| Safe Haskell | None |
|---|---|
| Language | Haskell2010 |
Data.Atomics.Counter
Contents
Description
Integer counters providing thread-safe, lock-free mutation functions.
Atomic counters are represented by a single memory location, such that built-in processor instructions are sufficient to perform fetch-and-add or compare-and-swap.
Remember, contention on such counters should still be minimized!
- data AtomicCounter
- newCounter :: Int -> IO AtomicCounter
- type CTicket = Int
- peekCTicket :: CTicket -> Int
- casCounter :: AtomicCounter -> CTicket -> Int -> IO (Bool, CTicket)
- incrCounter :: Int -> AtomicCounter -> IO Int
- incrCounter_ :: Int -> AtomicCounter -> IO ()
- readCounter :: AtomicCounter -> IO Int
- readCounterForCAS :: AtomicCounter -> IO CTicket
- writeCounter :: AtomicCounter -> Int -> IO ()
Type of counters of counters and tickets
data AtomicCounter
The type of mutable atomic counters.
Creating counters
newCounter :: Int -> IO AtomicCounter
Create a new counter initialized to the given value.
Tickets, used for compare-and-swap
See the documentation for Data.Atomics for more explanation of the ticket abstraction. The same ideas apply here for counters as for general mutable locations (IORefs).
You should not depend on this type. It varies between different implementations of atomic counters.
peekCTicket :: CTicket -> Int
Opaque tickets cannot be constructed, but they can be destructed into values.
Atomic memory operations
casCounter :: AtomicCounter -> CTicket -> Int -> IO (Bool, CTicket)
Compare and swap for the counter ADT. Similar behavior to
casIORef, in particular, in both success and failure cases it
returns a ticket that you should use for the next attempt. (That is, in the
success case, it actually returns the new value that you provided as input, but in
ticket form.)
incrCounter :: Int -> AtomicCounter -> IO Int
Increment the counter by a given amount. Returns the value AFTER the increment (in contrast with the behavior of the underlying instruction on architectures like x86.)
Note that UNLIKE with boxed implementations of counters, where increment is based on CAS, this increment is O(1). Fetch-and-add does not require a retry loop like CAS.
incrCounter_ :: Int -> AtomicCounter -> IO ()
An alternate version for when you don't care about the old value.
Non-atomic operations
readCounter :: AtomicCounter -> IO Int
Equivalent to readCounterForCAS followed by peekCTicket.
readCounterForCAS :: AtomicCounter -> IO CTicket
Just like the Data.Atomics CAS interface, this routine returns an opaque
ticket that can be used in CAS operations. Except for the difference in return
type, the semantics of this are the same as readCounter.
writeCounter :: AtomicCounter -> Int -> IO ()
Make a non-atomic write to the counter. No memory-barrier.