github.com/benz9527/xboot@v0.0.0-20240504061247-c23f15593274/lib/id/monotonic_id.go (about)

     1  package id
     2  
     3  import (
     4  	"strconv"
     5  	"sync/atomic"
     6  	"unsafe"
     7  
     8  	"golang.org/x/sys/cpu"
     9  )
    10  
    11  const cacheLinePadSize = unsafe.Sizeof(cpu.CacheLinePad{})
    12  
    13  // monotonicNonZeroID is an ID generator.
    14  // Only increase, if it overflows, it will be reset to 1.
    15  // Occupy a whole cache line (flag+tag+data), and a cache line data is 64 bytes.
    16  // L1D cache: cat /sys/devices/system/cpu/cpu0/cache/index0/coherency_line_size
    17  // L1I cache: cat /sys/devices/system/cpu/cpu0/cache/index1/coherency_line_size
    18  // L2 cache: cat /sys/devices/system/cpu/cpu0/cache/index2/coherency_line_size
    19  // L3 cache: cat /sys/devices/system/cpu/cpu0/cache/index3/coherency_line_size
    20  // MESI (Modified-Exclusive-Shared-Invalid)
    21  // RAM data -> L3 cache -> L2 cache -> L1 cache -> CPU register.
    22  // CPU register (cache hit) -> L1 cache -> L2 cache -> L3 cache -> RAM data.
    23  type monotonicNonZeroID struct {
    24  	// sequence consistency data race free program
    25  	// avoid load into cpu cache will be broken by others data
    26  	// to compose a data race cache line
    27  	_   [cacheLinePadSize - unsafe.Sizeof(*new(uint64))]byte // padding for CPU cache line, avoid false sharing
    28  	val uint64                                               // space waste to exchange for performance
    29  	_   [cacheLinePadSize - unsafe.Sizeof(*new(uint64))]byte // padding for CPU cache line, avoid false sharing
    30  }
    31  
    32  func (id *monotonicNonZeroID) next() uint64 {
    33  	// Golang atomic store with LOCK prefix, it means that
    34  	// it implements the Happens-Before relationship.
    35  	// But it is not clearly that atomic add satisfies the
    36  	// Happens-Before relationship.
    37  	// https://go.dev/ref/mem
    38  	var v uint64
    39  	if v = atomic.AddUint64(&id.val, 1); v == 0 {
    40  		v = atomic.AddUint64(&id.val, 1)
    41  	}
    42  	return v
    43  }
    44  
    45  func MonotonicNonZeroID() (UUIDGen, error) {
    46  	src := &monotonicNonZeroID{val: 0}
    47  	id := new(uuidDelegator)
    48  	id.number = func() uint64 {
    49  		return src.next()
    50  	}
    51  	id.str = func() string {
    52  		return strconv.FormatUint(src.next(), 10)
    53  	}
    54  	return id, nil
    55  }