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.