在做一些关于无锁/无等待算法的研究时,我偶然发现了错误的共享问题.挖掘更多的东西让我得到了Folly的源代码(Facebook的C++库),更具体地说是这个头文件和FOLLY_ALIGN_TO_AVOID_FALSE_SHARING宏的定义(目前在第130行).乍一看最令我惊讶的是价值:128(即:而不是64)......
/// An attribute that will cause a variable or field to be aligned so that
/// it doesn't have false sharing with anything at a smaller memory address.
#define FOLLY_ALIGN_TO_AVOID_FALSE_SHARING __attribute__((__aligned__(128)))
AFAIK,现代CPU上的缓存块长度为64字节,实际上,到目前为止,我发现的每一项资源,包括英特尔的这篇文章,都讨论了64字节对齐和填充,以帮助解决错误共享问题.
尽管如此,Facebook的人们在需要时将他们的班级成员对齐并填充为128字节.然后我在上面FOLLY_ALIGN_TO_AVOID_FALSE_SHARING的定义中找到了解释的开头:
enum {
/// Memory locations on the same cache line are subject to false
/// sharing, which is very bad for performance. Microbenchmarks
/// indicate that pairs of cache lines also see …