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    SubjectRe: [2.4.17/18pre] VM and swap - it's really unusable
    Remember, the -aa kernels are dbench champ -

    They don't use preempt, but do use mini-ll...

    Just a rant from the peanut gallery....

    cu

    jjs

    Andrew Morton wrote:

    >Robert Love wrote:
    >
    >>On Mon, 2002-01-21 at 11:06, yodaiken@fsmlabs.com wrote:
    >>
    >>>I have not seen a single well structured benchmark that shows a significant
    >>>difference. I've seen lots of benchmarks with odd mixes of different patches
    >>>showing something unknown. How about a simple clear dbench?
    >>>
    >>I and many others have been posting benchmarks for months.
    >>
    >>Here:
    >>
    >>(average of 4 runs of `dbench 16')
    >>2.5.3-pre1: 25.7608 MB/s
    >>2.5.3-pre1-preempt: 32.341 MB/s
    >>
    >
    >Well I spent several hours last week trying to reproduce and
    >account for these observations and basically came up with
    >nothing. The patch-induced variation was of the same order
    >as the inter-run variation.
    >
    >You should publish the dbench dots. They're most informative.
    >Look at these two:
    >
    >16 clients started
    >..................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................+..............................................................................................+++++++++++++++****************
    >Throughput 6.48209 MB/sec (NB=8.10261 MB/sec 64.8209 MBit/sec)
    >
    >16 clients started
    >.................................................................................................................................................................................................................................+...................................................................................................................................................................................................................................................................+................................................................................+............................+++++++++++++****************
    >Throughput 7.76901 MB/sec (NB=9.71126 MB/sec 77.6901 MBit/sec)
    >
    >These are identical runs. Empty filesystem, 64 megs of memory.
    >
    >Note how in the second run, a few clients terminated early,
    >and the throughput numbers increased quite markedly.
    >
    >I don't know what causes this, and frankly I'm not really
    >interested. It's some bizarre freaky dbench instability.
    >
    >Similar effects occur with the `make -j12 bzImage' swapstorm
    >test. After a while, all the `cc' instances start to
    >get synchronised at the onset of their peak memory demand.
    >The earlier and longer this happens, the worse the runtime.
    >It's an unstable system and tiny input perturbations create
    >large effects on the output.
    >
    >Sorry, but these are not interesting, repeatable or stable workloads,
    >and I remain unconvinced about claims that low-latency or preempt
    >aid I/O throughput. And even if a statistically significant
    >benefit _can_ be empirically demonstrated, it would be incautious
    >to claim a general benefit without a solid explanation of what
    >is causing it. (Apart from the RAID5 kernel thread starvation
    >effect, which _is_ explained).
    >
    >If someone can show me a sensible and repeatable I/O gain then
    >great, I can go in and work out exactly where it's coming from
    >and then we finally have a real, tangible, non-hand-wavy
    >explanation. It may be there, but I just don't see it yet.
    >
    >-
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