不同级别的延迟估算
Order-of-Magnitude Estimates
read more from «Understanding Software Dynamics», Chapter 1, section 6.
The phrase order of magnitude refers to an approximate measure of the size of a number. A decimal order of magnitude gives an estimate that is the nearest power of 10 (1, 10, 100, …), while a binary order of magnitude gives an estimate that is the nearest power of 2 (1, 2, 4, 8, and so on). we use the notation O(n) for “on the order of n,” with the units always specified. It matters a lot whether you are talking about O(10) nanoseconds or O(10) milliseconds or O(10) bytes. we will also use nsec, usec, and msec to abbreviate nanoseconds, microseconds, and milliseconds, respectively.
Numbers Everyone Should Know [Dean 2009]
| Action | Time | O(n) |
|---|---|---|
| L1 cache reference | 0.5 nsec | O(1) nsec |
| Branch mispredict | 5 nsec | O(10) nsec |
| L2 cache reference | 7 nsec | O(10) nsec |
| Mutex lock/unlock | 25 nsec | O(10) nsec |
| Main memory reference | 100 nsec | O(100) nsec |
| Compress 1K bytes with Zippy | 3,000 nsec | O(1) usec |
| Send 2K bytes over 1 Gbps network | 20,000 nsec | O(10) usec |
| Read 1 MB sequentially from memory | 250,000 nsec | O(100) usec |
| Round trip within same datacenter | 500,000 nsec | O(1) msec |
| Disk seek | 10,000,000 nsec | O(10) msec |
| Read 1 MB sequentially from disk | 20,000,000 nsec | O(10) msec |
| Send packet CA->Netherlands->CA | 150,000,000 nsec | O(100) msec |
Knowing the estimates in Table 1.1 will also guide you in identifying the likely source of a performance bug.