View Source counters (erts v14.3)
Counter Functions
This module provides a set of functions to do operations towards shared mutable counter variables. The implementation does not utilize any software level locking, which makes it very efficient for concurrent access. The counters are organized into arrays with the following semantics:
- Counters are 64 bit signed integers.
- Counters wrap around at overflow and underflow operations.
- Counters are initialized to zero.
- Write operations guarantee atomicity. No intermediate results can be seen from a single write operation.
- Two types of counter arrays can be created with options
atomics
orwrite_concurrency
. Theatomics
counters have good allround performance with nice consistent semantics whilewrite_concurrency
counters offers even better concurrent write performance at the expense of some potential read inconsistencies. Seenew/2
. - Indexes into counter arrays are one-based. A counter array of size N contains N counters with index from 1 to N.
Summary
Functions
Add Incr
to counter at index Ix
.
Read counter value.
Return information about a counter array in a map.
Create a new counter array of Size
counters. All counters in the array are
initially set to zero.
Write Value
to counter at index Ix
.
Subtract Decr
from counter at index Ix
.
Types
-opaque counters_ref()
Identifies a counter array returned from new/2
.
Functions
-spec add(Ref, Ix, Incr) -> ok when Ref :: counters_ref(), Ix :: integer(), Incr :: integer().
Add Incr
to counter at index Ix
.
-spec get(Ref, Ix) -> integer() when Ref :: counters_ref(), Ix :: integer().
Read counter value.
-spec info(Ref) -> Info when Ref :: counters_ref(), Info :: #{size := Size, memory := Memory}, Size :: non_neg_integer(), Memory :: non_neg_integer().
Return information about a counter array in a map.
The map has the following keys (at least):
size
- The number of counters in the array.memory
- Approximate memory consumption for the array in bytes.
-spec new(Size, Opts) -> counters_ref() when Size :: pos_integer(), Opts :: [Opt], Opt :: atomics | write_concurrency.
Create a new counter array of Size
counters. All counters in the array are
initially set to zero.
Argument Opts
is a list of the following possible options:
atomics
(Default) - Counters will be sequentially consistent. If write operation A is done sequentially before write operation B, then a concurrent reader may see the result of none of them, only A, or both A and B. It cannot see the result of only B.write_concurrency
- This is an optimization to achieve very efficient concurrentadd
andsub
operations at the expense of potential read inconsistency and memory consumption per counter.Read operations may see sequentially inconsistent results with regard to concurrent write operations. Even if write operation A is done sequentially before write operation B, a concurrent reader may see any combination of A and B, including only B. A read operation is only guaranteed to see all writes done sequentially before the read. No writes are ever lost, but will eventually all be seen.
The typical use case for
write_concurrency
is when concurrent calls toadd
andsub
toward the same counters are very frequent, while calls toget
andput
are much less frequent. The lack of absolute read consistency must also be acceptable.
Counters are not tied to the current process and are automatically garbage collected when they are no longer referenced.
-spec put(Ref, Ix, Value) -> ok when Ref :: counters_ref(), Ix :: integer(), Value :: integer().
Write Value
to counter at index Ix
.
Note
Despite its name, the
write_concurrency
optimization does not improveput
. A call toput
is a relatively heavy operation compared to the very lightweight and scalableadd
andsub
. The cost for aput
withwrite_concurrency
is like aget
plus aput
withoutwrite_concurrency
.
-spec sub(Ref, Ix, Decr) -> ok when Ref :: counters_ref(), Ix :: integer(), Decr :: integer().
Subtract Decr
from counter at index Ix
.