# Zero Copy

## `sendfile`

一次`sendfile()`有三次内存拷贝:
- 磁盘 -> kernel buffer
- kernel buffer -> socket发送缓冲区
- socket发送缓冲区 -> NIC

先比`read()`和`write()`的方式, 少了一次内存拷贝, 两次上下文切换.

如果是`Scatter-Gatther NIC`, 那么os缓存区会直接拷贝到网卡, 少两次内存拷贝. ([`TCP Segmentation Offloading`](https://lwn.net/Articles/9123/))


## `splice`

和`sendfile`的区别是:

- 磁盘 -> kernel buffer
- kernel buffer -- pipe --> socket buffer
- socket buffer -> NIC

这里的`pipe`不做拷贝,只是建立一个引用(指针).

> splice() moves data between two file descriptors without copying between kernel address space and user address space.

http://man7.org/linux/man-pages/man2/splice.2.html


## 补充参考

- preformance review https://pdfs.semanticscholar.org/6a35/60046cb8d3258669c86072a7cab05e1d2300.pdf
- https://www.linuxjournal.com/article/6345