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    SubjectRe: [00/17] Large Blocksize Support V3
    On (27/04/07 20:05), Nick Piggin didst pronounce:
    > Christoph Hellwig wrote:
    > >On Thu, Apr 26, 2007 at 05:48:12PM +1000, Nick Piggin wrote:
    > >
    > >>>Well maybe you could explain what you want. Preferably without
    > >>>redefining the established terms?
    > >>
    > >>Support for larger buffers than page cache pages.
    > >
    > >
    > >I don't think you really want this :) The whole non-pagecache I/O
    > >path before 2.3 was a toal pain just because it used buffers to drive
    > >I/O. Add to that buffers bigger than a page and you add another
    > >two mangnitudes of complexity. If you want to see a mess like that
    > >download on of the eary XFS/Linux releases that had an I/O path
    > >like that. I _really_ _really_ don't want to go there.
    > I'm not actually suggesting to add anything like that. But I think
    > larger blocks can be doable while retaining the "buffer" layer as a
    > relatively simple pagecache to block translation.
    > Anyway, I'm working on patches... they might crash and burn, but we
    > might have something to talk about later.
    > >Linux has a long tradition of trading a tiny bit of efficieny for
    > >much cleaner code, and I'd for 100% go down Christoph's route here.
    > >Then again I'd actually be rather surprised if > page buffers
    > >were more efficient - you'd run into shitloads over overhead due to
    > >them beeing non-contingous like calling vmap all over the place,
    > >reprogramming iommus to at least make them look virtually contingous [1],
    > >etc..
    > I still think hardware should work reasonably well with 4K pages. The
    > SGI io controllers and/or the Linux block layer that doesn't allow more
    > than 128 sg entries is clearly suboptimal if the hardware runs twice as
    > fast with 2MB submissions.
    > >I also don't quite get what your problem with higher order allocations
    > >are. order 1 allocations are generally just fine, and in fact
    > >thread stacks are >= oder 1 on most architectures. And if the pagecache
    > >uses higher order allocations that means we'll finally fix our problems
    > >with them, which we have to do anyway. Workloads continue to grow and
    > >with them the kernel overhead to manage them, while the pagesize for
    > >many architectures is fixed. So we'll have to deal with order 1
    > >and order 2 allocations better just for backing kmalloc and co.
    > The pagecache is much bigger and often a lot more activity than these
    > other things though. Also, the more things you add to higher order
    > allocations, the more pressure you have.
    > I like PAGE_SIZE pagecache, because it is reliable and really fast, if
    > you need to reclaim a page it should be almost O(1).
    > >Or think jumboframes for that matter.
    > They can actually run into problems if the hardware wants contiguous
    > memory.
    > I don't know why you think the fragmentation issues are just magically
    > fixed. It is hard and inefficient to reclaim larger order blocks (even
    > with lumpy reclaim), and Mel's patches aren't perfect. Actually, last
    > time I looked, they needed to keep at least 16MB of pages free to be
    > reasonably effective (or do we just say that people with less than XMB
    > of memory shouldn't be accessing these filesystems anyway?)

    It'll work without adjusting the min_free_kbytes at all. The 16MB free had
    better results after fragmentation stress tests but this was a few percent
    of memory when allocating as huge pages as opposed to it falling apart. The
    success rates were still way way higher than the vanilla kernel.

    >, and I'm
    > not sure if they have been tested for long term stability in the
    > presence of a reasonable amount of higher order allocations.

    I don't have a sample workload that has reasonable amount of higher order
    allocations over longer period of time. When the next -mm comes out, SLUB will
    be able to use high-order pages so I'll boot my machine with less memory to
    pressure it more. Assuming the kernel boots on my desktop machine, I should
    get some idea of what its long-term behaviour looks like.

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