• Stratos Karafotis's avatar
    cpufreq: ondemand: Eliminate the deadband effect · 6393d6a1
    Stratos Karafotis authored
    Currently, ondemand calculates the target frequency proportional to load
    using the formula:
    	Target frequency = C * load
    	where C = policy->cpuinfo.max_freq / 100
    
    Though, in many cases, the minimum available frequency is pretty high and
    the above calculation introduces a dead band from load 0 to
    100 * policy->cpuinfo.min_freq / policy->cpuinfo.max_freq where the target
    frequency is always calculated to less than policy->cpuinfo.min_freq and
    the minimum frequency is selected.
    
    For example: on Intel i7-3770 @ 3.4GHz the policy->cpuinfo.min_freq = 1600000
    and the policy->cpuinfo.max_freq = 3400000 (without turbo). Thus, the CPU
    starts to scale up at a load above 47.
    On quad core 1500MHz Krait the policy->cpuinfo.min_freq = 384000
    and the policy->cpuinfo.max_freq = 1512000. Thus, the CPU starts to scale
    at load above 25.
    
    Change the calculation of target frequency to eliminate the above effect using
    the formula:
    
    	Target frequency = A + B * load
    	where A = policy->cpuinfo.min_freq and
    	      B = (policy->cpuinfo.max_freq - policy->cpuinfo->min_freq) / 100
    
    This will map load values 0 to 100 linearly to cpuinfo.min_freq to
    cpuinfo.max_freq.
    
    Also, use the CPUFREQ_RELATION_C in __cpufreq_driver_target to select the
    closest frequency in frequency_table. This is necessary to avoid selection
    of minimum frequency only when load equals to 0. It will also help for selection
    of frequencies using a more 'fair' criterion.
    
    Tables below show the difference in selected frequency for specific values
    of load without and with this patch. On Intel i7-3770 @ 3.40GHz:
    	Without			With
    Load	Target	Selected	Target	Selected
    0	0	1600000		1600000	1600000
    5	170050	1600000		1690050	1700000
    10	340100	1600000		1780100	1700000
    15	510150	1600000		1870150	1900000
    20	680200	1600000		1960200	2000000
    25	850250	1600000		2050250	2100000
    30	1020300	1600000		2140300	2100000
    35	1190350	1600000		2230350	2200000
    40	1360400	1600000		2320400	2400000
    45	1530450	1600000		2410450	2400000
    50	1700500	1900000		2500500	2500000
    55	1870550	1900000		2590550	2600000
    60	2040600	2100000		2680600	2600000
    65	2210650	2400000		2770650	2800000
    70	2380700	2400000		2860700	2800000
    75	2550750	2600000		2950750	3000000
    80	2720800	2800000		3040800	3000000
    85	2890850	2900000		3130850	3100000
    90	3060900	3100000		3220900	3300000
    95	3230950	3300000		3310950	3300000
    100	3401000	3401000		3401000	3401000
    
    On ARM quad core 1500MHz Krait:
    	Without			With
    Load	Target	Selected	Target	Selected
    0	0	384000		384000	384000
    5	75600	384000		440400	486000
    10	151200	384000		496800	486000
    15	226800	384000		553200	594000
    20	302400	384000		609600	594000
    25	378000	384000		666000	702000
    30	453600	486000		722400	702000
    35	529200	594000		778800	810000
    40	604800	702000		835200	810000
    45	680400	702000		891600	918000
    50	756000	810000		948000	918000
    55	831600	918000		1004400	1026000
    60	907200	918000		1060800	1026000
    65	982800	1026000		1117200	1134000
    70	1058400	1134000		1173600	1134000
    75	1134000	1134000		1230000	1242000
    80	1209600	1242000		1286400	1242000
    85	1285200	1350000		1342800	1350000
    90	1360800	1458000		1399200	1350000
    95	1436400	1458000		1455600	1458000
    100	1512000	1512000		1512000	1512000
    
    Tested on Intel i7-3770 CPU @ 3.40GHz and on ARM quad core 1500MHz Krait
    (Android smartphone).
    Benchmarks on Intel i7 shows a performance improvement on low and medium
    work loads with lower power consumption. Specifics:
    
    Phoronix Linux Kernel Compilation 3.1:
    Time: -0.40%, energy: -0.07%
    Phoronix Apache:
    Time: -4.98%, energy: -2.35%
    Phoronix FFMPEG:
    Time: -6.29%, energy: -4.02%
    
    Also, running mp3 decoding (very low load) shows no differences with and
    without this patch.
    Signed-off-by: default avatarStratos Karafotis <stratosk@semaphore.gr>
    Signed-off-by: default avatarRafael J. Wysocki <rafael.j.wysocki@intel.com>
    6393d6a1
cpufreq_ondemand.c 17.4 KB