从uboot 的引导到kenrel，以及之后的init 进程的运行，中间耗费的时间，当然越短越是理想。
1. Method
常见的方法有：
- grabserial: add timestamps to all serial console output
- bootgraph: measuring time of kernel functions
- bootchart: measuring time of user-space processes
2. Kernel optimization
kernel 启动优化的建议, 主要是：越少的模块，打印更少。
quietboot argument- slim down useless driers file system
- slim down device tree by removing hardware interfaces that not used
3. user-space processes optimization
用户空间优化建议：
- apps startup or running order(应用执行顺序，进程之间可能有等待资源情况)
- build optimization e.g. compile flags
- library optimizations to reduce load time
Reference
A Pragmatic Guide to Boot-Time Optimization - Chris Simmonds, Consultant