APPLIED [I]: [PATCH][IMPISH] UBUNTU: SAUCE: md/raid6 algorithms: scale test duration for speedier boots

[Andrea Righi]

On Mon, Sep 06, 2021 at 11:17:46AM +0100, Colin King wrote:
> From: Colin Ian King <colin.king at canonical.com>
> 
> The original code runs for a set run time based on the duration of
> 2^RAID6_TIME_JIFFIES_LG2. The default kernel value for
> RAID6_TIME_JIFFIES_LG2 is 4, however, emperical testing shows that a
> value of 3.5 is the sweet spot for getting consistent benchmarking
> results and speeding up the run time of the benchmarking.
> 
> To achieve 2^3.5 we use the following:
>    2^3.5 = 2^4 / 2^0.5
>          = 2^4 / sqrt(2)
>          = 2^4 * 0.707106781
> 
> Too keep this as integer math that is as accurate as required and avoiding
> overflow, this becomes:
>          = 2^4 * 181 / 256
>          = (2^4 * 181) >> 8
> 
> We also need to scale down perf by the same factor, however, to
> get a good approximate integer result without an overflow we scale
> by 2^4.0 * sqrt(2) =
>          = 2 ^ 4 * 1.41421356237
>          = 2 ^ 4 * 1448 / 1024
>          = (2 ^ 4 * 1448) >> 10
> 
> This has been tested on 2 AWS instances, a small t2 and a medium m3
> with 30 boot tests each and compared to the same instances booted 30
> times on an umodified kernel. In all results, we get the same
> algorithms being selected and a 100% consistent result over the 30
> boots, showing that this optimised jiffy timing scaling does not break
> the original functionality.
> 
> On the t2.small we see a saving of ~0.126 seconds and t3.medium a saving of
> ~0.18 seconds.
> 
> Tested on a 4 CPU VM on an 8 thread Xeon server; seeing a saving of ~0.35
> seconds (average over 50 boots).
> 
> The testing included double checking the algorithm chosen by the optimized
> selection and seeing the same as pre-optimised version.
> 
> Signed-off-by: Colin Ian King <colin.king at canonical.com>

Applied to impish/linux (5.13).

Thanks,
-Andrea