Developer
University of Limerick                         IBM Dublin Software Lab

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