1persistent_term(3) Erlang Module Definition persistent_term(3)
2
6 persistent_term - Persistent terms.
7
9 This module is similar to ets in that it provides a storage for Erlang
10 terms that can be accessed in constant time, but with the difference
11 that persistent_term has been highly optimized for reading terms at the
12 expense of writing and updating terms. When a persistent term is up‐
13 dated or deleted, a global garbage collection pass is run to scan all
14 processes for the deleted term, and to copy it into each process that
15 still uses it. Therefore, persistent_term is suitable for storing Er‐
16 lang terms that are frequently accessed but never or infrequently up‐
17 dated.
18 Warning:
20 Persistent terms is an advanced feature and is not a general replace‐
21 ment for ETS tables. Before using persistent terms, make sure to fully
22 understand the consequence to system performance when updating or
23 deleting persistent terms.
24 Term lookup (using get/1), is done in constant time and without taking
27 any locks, and the term is not copied to the heap (as is the case with
28 terms stored in ETS tables).
29 Storing or updating a term (using put/2) is proportional to the number
31 of already created persistent terms because the hash table holding the
32 keys will be copied. In addition, the term itself will be copied.
33 When a (complex) term is deleted (using erase/1) or replaced by another
35 (using put/2), a global garbage collection is initiated. It works like
36 this:
37 * All processes in the system will be scheduled to run a scan of
39 their heaps for the term that has been deleted. While such scan is
40 relatively light-weight, if there are many processes, the system
41 can become less responsive until all process have scanned their
42 heaps.
43 * If the deleted term (or any part of it) is still used by a process,
45 that process will do a major (fullsweep) garbage collection and
46 copy the term into the process. However, at most two processes at a
47 time will be scheduled to do that kind of garbage collection.
48 Deletion of atoms and other terms that fit in one machine word is spe‐
50 cially optimized to avoid doing a global GC. It is still not recom‐
51 mended to update persistent terms with such values too frequently be‐
52 cause the hash table holding the keys is copied every time a persistent
53 term is updated.
54 Some examples are suitable uses for persistent terms are:
56 * Storing of configuration data that must be easily accessible by all
58 processes.
59 * Storing of references for NIF resources.
61 * Storing of references for efficient counters.
63 * Storing an atom to indicate a logging level or whether debugging is
65 turned on.
66
68 The current implementation of persistent terms uses the literal alloca‐
69 tor also used for literals (constant terms) in BEAM code. By default, 1
70 GB of virtual address space is reserved for literals in BEAM code and
71 persistent terms. The amount of virtual address space reserved for lit‐
72 erals can be changed by using the +MIscs option when starting the emu‐
73 lator.
74 Here is an example how the reserved virtual address space for literals
76 can be raised to 2 GB (2048 MB):
77 erl +MIscs 2048
79
81 It is recommended to use keys like ?MODULE or {?MODULE,SubKey} to avoid
82 name collisions.
83 Prefer creating a few large persistent terms to creating many small
85 persistent terms. The execution time for storing a persistent term is
86 proportional to the number of already existing terms.
87 Updating a persistent term with the same value as it already has is
89 specially optimized to do nothing quickly; thus, there is no need com‐
90 pare the old and new values and avoid calling put/2 if the values are
91 equal.
92 When atoms or other terms that fit in one machine word are deleted, no
94 global GC is needed. Therefore, persistent terms that have atoms as
95 their values can be updated more frequently, but note that updating
96 such persistent terms is still much more expensive than reading them.
97 Updating or deleting a persistent term will trigger a global GC if the
99 term does not fit in one machine word. Processes will be scheduled as
100 usual, but all processes will be made runnable at once, which will make
101 the system less responsive until all process have run and scanned their
102 heaps for the deleted terms. One way to minimize the effects on respon‐
103 siveness could be to minimize the number of processes on the node be‐
104 fore updating or deleting a persistent term. It would also be wise to
105 avoid updating terms when the system is at peak load.
106 Avoid storing a retrieved persistent term in a process if that persis‐
108 tent term could be deleted or updated in the future. If a process holds
109 a reference to a persistent term when the term is deleted, the process
110 will be garbage collected and the term copied to process.
111 Avoid updating or deleting more than one persistent term at a time.
113 Each deleted term will trigger its own global GC. That means that
114 deleting N terms will make the system less responsive N times longer
115 than deleting a single persistent term. Therefore, terms that are to be
116 updated at the same time should be collected into a larger term, for
117 example, a map or a tuple.
118
120 The following example shows how lock contention for ETS tables can be
121 minimized by having one ETS table for each scheduler. The table identi‐
122 fiers for the ETS tables are stored as a single persistent term:
123 %% There is one ETS table for each scheduler.
125 Sid = erlang:system_info(scheduler_id),
126 Tid = element(Sid, persistent_term:get(?MODULE)),
127 ets:update_counter(Tid, Key, 1).
128
130 key() = term()
131 Any Erlang term.
133 value() = term()
135 Any Erlang term.
137
139 erase(Key) -> Result
140 Types:
142 Key = key()
144 Result = boolean()
145 Erase the name for the persistent term with key Key. The return
147 value will be true if there was a persistent term with the key
148 Key, and false if there was no persistent term associated with
149 the key.
150 If there existed a previous persistent term associated with key
152 Key, a global GC has been initiated when erase/1 returns. See
153 Description.
154 get() -> List
156 Types:
158 List = [{key(), value()}]
160 Retrieve the keys and values for all persistent terms. The keys
162 will be copied to the heap for the process calling get/0, but
163 the values will not.
164 get(Key) -> Value
166 Types:
168 Key = key()
170 Value = value()
171 Retrieve the value for the persistent term associated with the
173 key Key. The lookup will be made in constant time and the value
174 will not be copied to the heap of the calling process.
175 This function fails with a badarg exception if no term has been
177 stored with the key Key.
178 If the calling process holds on to the value of the persistent
180 term and the persistent term is deleted in the future, the term
181 will be copied to the process.
182 get(Key, Default) -> Value
184 Types:
186 Key = key()
188 Default = Value = value()
189 Retrieve the value for the persistent term associated with the
191 key Key. The lookup will be made in constant time and the value
192 will not be copied to the heap of the calling process.
193 This function returns Default if no term has been stored with
195 the key Key.
196 If the calling process holds on to the value of the persistent
198 term and the persistent term is deleted in the future, the term
199 will be copied to the process.
200 info() -> Info
202 Types:
204 Info = #{count := Count, memory := Memory}
206 Count = Memory = integer() >= 0
207 Return information about persistent terms in a map. The map has
209 the following keys:
210 count:
212 The number of persistent terms.
213 memory:
215 The total amount of memory (measured in bytes) used by all
216 persistent terms.
217 put(Key, Value) -> ok
219 Types:
221 Key = key()
223 Value = value()
224 Store the value Value as a persistent term and associate it with
226 the key Key.
227 If the value Value is equal to the value previously stored for
229 the key, put/2 will do nothing and return quickly.
230 If there existed a previous persistent term associated with key
232 Key, a global GC has been initiated when put/2 returns. See De‐
233 scription.
234Ericsson AB erts 11.2.2.2 persistent_term(3)