github.com/petermattis/pebble@v0.0.0-20190905164901-ab51a2166067/internal/bytealloc/bytealloc.go (about)

     1  // Copyright 2016 The Cockroach Authors.
     2  //
     3  // Licensed under the Apache License, Version 2.0 (the "License");
     4  // you may not use this file except in compliance with the License.
     5  // You may obtain a copy of the License at
     6  //
     7  //     http://www.apache.org/licenses/LICENSE-2.0
     8  //
     9  // Unless required by applicable law or agreed to in writing, software
    10  // distributed under the License is distributed on an "AS IS" BASIS,
    11  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
    12  // implied. See the License for the specific language governing
    13  // permissions and limitations under the License.
    14  
    15  package bytealloc
    16  
    17  import "github.com/petermattis/pebble/internal/rawalloc"
    18  
    19  // An A provides chunk allocation of []byte, amortizing the overhead of each
    20  // allocation. Because the underlying storage for the slices is shared, they
    21  // should share a similar lifetime in order to avoid pinning large amounts of
    22  // memory unnecessarily. The allocator itself is a []byte where cap() indicates
    23  // the total amount of memory and len() is the amount already allocated. The
    24  // size of the buffer to allocate from is grown exponentially when it runs out
    25  // of room up to a maximum size (chunkAllocMaxSize).
    26  type A []byte
    27  
    28  const chunkAllocMinSize = 512
    29  const chunkAllocMaxSize = 16384
    30  
    31  func (a A) reserve(n int) A {
    32  	allocSize := cap(a) * 2
    33  	if allocSize < chunkAllocMinSize {
    34  		allocSize = chunkAllocMinSize
    35  	} else if allocSize > chunkAllocMaxSize {
    36  		allocSize = chunkAllocMaxSize
    37  	}
    38  	if allocSize < n {
    39  		allocSize = n
    40  	}
    41  	return rawalloc.New(0, allocSize)
    42  }
    43  
    44  // Alloc allocates a new chunk of memory with the specified length.
    45  func (a A) Alloc(n int) (A, []byte) {
    46  	if cap(a)-len(a) < n {
    47  		a = a.reserve(n)
    48  	}
    49  	p := len(a)
    50  	r := a[p : p+n : p+n]
    51  	a = a[:p+n]
    52  	return a, r
    53  }
    54  
    55  // Copy allocates a new chunk of memory, initializing it from src.
    56  func (a A) Copy(src []byte) (A, []byte) {
    57  	var alloc []byte
    58  	a, alloc = a.Alloc(len(src))
    59  	copy(alloc, src)
    60  	return a, alloc
    61  }