Messages in this thread | | | Subject | Re: [PATCH] sched: Avoid side-effect of tickless idle on update_cpu_load (v2) | From | Peter Zijlstra <> | Date | Mon, 17 May 2010 10:19:59 +0200 |
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On Fri, 2010-05-14 at 18:21 -0700, Venkatesh Pallipadi wrote:
> /* > + * The exact cpu_load decay at various idx values would be > + * [0] new = 0 * old + 1 * new > + * [1] new = 1/2 * old + 1/2 * new > + * [2] new = 3/4 * old + 1/4 * new > + * [3] new = 7/8 * old + 1/8 * new > + * [4] new = 15/16 * old + 1/16 * new
Would that be something like?
load_i = ((2^i)-1)/(2^i) * load_i + 1/(2^i) * load_(i-1)
Where load_-1 == current load.
> + * Load degrade calculations below are approximated on a 128 point scale. > + * degrade_zero_ticks is the number of ticks after which old_load at any > + * particular idx is approximated to be zero. > + * degrade_factor is a precomputed table, a row for each load idx. > + * Each column corresponds to degradation factor for a power of two ticks, > + * based on 128 point scale. > + * Example: > + * row 2, col 3 (=12) says that the degradation at load idx 2 after > + * 8 ticks is 12/128 (which is an approximation of exact factor 3^8/4^8).
So because we're in no_hz, current load == 0 and we could approximate the thing by:
load_i = ((2^i)-1)/(2^i) * load_i
Because for i ~ 1, there is no new input, and for i >> 1 the fraction is small.
But why then do we precalculate these factors? It seems to me ((2^i)-1)/(2^i) is something that is trivial to compute and doesn't warrant a lookup table?
> + * With this power of 2 load factors, we can degrade the load n times > + * by looking at 1 bits in n and doing as many mult/shift instead of > + * n mult/shifts needed by the exact degradation. > + */ > +#define DEGRADE_SHIFT 7 > +static const unsigned char > + degrade_zero_ticks[CPU_LOAD_IDX_MAX] = {0, 8, 32, 64, 128}; > +static const unsigned char > + degrade_factor[CPU_LOAD_IDX_MAX][DEGRADE_SHIFT + 1] = { > + {0, 0, 0, 0, 0, 0, 0, 0}, > + {64, 32, 8, 0, 0, 0, 0, 0}, > + {96, 72, 40, 12, 1, 0, 0}, > + {112, 98, 75, 43, 15, 1, 0}, > + {120, 112, 98, 76, 45, 16, 2} };
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