Re: [2.4.17/18pre] VM and swap - it's really unusable

J Sloan
2002-01-21T22:00:04Z

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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