1#!/bin/sh
2
3# Script implements watermark_scale calculation which results in the same low
4# watermark as if extra_free_kbytes tunable were to be used.
5#
6# Usage: extra_free_kbytes.sh <extra_free_kbytes value>
7#
8# extra_free_kbytes is distributed between zones based on
9# zone.managed_pages/vm_total_pages ratio, where vm_total_pages is the sum of
10# zone.managed_pages for all zones (zone.high used in this calculation is 0
11# when this is calculated). Therefore for each zone its share is calculated as:
12#
13# extra_free_pages = extra_free_kbytes / page_size
14# extra_share = extra_free_pages * managed_pages / vm_total_pages
15#
16# This extra_share is added to the low and high watermarks:
17#
18# low = min + max(min / 4, managed_pages * (watermark_scale / 10000)) + extra_share
19# high = min + 2 * max(min / 4, managed_pages * (watermark_scale / 10000)) + extra_share
20#
21# Because Android uses extra_free_kbytes to adjust the low watermark, we ignore
22# the difference in how watermark_scale and extra_free_kbytes affect the high
23# watermark and will match the low watermark only.
24#
25# To eliminate extra_share and compansate the difference with watermark_scale,
26# a new watermark_scale_new is calculated as:
27#
28# (1) max(min / 4, managed_pages * (watermark_scale / 10000)) + extra_share =
29#   max(min / 4, managed_pages * (watermark_scale_new / 10000))
30#
31# Two cases to consider:
32# A. managed_pages * (watermark_scale / 10000) > min / 4
33# The formula (1) becomes:
34#
35# managed_pages * (watermark_scale / 10000) + extra_share =
36#   managed_pages * (watermark_scale_new / 10000)
37#
38# after simplifying and substituting extra_share formula becomes:
39#
40# (2) watermark_scale_new = watermark_scale + extra_free_pages / vm_total_pages * 10000
41#
42# B. managed_pages * (watermark_scale / 10000) < min / 4
43# The formula (1) becomes:
44#
45# min / 4 + extra_share = max(min / 4, managed_pages * (watermark_scale_new / 10000))
46#
47# after calculating watermark_scale_new, if (managed_pages * (watermark_scale_new / 10000))
48# is still smaller than min / 4 then we can't compensate extra_share with
49# watermark_scale anyway. Therefore calculation becomes:
50#
51# watermark_scale_new = (min / 4 + extra_share) / managed_pages * 10000
52#
53# after simplifying and substituting extra_share formula becomes:
54#
55# (3) watermark_scale_new = (min / 4) * 10000 / managed_pages + extra_free_pages / vm_total_pages * 10000
56#
57# After defining watermark_delta = extra_free_pages / vm_total_pages * 10000:
58#
59# if (managed_pages * (watermark_scale / 10000) > min / 4)
60#     watermark_scale_new = watermark_scale + watermark_delta
61# else
62#     watermark_scale_new = (min / 4) * 10000 / managed_pages + watermark_delta
63#
64
65if [ "$#" -ne 1 ]
66then
67    echo "Usage: $0 <extra_free_kbytes value>"
68    exit
69fi
70
71extra_free_kbytes=$1
72
73# if extra_free_kbytes knob exists, use it and exit
74if [ -e /proc/sys/vm/extra_free_kbytes ]
75then
76    echo $extra_free_kbytes > /proc/sys/vm/extra_free_kbytes
77    exit
78fi
79
80# record the original watermark_scale_factor value
81watermark_scale=$(getprop "ro.kernel.watermark_scale_factor")
82if [ -z "$watermark_scale" ]
83then
84    watermark_scale=$(cat /proc/sys/vm/watermark_scale_factor)
85    setprop "ro.kernel.watermark_scale_factor" "$watermark_scale"
86    # On older distributions with no policies configured setprop may fail.
87    # If that happens, use the kernel default of 10.
88    if [ -z $(getprop "ro.kernel.watermark_scale_factor") ]
89    then
90        watermark_scale=10
91    fi
92fi
93
94# convert extra_free_kbytes to pages
95page_size=$(getconf PAGESIZE)
96page_size_kb=$((page_size/1024))
97extra_free_pg=$((extra_free_kbytes/page_size_kb))
98
99managed=($(grep managed /proc/zoneinfo | awk '{print $2}'))
100length=${#managed[@]}
101min=($(grep "min" /proc/zoneinfo | awk '{print $2}'))
102
103# calculate vm_total_pages.
104# WARNING: if the final low watermark differs from the original, the source of
105# the error is likely vm_total_pages which is impossible to get exact from the
106# userspace. Grep for "Total pages" in the kernel logs to see the actual
107# vm_total_pages and plug it in the calculation to confirm the source of the
108# error. Error caused by this inaccuracy is normally within 1% range.
109vm_total_pages=0
110i=0
111while [ $i -lt $length ]
112do
113    vm_total_pages=$((vm_total_pages + managed[i]))
114    i=$((i+1))
115done
116
117# calculate watermark_scale_new for each zone and choose the max
118max_watermark_scale=0
119i=0
120while [ $i -lt $length ]
121do
122    # skip unmanaged zones
123    if [ ${managed[i]} -eq 0 ]
124    then
125        i=$((i+1))
126        continue
127    fi
128
129    base_margin=$((min[i] / 4))
130    calc_margin=$(echo "${managed[i]} * $watermark_scale / 10000" | bc)
131    # round the value by adding 0.5 and truncating the decimal part
132    watermark_delta=$(echo "x=($extra_free_pg / ($vm_total_pages / 10000) + 0.5); scale = 0; x/1" | bc -l)
133    if [ $calc_margin -gt $base_margin ]
134    then
135        watermark_scale_new=$(echo "$watermark_scale + $watermark_delta" | bc)
136    else
137        watermark_scale_new=$(echo "$base_margin / (${managed[i]} / 10000) + $watermark_delta" | bc)
138    fi
139
140    if [ $max_watermark_scale -lt $watermark_scale_new ]
141    then
142        max_watermark_scale=$watermark_scale_new
143    fi
144
145    i=$((i+1))
146done
147
148echo $max_watermark_scale > /proc/sys/vm/watermark_scale_factor
149