(adding Anshul Garg) On Thu, 2016-01-28 at 13:42 -0800, Andi Kleen wrote: > From: Andi Kleen <a...@linux.intel.com> > > The menu cpuidle governor does at least two int_sqrt() each time > we go into idle in get_typical_interval to compute stddev > > int_sqrts take 100-120 cycles each. Short idle latency is important > for many workloads. > > I instrumented the function on my workstation and most values are > 16bit only and most others 32bit (50% percentile is 122094, > 75% is 3699533). > > sqrt is implemented by starting with an initial estimation, > and then iterating. int_sqrt currently only uses a fixed > estimating which is good for 64bits worth of input. > > This patch adds some checks at the beginning to start with > a better estimate for values fitting in 8, 16bit and 32bit. > This makes int_sqrt between 60+% faster for values in 16bit, > and still somewhat faster (between 10 and 30%) for larger values > upto 32bit. Full 64bit is slightly slower. > > This optimizes the short idle calls and does not hurt the > long sleep (which probably do not care) much. > > An alternative would be a full table drive approach, or > trying some inverted sqrt optimization, but this simple change > already seems to have a good payoff.
This thread might be relevant: https://lkml.org/lkml/2015/2/2/600 and perhaps using fls might still be a good approach.