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    SubjectRe: Making diff(1) of linux kernels faster


    On Wed, 17 Oct 2001, Linus Torvalds wrote:

    >
    > On Wed, 17 Oct 2001, Paul Gortmaker wrote:
    > >
    > > Oh, and prereading the dirs of both trees (vs. just one and letting
    > > normal execution read in the 2nd) seems to offer better improvements.
    > > (Steady stream of requests results in better merging perhaps?)
    >
    > That doesn't make much sense, but I'll take your word for it. Does this
    > behaviour show up on 2.4.x too? It sounds like a performance buglet in the
    > kernel or some infrastructure, really.
    >
    > The one problem with pre-reading is that it will now artificially touch
    > the data twice, and when running on 2.4.x it will activate the pages.
    > That's going to be exactly what _I_ want it to do on my machine, but
    > others are likely to be less happy about it.
    >
    > Btw, why use "slurp()" and actually doing the memory allocations etc, only
    > to throw it away again? It would be better to either really keep the
    > allocation around (which would also fix the touch-twice issue but would
    > cause much more changes to 'diff'), or to just read into the same buffer
    > over and over again..
    >
    > And I've for a long time thought about adding a "readahead()" system call.
    > There are just too many uses for it, it has come up in many different
    > areas..

    There is a paper on USENIX 2001 which does implement directory readahead
    and it shows huge improvements for some workload.

    I'll dig it down and see if I can find that.

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