Numerical Methods Contents - SAM

• Windows: for the exercises it is recommend to use a linux emulator like cygwin or VirtualBox:
http://www.virtualbox.org/ http://www.cygwin.com/
Time Measurement:
In order to compare the efficiency of different implementations we need to be able to measure the
time spent on a computation. The following definitions are commonly used in this context:
• the wall time or real time denotes the time an observer would measure between the program
start and end. (c.f. wall clock)
• the user time denotes the cpu-time spent in executing the user’s code.
• the system time denotes the cpu-time that the system spent on behalf of the user’s code (e.g.
memory allocation, i/o handling etc.)
Unix-based systems provide the time command for measuring the time of a whole runnable, e.g.: time
./runnable. For the measurement of the runtimes in c++, the clock()-command provided in the time.h
can be used. These methods will not provide correct results for the time-measurement of parallelized
code, where the routines from the parallelization framework should be used. (e.g. MPI_WTIME for
MPI-programs)
22 void r e s e t ( )
23 {
24 time =0;
25 bStarted = false ;
26 }
27 private :
28 double time ;
29 bool bStarted ;
30 } ;
1 /∗
Code 1.4.12: Measuring real, user and system time from C++ on unix based systems
2 ∗ unixTimer Class using times ( )−command from the unixbased times . h
3 ∗ t h i s class w i l l r e p o r t the user , system and r e a l time .
4 ∗/
5 #include
6 #include
7 #include
8 class u n i x t i m e r {
9 public :
10 u n i x t i m e r ( ) : utime ( 0 ) , stime ( 0 ) , r t i m e ( 0 ) , bStarted ( false )
11 { }
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12
Code 1.4.11: Measuring CPU time from C++ using clock command
1 #include
2 #include / / header for clock()
3
4 /∗
5 ∗ simple Timer Class , using clock ( )−command from the time . h ( should work
on a l l p l a t f o r m s )
6 ∗ t h i s class w i l l only r e p o r t the cputime ( not w a l l t i m e )
7 ∗/
8 class simpleTimer {
9 public :
10 simpleTimer ( ) : time ( 0 ) , bStarted ( false )
11 { }
12 void s t a r t ( )
13 {
14 time= clock ( ) ;
15 bStarted =true ;
16 }
17 double getTime ( )
18 {
19 a ssert ( bStarted ) ;
20 return ( clock ( )−time ) / ( double )CLOCKS_PER_SEC; ;
13 void s t a r t ( ) { r t 0 =times (& t0 ) ; bStarted =true ; }
14
15 double stop ( ) {
16 tms t1 ;
17 long r t 1 ;
18 a ssert ( bStarted ) ;
19 r t 1 =times (& t1 ) ;
20 utime = ( ( double ) ( t1 . tms_utime−t0 . tms_utime ) ) /
CLOCKS_PER_SEC∗10000;
21 stime = ( ( double ) ( t1 . tms_stime−t0 . tms_stime ) ) /
CLOCKS_PER_SEC∗10000;
22 r t i m e = ( ( double ) ( r t1−r t 0 ) ) / CLOCKS_PER_SEC∗10000;
23 bStarted = false ;
24 return r t i m e ;
25 }
26
27 double user ( ) { a ssert ( ! bStarted ) ; return utime ; }
28 double system ( ) { a ssert ( ! bStarted ) ; return stime ; }
29 double r e a l ( ) { a ssert ( ! bStarted ) ; return r t i m e ; }
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21 } 33 tms t0 ;
30
31 private :
32 double utime , stime , r t i m e ;