sync.Pool
A Pool is a set of temporary objects that may be individually saved and retrieved.
Any item stored in the Pool may be removed automatically at any time without notification. If the Pool holds the only reference when this happens, the item might be deallocated.
A Pool is safe for use by multiple goroutines simultaneously.
Pool’s purpose is to cache allocated but unused items for later reuse, relieving pressure on the garbage collector. That is, it makes it easy to build efficient, thread-safe free lists. However, it is not suitable for all free lists.
An appropriate use of a Pool is to manage a group of temporary items silently shared among and potentially reused by concurrent independent clients of a package. Pool provides a way to amortize allocation overhead across many clients.
An example of good use of a Pool is in the fmt package, which maintains a dynamically-sized store of temporary output buffers. The store scales under load (when many goroutines are actively printing) and shrinks when quiescent.
On the other hand, a free list maintained as part of a short-lived object is not a suitable use for a Pool, since the overhead does not amortize well in that scenario. It is more efficient to have such objects implement their own free list.
A Pool must not be copied after first use.
type Pool struct {
noCopy noCopy
local unsafe.Pointer // local fixed-size per-P pool, actual type is [P]poolLocal
localSize uintptr // size of the local array
victim unsafe.Pointer // local from previous cycle
victimSize uintptr // size of victims array
// New optionally specifies a function to generate
// a value when Get would otherwise return nil.
// It may not be changed concurrently with calls to Get.
New func() interface{}
}
func (*Pool) Get
func (p *Pool) Get() interface{} {
if race.Enabled {
race.Disable()
}
l, pid := p.pin()
x := l.private
l.private = nil
if x == nil {
// Try to pop the head of the local shard. We prefer
// the head over the tail for temporal locality of
// reuse.
x, _ = l.shared.popHead()
if x == nil {
x = p.getSlow(pid)
}
}
runtime_procUnpin()
if race.Enabled {
race.Enable()
if x != nil {
race.Acquire(poolRaceAddr(x))
}
}
if x == nil && p.New != nil {
x = p.New()
}
return x
}
Get selects an arbitrary item from the Pool, removes it from the Pool, and returns it to the caller. Get may choose to ignore the pool and treat it as empty. Callers should not assume any relation between values passed to Put and the values returned by Get.
If Get would otherwise return nil and p.New is non-nil, Get returns the result of calling p.New.
func (*Pool) Put
func (p *Pool) Put(x interface{}) {
if x == nil {
return
}
if race.Enabled {
if fastrand()%4 == 0 {
// Randomly drop x on floor.
return
}
race.ReleaseMerge(poolRaceAddr(x))
race.Disable()
}
l, _ := p.pin()
if l.private == nil {
l.private = x
x = nil
}
if x != nil {
l.shared.pushHead(x)
}
runtime_procUnpin()
if race.Enabled {
race.Enable()
}
}
Put adds x to the pool.