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

    I posted patches to make ZONE_REMOVABLE to LHMS.
    I don't say they are better than Mel-san's patch.
    I hope this will be base of good discussion.

    There were 2 types.
    One was just add ZONE_REMOVABLE.
    This patch came from early implementation of memory hotplug VA-Linux

    ZONE_HIGHMEM is used for this purpose at early implementation.
    We thought ZONE_HIGHMEM is easier removing than other zone.
    But some of archtecture don't use it. That is why ZONE_REMOVABLE
    was born.
    (And I remember that ZONE_DMA32 was defined after this patch.
    So, number of zone became 5, and one more bit was necessary in
    page->flags. (I don't know recent progress of ZONE_DMA32)).

    Another one was a bit similar than Mel-san's one.
    One of motivation of this patch was to create orthogonal relationship
    between Removable and DMA/Normal/Highmem. I thought it is desirable.
    Because, ppc64 can treat that all of memory is same (DMA) zone.
    I thought that new zone spoiled its good feature.


    P.S. to Mel-san.
    I'm sorry for late writing of this. This threads was mail bomb for me
    to read with my poor English skill. :-(

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

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