C and pointer Chapter 18 runtime efficiency 18.3 runtime efficiency

18.3 Runtime efficiency
     When the capacity of the program after growth exceeds the amount of memory , It won't work , This belongs to “ Too big ”. Even on some modern machines , One must be stored in ROM The program must be quite small , It is possible to load into the limited memory space .
     But many modern computer systems are far less restrictive in this regard , This is because virtual memory is provided （virtual memory）. Virtual memory is implemented by the operating system , It needs to put the active part of the program into memory and copy the inactive part to disk , This allows the system to run large programs . But the bigger the program , The more replication is required . So large programs are not as impossible to run as before , But as the program grows , Execution efficiency gradually decreases . therefore , When does it appear “ Too big ” Well ？ When it runs too slowly .
     The execution speed of a program is obviously related to its volume . The slower the program executes , The more uncomfortable it will be to use this program . It is difficult for us to define at what point a program is suddenly detained “ Too slow ” The hat of . Unless it has to respond to some physical events beyond its control . for example , One operation CD If the player's program can't process the data as fast as the data from CD The speed of transmission , It's obviously too slow .
     Increase of efficiency
     Modern code is optimized from a C The program is very good at generating efficient object code . therefore , Spend time making small changes to the code to make it more efficient , It's often not very cost-effective .
     Tips ：
     If a program is too big or too slow , Instead of delving into each variable , Look at declaring them register Can we improve efficiency , It is far better to choose a more efficient algorithm or data structure , This effect is more satisfactory . then , This is not to say that you can misbehave in your code , Because bad code always makes things worse . If a program is too big , It's easy to think of where to start and make the program smaller ： The largest function and data structure . But if a program is too slow , Where should we start to improve its speed ？ The answer is to evaluate the performance of the program , Simply put, it is to measure the time spent in the execution of each part of the program . The part that takes the most time is obviously the goal of optimization . The most frequently used part of the program , If you can run faster , It will greatly improve the overall running speed of the program .
     You can find from this list 3 An interesting place .
    1. Among the most time-consuming functions , Some are library functions . In this case ,malloc and free Take the top two . We cannot modify their implementation , But when redesigning the program , If there is no or less dynamic memory allocation , The execution speed of the program can be improved at most 25%.
    2. Some functions take a lot of time , Because they are called many times . It's fast when called individually several times , But due to frequent calls , Therefore, there is a lot of time . Add a few wise register Declaring slightly improves the efficiency of the function , The overall performance of the program may be greatly improved .
    3. Some functions are not called many times , But each call takes a long time . Finding a faster algorithm for this function can at most improve the speed of the program .

    As a last resort , We can recode a single function with assembly statements , Smaller function , The easier it is to recode . This method may work well , Because in small functions ,C The fixed cost of function order and function postscript of takes a large proportion in the execution time . Recoding larger functions is much more difficult , Therefore, it is not very efficient to spend time in this place .
     Performance evaluation often doesn't tell you what you didn't know , But sometimes its results can be quite unexpected . The advantage of performance evaluation is that you will find out which part of the program you are spending time studying , It may bring maximum performance improvement .