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Pavel Emelyanovfaebe9f2008-04-29 01:00:18 -07001
2 The Resource Counter
3
4The resource counter, declared at include/linux/res_counter.h,
5is supposed to facilitate the resource management by controllers
6by providing common stuff for accounting.
7
8This "stuff" includes the res_counter structure and routines
9to work with it.
10
11
12
131. Crucial parts of the res_counter structure
14
15 a. unsigned long long usage
16
17 The usage value shows the amount of a resource that is consumed
18 by a group at a given time. The units of measurement should be
19 determined by the controller that uses this counter. E.g. it can
20 be bytes, items or any other unit the controller operates on.
21
22 b. unsigned long long max_usage
23
24 The maximal value of the usage over time.
25
26 This value is useful when gathering statistical information about
27 the particular group, as it shows the actual resource requirements
28 for a particular group, not just some usage snapshot.
29
30 c. unsigned long long limit
31
32 The maximal allowed amount of resource to consume by the group. In
33 case the group requests for more resources, so that the usage value
34 would exceed the limit, the resource allocation is rejected (see
35 the next section).
36
37 d. unsigned long long failcnt
38
39 The failcnt stands for "failures counter". This is the number of
40 resource allocation attempts that failed.
41
42 c. spinlock_t lock
43
44 Protects changes of the above values.
45
46
47
482. Basic accounting routines
49
Andrea Righi5341cfa2009-04-13 14:39:58 -070050 a. void res_counter_init(struct res_counter *rc,
51 struct res_counter *rc_parent)
Pavel Emelyanovfaebe9f2008-04-29 01:00:18 -070052
53 Initializes the resource counter. As usual, should be the first
54 routine called for a new counter.
55
Andrea Righi5341cfa2009-04-13 14:39:58 -070056 The struct res_counter *parent can be used to define a hierarchical
57 child -> parent relationship directly in the res_counter structure,
58 NULL can be used to define no relationship.
59
60 c. int res_counter_charge(struct res_counter *rc, unsigned long val,
61 struct res_counter **limit_fail_at)
Pavel Emelyanovfaebe9f2008-04-29 01:00:18 -070062
63 When a resource is about to be allocated it has to be accounted
64 with the appropriate resource counter (controller should determine
65 which one to use on its own). This operation is called "charging".
66
67 This is not very important which operation - resource allocation
68 or charging - is performed first, but
69 * if the allocation is performed first, this may create a
70 temporary resource over-usage by the time resource counter is
71 charged;
72 * if the charging is performed first, then it should be uncharged
73 on error path (if the one is called).
74
Andrea Righi5341cfa2009-04-13 14:39:58 -070075 If the charging fails and a hierarchical dependency exists, the
76 limit_fail_at parameter is set to the particular res_counter element
77 where the charging failed.
78
79 d. int res_counter_charge_locked
80 (struct res_counter *rc, unsigned long val)
81
82 The same as res_counter_charge(), but it must not acquire/release the
83 res_counter->lock internally (it must be called with res_counter->lock
84 held).
85
86 e. void res_counter_uncharge[_locked]
Pavel Emelyanovfaebe9f2008-04-29 01:00:18 -070087 (struct res_counter *rc, unsigned long val)
88
89 When a resource is released (freed) it should be de-accounted
90 from the resource counter it was accounted to. This is called
91 "uncharging".
92
Andrea Righi5341cfa2009-04-13 14:39:58 -070093 The _locked routines imply that the res_counter->lock is taken.
Pavel Emelyanovfaebe9f2008-04-29 01:00:18 -070094
95 2.1 Other accounting routines
96
97 There are more routines that may help you with common needs, like
98 checking whether the limit is reached or resetting the max_usage
99 value. They are all declared in include/linux/res_counter.h.
100
101
102
1033. Analyzing the resource counter registrations
104
105 a. If the failcnt value constantly grows, this means that the counter's
106 limit is too tight. Either the group is misbehaving and consumes too
107 many resources, or the configuration is not suitable for the group
108 and the limit should be increased.
109
110 b. The max_usage value can be used to quickly tune the group. One may
111 set the limits to maximal values and either load the container with
112 a common pattern or leave one for a while. After this the max_usage
113 value shows the amount of memory the container would require during
114 its common activity.
115
116 Setting the limit a bit above this value gives a pretty good
117 configuration that works in most of the cases.
118
119 c. If the max_usage is much less than the limit, but the failcnt value
120 is growing, then the group tries to allocate a big chunk of resource
121 at once.
122
123 d. If the max_usage is much less than the limit, but the failcnt value
124 is 0, then this group is given too high limit, that it does not
125 require. It is better to lower the limit a bit leaving more resource
126 for other groups.
127
128
129
1304. Communication with the control groups subsystem (cgroups)
131
132All the resource controllers that are using cgroups and resource counters
133should provide files (in the cgroup filesystem) to work with the resource
134counter fields. They are recommended to adhere to the following rules:
135
136 a. File names
137
138 Field name File name
139 ---------------------------------------------------
140 usage usage_in_<unit_of_measurement>
141 max_usage max_usage_in_<unit_of_measurement>
142 limit limit_in_<unit_of_measurement>
143 failcnt failcnt
144 lock no file :)
145
146 b. Reading from file should show the corresponding field value in the
147 appropriate format.
148
149 c. Writing to file
150
151 Field Expected behavior
152 ----------------------------------
153 usage prohibited
154 max_usage reset to usage
155 limit set the limit
156 failcnt reset to zero
157
158
159
1605. Usage example
161
162 a. Declare a task group (take a look at cgroups subsystem for this) and
163 fold a res_counter into it
164
165 struct my_group {
166 struct res_counter res;
167
168 <other fields>
169 }
170
171 b. Put hooks in resource allocation/release paths
172
173 int alloc_something(...)
174 {
175 if (res_counter_charge(res_counter_ptr, amount) < 0)
176 return -ENOMEM;
177
178 <allocate the resource and return to the caller>
179 }
180
181 void release_something(...)
182 {
183 res_counter_uncharge(res_counter_ptr, amount);
184
185 <release the resource>
186 }
187
188 In order to keep the usage value self-consistent, both the
189 "res_counter_ptr" and the "amount" in release_something() should be
190 the same as they were in the alloc_something() when the releasing
191 resource was allocated.
192
193 c. Provide the way to read res_counter values and set them (the cgroups
194 still can help with it).
195
196 c. Compile and run :)