Re: Default cache_hot_time value back to 10ms

[Con Kolivas]

Chen, Kenneth W writes:

> 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.
>
> 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.

Should it not be based on the cache flush time? We measure that and set the 
cache_decay_ticks and can base it on that. What is the cache_decay_ticks 
value reported in the dmesg of your hardware?

Cheers,
Con

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