Go语言切片扩容规则闲谈
2022-01-10 16:20 天心PHP 阅读(428) 评论(0) 编辑 收藏 举报s := []string{"a", "b"} // 此时切片长度为2，容量也为2。 s = append(s, "c") s = append(s, "d") s = append(s, "e") fmt.Printf("len=%d, cap=%d\n", len(s), cap(s)) // 结果：len=5, cap=8 s1 := []string{"a", "b"} // 此时切片长度为2，容量也为2。 s1 = append(s1, "c", "d", "e") fmt.Printf("len=%d, cap=%d\n", len(s1), cap(s1)) // 结果：len=5, cap=5 s2 := []int{1, 2} // 此时切片长度为2，容量也为2。 s2 = append(s2, 3, 4, 5) fmt.Printf("len=%d, cap=%d\n", len(s2), cap(s2)) // 结果：len=5, cap=6
源码位置在go/src/runtime/slice.go：
func growslice(et *_type, old slice, cap int) slice { if raceenabled { callerpc := getcallerpc() racereadrangepc(old.array, uintptr(old.len*int(et.size)), callerpc, funcPC(growslice)) } if msanenabled { msanread(old.array, uintptr(old.len*int(et.size))) } if cap < old.cap { panic(errorString("growslice: cap out of range")) } if et.size == 0 { // append should not create a slice with nil pointer but non-zero len. // We assume that append doesn't need to preserve old.array in this case. return slice{unsafe.Pointer(&zerobase), old.len, cap} } newcap := old.cap doublecap := newcap + newcap if cap > doublecap { newcap = cap } else { if old.len < 1024 { newcap = doublecap } else { // Check 0 < newcap to detect overflow // and prevent an infinite loop. for 0 < newcap && newcap < cap { newcap += newcap / 4 } // Set newcap to the requested cap when // the newcap calculation overflowed. if newcap <= 0 { newcap = cap } } } var overflow bool var lenmem, newlenmem, capmem uintptr // Specialize for common values of et.size. // For 1 we don't need any division/multiplication. // For sys.PtrSize, compiler will optimize division/multiplication into a shift by a constant. // For powers of 2, use a variable shift. switch { case et.size == 1: lenmem = uintptr(old.len) newlenmem = uintptr(cap) capmem = roundupsize(uintptr(newcap)) overflow = uintptr(newcap) > maxAlloc newcap = int(capmem) case et.size == sys.PtrSize: lenmem = uintptr(old.len) * sys.PtrSize newlenmem = uintptr(cap) * sys.PtrSize capmem = roundupsize(uintptr(newcap) * sys.PtrSize) overflow = uintptr(newcap) > maxAlloc/sys.PtrSize newcap = int(capmem / sys.PtrSize) case isPowerOfTwo(et.size): var shift uintptr if sys.PtrSize == 8 { // Mask shift for better code generation. shift = uintptr(sys.Ctz64(uint64(et.size))) & 63 } else { shift = uintptr(sys.Ctz32(uint32(et.size))) & 31 } lenmem = uintptr(old.len) << shift newlenmem = uintptr(cap) << shift capmem = roundupsize(uintptr(newcap) << shift) overflow = uintptr(newcap) > (maxAlloc >> shift) newcap = int(capmem >> shift) default: lenmem = uintptr(old.len) * et.size newlenmem = uintptr(cap) * et.size capmem, overflow = math.MulUintptr(et.size, uintptr(newcap)) capmem = roundupsize(capmem) newcap = int(capmem / et.size) } // The check of overflow in addition to capmem > maxAlloc is needed // to prevent an overflow which can be used to trigger a segfault // on 32bit architectures with this example program: // // type T [1<<27 + 1]int64 // // var d T // var s []T // // func main() { // s = append(s, d, d, d, d) // print(len(s), "\n") // } if overflow || capmem > maxAlloc { panic(errorString("growslice: cap out of range")) } var p unsafe.Pointer if et.ptrdata == 0 { p = mallocgc(capmem, nil, false) // The append() that calls growslice is going to overwrite from old.len to cap (which will be the new length). // Only clear the part that will not be overwritten. memclrNoHeapPointers(add(p, newlenmem), capmem-newlenmem) } else { // Note: can't use rawmem (which avoids zeroing of memory), because then GC can scan uninitialized memory. p = mallocgc(capmem, et, true) if lenmem > 0 && writeBarrier.enabled { // Only shade the pointers in old.array since we know the destination slice p // only contains nil pointers because it has been cleared during alloc. bulkBarrierPreWriteSrcOnly(uintptr(p), uintptr(old.array), lenmem) } } memmove(p, old.array, lenmem) return slice{p, old.len, newcap} }
这里我把所有代码都拿过来了，在详细分析这部分代码时，我们先准备一下必备的基础知识。
go 语言的内存分配也是比较复杂的，这篇文章不会把所有内存分配的细节讲出来，只会把和切片扩容相关的知识点说一下，首先我们先看一个源码go/src/runtime/sizeclasses.go ：
// Code generated by mksizeclasses.go; DO NOT EDIT. //go:generate go run mksizeclasses.go package runtime // class bytes/obj bytes/span objects tail waste max waste // 1 8 8192 1024 0 87.50% // 2 16 8192 512 0 43.75% // 3 32 8192 256 0 46.88% // 4 48 8192 170 32 31.52% // 5 64 8192 128 0 23.44% // 6 80 8192 102 32 19.07% // 7 96 8192 85 32 15.95% // 8 112 8192 73 16 13.56% // 9 128 8192 64 0 11.72% // 10 144 8192 56 128 11.82% // 11 160 8192 51 32 9.73% // 12 176 8192 46 96 9.59% // 13 192 8192 42 128 9.25% // 14 208 8192 39 80 8.12% // 15 224 8192 36 128 8.15% // 16 240 8192 34 32 6.62% // 17 256 8192 32 0 5.86% // 18 288 8192 28 128 12.16% // 19 320 8192 25 192 11.80% // 20 352 8192 23 96 9.88% // 21 384 8192 21 128 9.51% // 22 416 8192 19 288 10.71% // 23 448 8192 18 128 8.37% // 24 480 8192 17 32 6.82% // 25 512 8192 16 0 6.05% // 26 576 8192 14 128 12.33% // 27 640 8192 12 512 15.48% // 28 704 8192 11 448 13.93% // 29 768 8192 10 512 13.94% // 30 896 8192 9 128 15.52% // 31 1024 8192 8 0 12.40% // 32 1152 8192 7 128 12.41% // 33 1280 8192 6 512 15.55% // 34 1408 16384 11 896 14.00% // 35 1536 8192 5 512 14.00% // 36 1792 16384 9 256 15.57% // 37 2048 8192 4 0 12.45% // 38 2304 16384 7 256 12.46% // 39 2688 8192 3 128 15.59% // 40 3072 24576 8 0 12.47% // 41 3200 16384 5 384 6.22% // 42 3456 24576 7 384 8.83% // 43 4096 8192 2 0 15.60% // 44 4864 24576 5 256 16.65% // 45 5376 16384 3 256 10.92% // 46 6144 24576 4 0 12.48% // 47 6528 32768 5 128 6.23% // 48 6784 40960 6 256 4.36% // 49 6912 49152 7 768 3.37% // 50 8192 8192 1 0 15.61% // 51 9472 57344 6 512 14.28% // 52 9728 49152 5 512 3.64% // 53 10240 40960 4 0 4.99% // 54 10880 32768 3 128 6.24% // 55 12288 24576 2 0 11.45% // 56 13568 40960 3 256 9.99% // 57 14336 57344 4 0 5.35% // 58 16384 16384 1 0 12.49% // 59 18432 73728 4 0 11.11% // 60 19072 57344 3 128 3.57% // 61 20480 40960 2 0 6.87% // 62 21760 65536 3 256 6.25% // 63 24576 24576 1 0 11.45% // 64 27264 81920 3 128 10.00% // 65 28672 57344 2 0 4.91% // 66 32768 32768 1 0 12.50% const ( _MaxSmallSize = 32768 smallSizeDiv = 8 smallSizeMax = 1024 largeSizeDiv = 128 _NumSizeClasses = 67 _PageShift = 13 ) var class_to_size = [_NumSizeClasses]uint16{0, 8, 16, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176, 192, 208, 224, 240, 256, 288, 320, 352, 384, 416, 448, 480, 512, 576, 640, 704, 768, 896, 1024, 1152, 1280, 1408, 1536, 1792, 2048, 2304, 2688, 3072, 3200, 3456, 4096, 4864, 5376, 6144, 6528, 6784, 6912, 8192, 9472, 9728, 10240, 10880, 12288, 13568, 14336, 16384, 18432, 19072, 20480, 21760, 24576, 27264, 28672, 32768} var class_to_allocnpages = [_NumSizeClasses]uint8{0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 2, 1, 2, 1, 3, 2, 3, 1, 3, 2, 3, 4, 5, 6, 1, 7, 6, 5, 4, 3, 5, 7, 2, 9, 7, 5, 8, 3, 10, 7, 4} type divMagic struct { shift uint8 shift2 uint8 mul uint16 baseMask uint16 } var class_to_divmagic = [_NumSizeClasses]divMagic{{0, 0, 0, 0}, {3, 0, 1, 65528}, {4, 0, 1, 65520}, {5, 0, 1, 65504}, {4, 11, 683, 0}, {6, 0, 1, 65472}, {4, 10, 205, 0}, {5, 9, 171, 0}, {4, 11, 293, 0}, {7, 0, 1, 65408}, {4, 13, 911, 0}, {5, 10, 205, 0}, {4, 12, 373, 0}, {6, 9, 171, 0}, {4, 13, 631, 0}, {5, 11, 293, 0}, {4, 13, 547, 0}, {8, 0, 1, 65280}, {5, 9, 57, 0}, {6, 9, 103, 0}, {5, 12, 373, 0}, {7, 7, 43, 0}, {5, 10, 79, 0}, {6, 10, 147, 0}, {5, 11, 137, 0}, {9, 0, 1, 65024}, {6, 9, 57, 0}, {7, 9, 103, 0}, {6, 11, 187, 0}, {8, 7, 43, 0}, {7, 8, 37, 0}, {10, 0, 1, 64512}, {7, 9, 57, 0}, {8, 6, 13, 0}, {7, 11, 187, 0}, {9, 5, 11, 0}, {8, 8, 37, 0}, {11, 0, 1, 63488}, {8, 9, 57, 0}, {7, 10, 49, 0}, {10, 5, 11, 0}, {7, 10, 41, 0}, {7, 9, 19, 0}, {12, 0, 1, 61440}, {8, 9, 27, 0}, {8, 10, 49, 0}, {11, 5, 11, 0}, {7, 13, 161, 0}, {7, 13, 155, 0}, {8, 9, 19, 0}, {13, 0, 1, 57344}, {8, 12, 111, 0}, {9, 9, 27, 0}, {11, 6, 13, 0}, {7, 14, 193, 0}, {12, 3, 3, 0}, {8, 13, 155, 0}, {11, 8, 37, 0}, {14, 0, 1, 49152}, {11, 8, 29, 0}, {7, 13, 55, 0}, {12, 5, 7, 0}, {8, 14, 193, 0}, {13, 3, 3, 0}, {7, 14, 77, 0}, {12, 7, 19, 0}, {15, 0, 1, 32768}} var size_to_class8 = [smallSizeMax/smallSizeDiv + 1]uint8{0, 1, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7, 7, 8, 8, 9, 9, 10, 10, 11, 11, 12, 12, 13, 13, 14, 14, 15, 15, 16, 16, 17, 17, 18, 18, 18, 18, 19, 19, 19, 19, 20, 20, 20, 20, 21, 21, 21, 21, 22, 22, 22, 22, 23, 23, 23, 23, 24, 24, 24, 24, 25, 25, 25, 25, 26, 26, 26, 26, 26, 26, 26, 26, 27, 27, 27, 27, 27, 27, 27, 27, 28, 28, 28, 28, 28, 28, 28, 28, 29, 29, 29, 29, 29, 29, 29, 29, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 30, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31, 31} var size_to_class128 = [(_MaxSmallSize-smallSizeMax)/largeSizeDiv + 1]uint8{31, 32, 33, 34, 35, 36, 36, 37, 37, 38, 38, 39, 39, 39, 40, 40, 40, 41, 42, 42, 43, 43, 43, 43, 43, 44, 44, 44, 44, 44, 44, 45, 45, 45, 45, 46, 46, 46, 46, 46, 46, 47, 47, 47, 48, 48, 49, 50, 50, 50, 50, 50, 50, 50, 50, 50, 50, 51, 51, 51, 51, 51, 51, 51, 51, 51, 51, 52, 52, 53, 53, 53, 53, 54, 54, 54, 54, 54, 55, 55, 55, 55, 55, 55, 55, 55, 55, 55, 55, 56, 56, 56, 56, 56, 56, 56, 56, 56, 56, 57, 57, 57, 57, 57, 57, 58, 58, 58, 58, 58, 58, 58, 58, 58, 58, 58, 58, 58, 58, 58, 58, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 59, 60, 60, 60, 60, 60, 61, 61, 61, 61, 61, 61, 61, 61, 61, 61, 61, 62, 62, 62, 62, 62, 62, 62, 62, 62, 62, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 63, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 65, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66, 66}
2.2.1.1 介绍内存对齐
首先我们需要知道 CPU 从内存度数据是按照每次固定大小读的，例如每次读8字节或者4字节，所以就有一个效率问题，每读取一次，就浪费一点时间，因此 Go 中也存在内存对齐，今天重点不是内存对齐，但是这里可以先和大家简单说一下：每个对象在内存中都要占一块空间，可能是1字节，可能是3字节，可能是8字节，所以对于一个struct 来说，其字段类型会觉得这个结构体对象所占的内存，但是所占内存大小可能不是所有字段类型所占内存的大小，因为为了让 CPU 读取方便，我们就会对每个字段进行内存对齐。点到为止，上面的内存对齐就不深说了，但是上面所说的内容大家还是需要理解的。
2.2.1.2. 分配内存事实
当我们创建一个对象时，需要分配一块内存。假设我们创建的对象需要52 byte，系统是不会真的就给我们52byte大小的内存的，首先会根据上面代码中第六行注释的部分来计算需要给你分配的内存大小，这里可以参考一下go/src/runtime/msize.go 中的roundupsize 函数。也就是需要向上取整，意思就是48<52<64，所以会分配64byte大小的内存。
这里我们需要明白一个事实，如果我们每次创建一个对象，程序都向计算机中申请一块内存，这样在程序运行时我们是会频繁的创建对象的，这样效率会大大降低，所以程序会预先申请好一些内存块，其大小就是：8、16、32、48等等，这样在我们让程序申请内存时，程序就可以把申请好的内存选一块给我们了，效率也就提高了。
我们使用下面的代码来分析扩容的规则：
s := []int64{1, 2}
s = append(s, 3, 4, 5)
fmt.Printf("len=%d, cap=%d", len(s), cap(s)) // len=5, cap=6
我们首先知道int64 类型大小为8字节。我们再看切片源码的参数：func growslice(et *_type, old slice, cap int) slice，在上面的代码中，本地扩容的参数第一个是int64类型，第二个就是扩容前切片a（元素为1和2），第三个参数就是预估容量5（因为原有切片容量加上新加元素个数就是5），我们再继续看源码中对容量计算的部分代码：
... newcap := old.cap doublecap := newcap + newcap if cap > doublecap { newcap = cap } else { if old.len < 1024 { newcap = doublecap } else { // Check 0 < newcap to detect overflow // and prevent an infinite loop. for 0 < newcap && newcap < cap { newcap += newcap / 4 } // Set newcap to the requested cap when // the newcap calculation overflowed. if newcap <= 0 { newcap = cap } } } ...
newcap 开始是就切片的容量2，doublecap 为2+2=4，cap 为5，此时cap>doublecap，因此newcap 最终就是5（剩下的逻辑自己可以看看，如果 cap<doublecap 的情况自己可以试一下）。
在 2.2.1.2 中我们介绍了 roundupsize，此时我们已经知道 newcap 为5，int64 占用8字节，那就是说我切片扩容后需要5*8=40字节，再通过 roundupsize 计算，最终给我们的内存大小应该是48字节（可以回顾一下2.2.1.2），48/8=6，所以结果我们就知道了，最终切片的 cap 就是6。
源码实现的方式中很多计算，包括左移右移等，但是想法和我上面说的是一样的，感兴趣大家可以参考源码计算一下。
4.总结
至此切片扩容就结束了，其中有一些地方这里只是简单说了下。根据上面的例子，大家可以手动试一下string 类型，或者int32等。如果对你有帮助点个在看哦~
