[Xen-devel] How does xentop.c code gets previous sample to calculate CPU utilization ?

[Mahendra Kutare <mahendra.kutare@xxxxxxxxx>]

Hi,

I am creating my sample code which takes most of the cpu utilization code from xentop.c under tools/xenstat/ for cpu and memory utilization calculation.

I see somehow the cpu utilization always shows zero because it always sees prev_node as zero of the following -

475 /* Computes the CPU percentage used for a specified domain */
 476 static double get_cpu_pct(xenstat_domain *domain)
 477 {                                     
 478     xenstat_domain *old_domain;       

 479     double us_elapsed;                
 480                                       
 481     /* Can't calculate CPU percentage without a previous sample. */
 482     if(prev_node == NULL)             

 483         return 0.0;                   
 484                                       
 485     old_domain = xenstat_node_domain(prev_node, xenstat_domain_id(domain));                                                                              

 486     if(old_domain == NULL)            
 487         return 0.0;                   
 488                                       
 489     /* Calculate the time elapsed in microseconds */
 490     us_elapsed = ((curtime.tv_sec-oldtime.tv_sec)*1000000.0

 491               +(curtime.tv_usec - oldtime.tv_usec));
 492                                       
 493     /* In the following, nanoseconds must be multiplied by 1000.0 to
 494      * convert to microseconds, then divided by 100.0 to get a percentage,

 495      * resulting in a multiplication by 10.0 */
 496     return ((xenstat_domain_cpu_ns(domain)
 497          -xenstat_domain_cpu_ns(old_domain))/10.0)/us_elapsed;
 498 }                                     

I have looked around in tools/xenstats/xentop.c code and have used the same functions with same code. But somehow I am missing something here. It is clearly mentioned in above code on line 481 that it requires previous samples and somehow in my code I am not supplying that and xentop is doing it. 

I am trying to figure out how does xentop.c provides the previous sample for CPU utilization...so that I can do the same in my code but no luck so far after I have looked and used the same code.

Any pointers would be useful ?

Thanks
Mahendra


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