Questions RE: /proc/stat

[Andrew Miehs]

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Dear list,

I had some questions regarding using /proc/stat (on the 2.6 kernel).

Can I assume that reading /proc/stat is 'atomic'? Can I assume that  
when I read
the file, that nothing will change the data while I am reading it?
IE:
  cpu  15948315 4687107 3321012 122412279 659908 43304 144288
  cpu0 15948315 4687107 3321012 122412279 659908 43304 144288

can I assume that while I am reading the third value (system), that  
the fourth value (idle)
will not be changed underneath me? Can I assume that all the lines?  
cpu, cpu0 will all be
generated at once?


My second question is regarding using these values to calculate usage...

Should I calculate usage by
(where T1, T2 are time)

A) read /proc/stat
   Fill variables UserT1, NiceT1, SystemT1, IdleT1, IOWaitT1, irqT1,  
irq2T1
   wait a time
   Fill variables UserT2, NiceT2, SystemT2, IdleT2, IOWaitT2, irqT2,  
irq2T2
   DeltaTotalTime=(UserT2-UserT1)+(NiceT2-NiceT1)+(SystemT2-SystemT1) 
+(.........)

   Then calculate the values I want as the delta  
variableOfInterest / total delta
   ie: (UserT2-UserT1)/DeltaTotalTime

   Can I assume that all these values added together should add up  
to 100%?


or


B) read /proc/stat
   read variable of interest, ie: UserT1 and btimeT1 and number of CPUs
   wait a time
   read variable of interest, ie: UserT2 and btimeT2 and number of CPUs

   Then calculate the values I want as
     ((UserT2-UserT1)/NumCPU)/(btimeT2-btimeT1)*100 (Jiffies)



Thanks for any comments

Regards

Andrew



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