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    SubjectRe: OOM killer gripe (was Re: What still uses the block layer?)
    Date
    david@lang.hm writes:

    >
    > on some kernel versions you are correct about needing swap > ram, but on current
    > versions you are not. the swap space gets allocated as needed, and re-used as
    > needed (I don't know the mechanism of this, but I remember the last time this
    > changed from vm=max(ram,swap) to vm=ram+swap)

    I don't think I can recall a linux kernel that required swap > ram.
    However for serious swapping under linux having swap > ram was very
    useful and pretty much a requirement for a workload that involved
    swapping heavily (not thrashing).

    >> I have not heard of many people swapping and not thrashing lately.
    >> I think part of the problem is that we do random access to the swap
    >> partition which makes us seek limited. And since the number of
    >> seeks per unit time has been increasing at a linear or slower rate
    >> that if we are doing random disk I/O then the amount we can use
    >> the disk for is very limited. I wonder if we could figure out
    >> how to push and pull 1M or bigger chunks into and out of swap?
    >
    > it has been noted by many people that linux is very slow to pull things back
    > into ram from swap, significantly slower then simple seed limiting would seem to
    > account for.

    Yes. It may be the large amount of random access (my current guess)
    or it may be something else.

    I'm wonder if I should build an application with a configurable
    data set and working set that can be used for swap testing. I don't
    think it would be very hard and it might help sort through some of
    the swap performance problems.

    Eric



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