1

Question: What is the maximum value of kernel statistic counters and how can I handle it in python code?

Context: I calculate some statistics based on kernel statistics (e.g. /proc/partitions - it'll be customized python iostat version). But I have a problem with overflowed values - negative values. Original iostat code https://github.com/sysstat/sysstat/blob/master/iostat.c comments:

* Counters overflows are possible, but don't need to be handled in
* a special way: The difference is still properly calculated if the
* result is of the same type as the two values.

My language is python and I need to care about overflow in my case. Probably it depends also on architecture (32/64). I've tried 2^64-1 (64bit system), but no success.

NPE
  • 486,780
  • 108
  • 951
  • 1,012
Jan Garaj
  • 25,598
  • 3
  • 38
  • 59

1 Answers1

1

The following function will work for 32-bit counters:

def calc_32bit_diff(old, new):
  return (new - old + 0x100000000) % 0x100000000

print calc_32bit_diff(1, 42)
print calc_32bit_diff(2147483647, -2147483648)
print calc_32bit_diff(-2147483648, 2147483647)

This obviously won't work is the counter wraps around more than once between two consecutive reads (but then no other method would work either, since the information has been irrecoverably lost).

Writing a 64-bit version of this is left as an exercise for the reader. :)

NPE
  • 486,780
  • 108
  • 951
  • 1,012