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    SubjectRe: ZONE_PADDING wastes 4 bytes of the new cacheline
    Andrea Arcangeli wrote:
    > On Sat, Oct 23, 2004 at 02:33:39PM +1000, Nick Piggin wrote:
    >
    >>Andrea Arcangeli wrote:
    >>
    >>>I don't see any benefit in limiting the high order, infact it seems a
    >>>bad bug. If something you should limit the _small_ order, so that the
    >>>high order will have a slight chance to succeed. You're basically doing
    >>>the opposite.
    >>>
    >>
    >>You need the order there so someone can't allocate a huge amount
    >>of memory and deplete all your reserves and crash the system.
    >
    >
    > what? the point of alloc_pages is to allow people to allocate memory,
    > what's the difference of 1 2M allocation and 512 4k allocations? No
    > difference at all. Infact if something the 512 4k allocations hurts a
    > lot more since they can fragment the memory, while the single 2M
    > allocation will not fragment the memory. So if you really want to limit
    > something you should do the exact opposite of what the allocator is
    > doing.
    >
    >
    >>For day to day running, it should barely make a difference because
    >>the watermarks will be an order of magnitude larger.
    >
    >
    > yes, it makes little difference, this is why it doesn't hurt that much.
    >

    It is an unlikely scenario, but it is definitely good for robustness.
    Especially on small memory systems where the amount allocated doesn't
    have to be that large.

    Let's say a 16MB system pages_low ~= 64K, so we'll also say we've
    currently got 64K free. Someone then wants to do an order 4 allocation
    OK they succeed (assuming memory isn't fragmented) and there's 0K free.

    Which is bad because you can now get deadlocks when trying to free
    memory.

    >
    >>AFAIKS, pages_min, pages_low and pages_high are all required for
    >>what we want to be doing. I don't see you you could remove any one
    >>of them and still have everything functioning properly....
    >>
    >>I haven't really looked at 2.4 or your patches though. Maybe I
    >>misunderstood you.
    >
    >
    > 2.4 has everything functionally properly but it has no
    > pages_min/low/high, it only has the watermarks. Infact the watermarks
    > _are_ low/min/high. That's what I'm forward porting to 2.6 (besides
    > fixing minor mostly not noticeable but harmful bits like the order
    > nosense described above).
    >

    Oh if you've still got the three watermarks then that may work -
    I thought you meant getting rid of one of the *completely*.

    But I'm still not sure what advantage you see in moving from
    pages_xxx + protection to a single watermark.
    -
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