Messages in this thread | | | From | Pratik Rajesh Sampat <> | Subject | [RFC 0/1] Alternate history mechanism for the TEO governor | Date | Mon, 11 May 2020 19:40:54 +0530 |
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First RFC posting: https://lkml.org/lkml/2020/2/22/27 Changelog 1. Upated benchmarks
Currently the TEO governor apart from the TEO timer and hit/miss/early hit buckets; especially for non-timer events, also gathers history of 8 intervals and if there are significant idle durations less than the current, then it decides if a shallower state must be chosen.
The current sliding history window does do fair job for the prediction of the non-timer events, so to improve upon the history mechanism, one may just hypothesize increasing the interval window so that more history can influence better decisions.
However, this may not be true. The current algorithm can be limited due to its linearity, while also average of interval durations may not be a suitable metric when the data presents skewness. This can especially be true for the when number of intervals taken into account increases.
Benchmark: Schbench Provides latency statistics for scheduler wakeups Machine - IBM Power 9
Metric of measurement 1. Performance - latency 99th percentile - usec 2. Power - watts Performance and Power statistics are normalized
Latency - Normalized 8 intervals is baseline +---------+-------------+--------------+--------------+--------------+ | Threads | 8 Intervals | 16 Intervals | 32 Intervals | 64 Intervals | +---------+-------------+--------------+--------------+--------------+ | 2 | 100 | 102.81 | 106.93 | 100.86 | +---------+-------------+--------------+--------------+--------------+ | 4 | 100 | 104.67 | 96.58 | 96.58 | +---------+-------------+--------------+--------------+--------------+ | 8 | 100 | 116.26 | 100 | 121.44 | +---------+-------------+--------------+--------------+--------------+ | 16 | 100 | 85.94 | 81.46 | 80.86 | +---------+-------------+--------------+--------------+--------------+ +---------+---------------+ | Threads | 128 Intervals | +---------+---------------+ | 2 | 83.09 | +---------+---------------+ | 4 | 74.75 | +---------+---------------+ | 8 | 87.15 | +---------+---------------+ | 16 | 76.98 | +---------+---------------+
The latency number are kind of a mixed bag. Some intervals perform similar, while some slightly better, while some considerably worse
Standout: 128 latency intervals always performs well in terms of latency
Power - Normalized 8 intervals in baseline +---------+-------------+--------------+--------------+--------------+ | Threads | 8 Intervals | 16 Intervals | 32 Intervals | 64 Intervals | +---------+-------------+--------------+--------------+--------------+ | 2 | 100 | 100 | 85.29 | 97.05 | +---------+-------------+--------------+--------------+--------------+ | 4 | 100 | 115.9 | 111.36 | 145.45 | +---------+-------------+--------------+--------------+--------------+ | 8 | 100 | 109.63 | 114.45 | 102.4 | +---------+-------------+--------------+--------------+--------------+ | 16 | 100 | 104.2 | 103.36 | 102.52 | +---------+-------------+--------------+--------------+--------------+ +---------+---------------+ | Threads | 128 Intervals | +---------+---------------+ | 2 | 123.52 | +---------+---------------+ | 4 | 159.09 | +---------+---------------+ | 8 | 114.45 | +---------+---------------+ | 16 | 106.72 | +---------+---------------+
Power considerably regresses in almost all cases, showing that with increasing history there seems to be skewness presented to shallow states.
Weighted TEO Governor ---------------------
To complement the current moving window history, an approach is devised where each idle state separately maintains a weight for itself and its counterpart idle states to form a probability distribution.
When a decision needs to be made, the TEO governor selects an idle state based on its timer and other hits/early hits metric. After which, the probability distribution of that selected idle state is looked at which gives insight into how probable that state is to occur if picked.
The probability distribution is nothing but a n*n matrix, where n - drv->state_count. Each entry in the array signifies a weight for that row. The weights can vary from the range [0-10000].
For example: state_mat[2][1] - 3000 means that when state 2 is entered idle with, the probability that the interval will last long enough to satisfy state 1's residency is 30%. The trailing zeros correspond to having more resolution while increasing or reducing the weights for correction.
Currently, for selection of an idle state based on probabilities, a weighted random number generator is used to choose one of the idle states. Naturally, the states with higher weights are more likely to be chosen.
On wakeup, the weights are updated. The state with which it should have woken up with (could be the hit / miss / early hit state) is increased in weight by the "LEARNING_RATE" % and the rest of the states for that index are reduced by the same factor.
A possible disadvantage of this approach is that history for that thread may make the weights stale and influence the choosing of idle states which may not be relevant anymore. Aging the weights could be a solution for that, where over time, the highest weight for that index can be decayed and the weight equally spread across the rest of the states. Although this RFC does not cover the implementation for that as there seemed to be too much run to run variance with this approach.
Having a finer view of the history in addition to weighted randomized salt seems to show some promise in terms of saving power without compromising performance.
Why not just pick the state with the highest probability? - If there are multiple states close in probabilities competing for dominance they should be given a fair chance Of course, if the prediction was wrong the algorithm will self correct, however by that time the pattern may have changed - If this contention pattern is exhibited often then the prediction algorithm will always stay playing catch-up.
Benchmarks:
Metric of measurement: 1. Performance (latency / throughput) 2. Power (watts) 3. Accuracy % a. Correct prediction - The idle state predicted aligns with the actual sleep duration b. Undershoot prediction (US) - The idle state predicted is deeper for the actual sleep duration c. Overshoot prediction (OS) - The idle state predicted is shallower for the actual sleep duration
Performance and Power numbers are normalized. However accuracy numbers are deliberately kept as-is to show how well the vanilla governor performed in the first place
Schbench -------- Benchmarks scheduler wakeup latencies
1. latency 99th percentile - usec 2. Power - watts Machine - IBM Power 9
Latency and Power - Normalized +---------+--------------+-----------------+---------------+ | Threads | TEO Baseline | Wt. TEO Latency | Wt. TEO Power | +---------+--------------+-----------------+---------------+ | 2 | 100 | 101.3 | 85.29 | +---------+--------------+-----------------+---------------+ | 4 | 100 | 105.06 | 113.63 | +---------+--------------+-----------------+---------------+ | 8 | 100 | 92.32 | 90.36 | +---------+--------------+-----------------+---------------+ | 16 | 100 | 99.1 | 92.43 | +---------+--------------+-----------------+---------------+
Accuracy
Vanilla TEO Governor - Prediction distribution % +---------+------+------+------+-------+-------+-------+---------+ | Threads | US 1 | US 2 | US 3 | US 4 | US 5 | US 6 | Correct | +---------+------+------+------+-------+-------+-------+---------+ | 2 | 6.12 | 1.08 | 1.76 | 20.41 | 9.2 | 28.74 | 22.51 | +---------+------+------+------+-------+-------+-------+---------+ | 4 | 8.54 | 1.56 | 1.25 | 20.24 | 10.75 | 25.17 | 22.67 | +---------+------+------+------+-------+-------+-------+---------+ | 8 | 5.88 | 2.67 | 1.09 | 13.72 | 17.08 | 32.04 | 22.95 | +---------+------+------+------+-------+-------+-------+---------+ | 16 | 6.29 | 2.43 | 0.86 | 13.21 | 15.33 | 26.52 | 29.34 | +---------+------+------+------+-------+-------+-------+---------+ +---------+------+------+------+ | Threads | OS 1 | OS 2 | OS 3 | +---------+------+------+------+ | 2 | 1.77 | 1.27 | 7.14 | +---------+------+------+------+ | 4 | 1.8 | 1.31 | 6.71 | +---------+------+------+------+ | 8 | 0.65 | 0.72 | 3.2 | +---------+------+------+------+ | 16 | 0.63 | 1.71 | 3.68 | +---------+------+------+------+
Weighted TEO Governor - Prediction distribution % +---------+------+------+------+-------+-------+-------+---------+ | Threads | US 1 | US 2 | US 3 | US 4 | US 5 | US 6 | Correct | +---------+------+------+------+-------+-------+-------+---------+ | 2 | 7.26 | 2.07 | 0.02 | 15.85 | 13.29 | 36.26 | 22.13 | +---------+------+------+------+-------+-------+-------+---------+ | 4 | 4.33 | 1.45 | 0.15 | 14.17 | 14.68 | 40.36 | 21.01 | +---------+------+------+------+-------+-------+-------+---------+ | 8 | 4.73 | 2.46 | 0.12 | 12.48 | 14.68 | 32.38 | 28.9 | +---------+------+------+------+-------+-------+-------+---------+ | 16 | 7.68 | 1.25 | 0.98 | 12.15 | 11.19 | 24.91 | 35.92 | +---------+------+------+------+-------+-------+-------+---------+ +---------+------+------+------+ | Threads | OS 1 | OS 2 | OS 3 | +---------+------+------+------+ | 2 | 0.39 | 0.42 | 2.31 | +---------+------+------+------+ | 4 | 0.45 | 0.51 | 2.89 | +---------+------+------+------+ | 8 | 0.53 | 0.66 | 3.06 | +---------+------+------+------+ | 16 | 0.97 | 1.9 | 3.05 | +---------+------+------+------+
Sleeping Ebizzy --------------- Program to generate workloads resembling web server workloads. The benchmark is customized to allow for a sleep interval -i
1. Number of records 2. Power - watts Machine - IBM Power 9
Parameters: 1. -m -> Always use mmap instead of malloc 2. -M -> Never use mmap 3. -S <seconds> -> Number of seconds to run 4. -i <interval> -> Sleep interval
Number of records and power normalized +-------------------+---------------+------------------+-----------------+ | Parameters | TEO baseline | Wt TEO records | Wt. TEO Power | +-------------------+---------------+------------------+-----------------+ | -S 60 -i 10000 | 100 | 106.56 | 93.95 | +-------------------+---------------+------------------+-----------------+ | -m -S 60 -i 10000 | 100 | 100.62 | 82.14 | +-------------------+---------------+------------------+-----------------+ | -M -S 60 -i 10000 | 100 | 104.97 | 95.19 | +-------------------+---------------+------------------+-----------------+
Accuracy
Vanilla TEO Governor - Prediction distribution % +-------------------+-------+------+------+-------+------+-------+ | Parameters | US 1 | US 2 | US 3 | US 4 | US 5 | US 6 | +-------------------+-------+------+------+-------+------+-------+ | -S 60 -i 10000 | 45.46 | 0.52 | 1.5 | 15.34 | 2.44 | 8.61 | +-------------------+-------+------+------+-------+------+-------+ | -m -S 60 -i 10000 | 4.22 | 2.08 | 0.71 | 90.01 | 0 | 0.01 | +-------------------+-------+------+------+-------+------+-------+ | -M -S 60 -i 10000 | 15.78 | 1.42 | 2.4 | 22.39 | 1.68 | 11.25 | +-------------------+-------+------+------+-------+------+-------+ +-------------------+---------+------+------+------+------+ | Parameters | Correct | OS 1 | OS 2 | OS 3 | OS 4 | +-------------------+---------+------+------+------+------+ | -S 60 -i 10000 | 17.03 | 1.73 | 1.1 | 6.27 | 0 | +-------------------+---------+------+------+------+------+ | -m -S 60 -i 10000 | 2.44 | 0.18 | 0.13 | 0.22 | 0 | +-------------------+---------+------+------+------+------+ | -M -S 60 -i 10000 | 31.65 | 3.45 | 1.8 | 8.18 | 0 | +-------------------+---------+------+------+------+------+
Weigted TEO Governor - Prediction distribution % +-------------------+-------+------+------+-------+------+-------+ | Parameters | US 1 | US 2 | US 3 | US 4 | US 5 | US 6 | +-------------------+-------+------+------+-------+------+-------+ | -S 60 -i 10000 | 8.25 | 0.87 | 0.98 | 19.23 | 4.05 | 26.35 | +-------------------+-------+------+------+-------+------+-------+ | -m -S 60 -i 10000 | 7.69 | 4.35 | 0.93 | 82.74 | 0.01 | 0.01 | +-------------------+-------+------+------+-------+------+-------+ | -M -S 60 -i 10000 | 3.73 | 3.29 | 0.73 | 13.33 | 7.38 | 18.61 | +-------------------+-------+------+------+-------+------+-------+ +-------------------+---------+------+------+------+------+ | Parameters | Correct | OS 1 | OS 2 | OS 3 | OS 4 | +-------------------+---------+------+------+------+------+ | -S 60 -i 10000 | 32.86 | 3.27 | 2.05 | 2.09 | 0 | +-------------------+---------+------+------+------+------+ | -m -S 60 -i 10000 | 3.4 | 0.29 | 0.28 | 0.3 | 0 | +-------------------+---------+------+------+------+------+ | -M -S 60 -i 10000 | 48.19 | 1.8 | 0.93 | 1.97 | 0.04 | +-------------------+---------+------+------+------+------+
Pgbench ------- pgbench is a simple program for running benchmark tests on PostgreSQL. It runs the same sequence of SQL commands over and over, possibly in multiple concurrent database sessions, and then calculates the average transaction rate (transactions per second). Scale of measurement: 1. Number of transactions 2. Power (watts) Machine - IBM Power 9
Number of transactions and power is normalized
+---------+---------------+---------------------+-----------------+ | Clients | TEO Baseline | Wt. TEO transations | Wt. TEO power | +---------+---------------+---------------------+-----------------+ | 4 | 100 | 105.93 | 85.18 | +---------+---------------+---------------------+-----------------+ | 8 | 100 | 98.11 | 100 | +---------+---------------+---------------------+-----------------+ | 16 | 100 | 98.73 | 104.16 | +---------+---------------+---------------------+-----------------+ | 32 | 100 | 101.05 | 95 | +---------+---------------+---------------------+-----------------+
Accuracy
Vanilla TEO Governor - Prediction distribution % +---------+-------+-------+-------+-------+------+------+---------+ | Clients | US 1 | US 2 | US 3 | US 4 | US 5 | US 6 | Correct | +---------+-------+-------+-------+-------+------+------+---------+ | 4 | 59.97 | 2.56 | 0.46 | 1.45 | 0.13 | 0.87 | 32.09 | +---------+-------+-------+-------+-------+------+------+---------+ | 8 | 82.76 | 7.31 | 0.12 | 1.23 | 0.08 | 1.22 | 7.2 | +---------+-------+-------+-------+-------+------+------+---------+ | 16 | 2 | 13.46 | 73.27 | 10.7 | 0.04 | 0.12 | 0.38 | +---------+-------+-------+-------+-------+------+------+---------+ | 32 | 1.33 | 0.76 | 44.9 | 40.8 | 0.07 | 12 | 0.12 | +---------+-------+-------+-------+-------+------+------+---------+ +---------+------+------+------+ | Clients | OS 1 | OS 2 | OS 3 | +---------+------+------+------+ | 4 | 2.44 | 0.01 | 0.02 | +---------+------+------+------+ | 8 | 0.04 | 0.02 | 0.02 | +---------+------+------+------+ | 16 | 0.02 | 0 | 0.01 | +---------+------+------+------+ | 32 | 0.02 | 0 | 0 | +---------+------+------+------+
Weigted TEO Governor - Prediction distribution % +---------+-------+-------+-------+-------+------+------+---------+ | Clients | US 1 | US 2 | US 3 | US 4 | US 5 | US 6 | Correct | +---------+-------+-------+-------+-------+------+------+---------+ | 4 | 42.79 | 1.79 | 0.01 | 0.72 | 0.1 | 0.78 | 46.99 | +---------+-------+-------+-------+-------+------+------+---------+ | 8 | 76.52 | 8.02 | 0.12 | 0.42 | 0.06 | 0.5 | 14.35 | +---------+-------+-------+-------+-------+------+------+---------+ | 16 | 0.72 | 51.95 | 42.02 | 4.65 | 0.05 | 0.31 | 0.3 | +---------+-------+-------+-------+-------+------+------+---------+ | 32 | 1.06 | 1.59 | 42.89 | 53.11 | 0.05 | 0.42 | 0.87 | +---------+-------+-------+-------+-------+------+------+---------+ +---------+------+------+------+ | Clients | OS 1 | OS 2 | OS 3 | +---------+------+------+------+ | 4 | 6.81 | 0 | 0.01 | +---------+------+------+------+ | 8 | 0 | 0 | 0.01 | +---------+------+------+------+ | 16 | 0 | 0 | 0 | +---------+------+------+------+ | 32 | 0 | 0 | 0.01 | +---------+------+------+------+
Hackbench --------- Creates a specified number of pairs of schedulable entities which communicate via either sockets or pipes and time how long it takes for each pair to send data back and forth.
Machine - IBM Power 9
Scale of measurement: 1. Time (s) 2. Power (watts) Time is normalized
+---------+----------+----------------------+-------------------+ | Loops | TEO Time | Wt. TEO Time Sockets | Wt. TEO Time Pipe | +---------+----------+----------------------+-------------------+ | 100 | 100 | 95.23 | 87.09 | +---------+----------+----------------------+-------------------+ | 1000 | 100 | 105.81 | 98.67 | +---------+----------+----------------------+-------------------+ | 10000 | 100 | 99.33 | 92.73 | +---------+----------+----------------------+-------------------+ | 100000 | 100 | 98.88 | 101.99 | +---------+----------+----------------------+-------------------+ | 1000000 | 100 | 100.04 | 100.2 | +---------+----------+----------------------+-------------------+
Power :Socket: Consistent between 135-140 watts for both TEO and Wt. TEO Pipe: Consistent between 125-130 watts for both TEO and Wt. TEO
Pratik Rajesh Sampat (1): Weighted approach to gather and use history in TEO governor
drivers/cpuidle/governors/teo.c | 96 +++++++++++++++++++++++++++++++-- 1 file changed, 91 insertions(+), 5 deletions(-)
-- 2.17.1
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