Messages in this thread | | | Date | Wed, 6 Oct 2004 09:48:15 +0200 | From | Ingo Molnar <> | Subject | Re: Default cache_hot_time value back to 10ms |
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* Chen, Kenneth W <kenneth.w.chen@intel.com> wrote:
> Chen, Kenneth W wrote on Tuesday, October 05, 2004 10:31 AM > > We have experimented with similar thing, via bumping up sd->cache_hot_time to > > a very large number, like 1 sec. What we measured was a equally low throughput. > > But that was because of not enough load balancing. > > Since we are talking about load balancing, we decided to measure various > value for cache_hot_time variable to see how it affects app performance. We > first establish baseline number with vanilla base kernel (default at 2.5ms), > then sweep that variable up to 1000ms. All of the experiments are done with > Ingo's patch posted earlier. Here are the result (test environment is 4-way > SMP machine, 32 GB memory, 500 disks running industry standard db transaction > processing workload): > > cache_hot_time | workload throughput > -------------------------------------- > 2.5ms - 100.0 (0% idle) > 5ms - 106.0 (0% idle) > 10ms - 112.5 (1% idle) > 15ms - 111.6 (3% idle) > 25ms - 111.1 (5% idle) > 250ms - 105.6 (7% idle) > 1000ms - 105.4 (7% idle) > > Clearly the default value for SMP has the worst application throughput (12% > below peak performance). When set too low, kernel is too aggressive on load > balancing and we are still seeing cache thrashing despite the perf fix. > However, If set too high, kernel gets too conservative and not doing enough > load balance.
could you please try the test in 1 msec increments around 10 msec? It would be very nice to find a good formula and the 5 msec steps are too coarse. I think it would be nice to test 7,9,11,13 msecs first, and then the remaining 1 msec slots around the new maximum. (assuming the workload measurement is stable.)
> This value was default to 10ms before domain scheduler, why does domain > scheduler need to change it to 2.5ms? And on what bases does that decision > take place? We are proposing change that number back to 10ms.
agreed. What value does cache_decay_ticks have on your box?
> > Signed-off-by: Ken Chen <kenneth.w.chen@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
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