/proc/stat interface and 32bit jiffies / kernel_stat
Hello,
my name is Mario Holbe.
Among others, I'm running a 2-CPU i386 system with a current uptime of 348 days.
Since around 250 days uptime, we recognize kernel statistics overflows, which result in serveral problems in HZ calculations in the procps utilities:
$ uptime Unknown HZ value! (28) Assume 0. 14:49:20 up 348 days, 33 min, ... $ cat /proc/stat cpu 259658341 5066163 336423925 1117681229 cpu0 129832988 2535697 168074057 2706455735 cpu1 129825353 2530466 168349868 2706192790
As you can see, the 'unused jiffies' counter in the cpu summary flows over, since 2706455735 + 2706192790 > 2^32.
Besides the general problem, that jiffies and kernel_stat components are 32bit on i386 platforms:
sched.h:extern unsigned long volatile jiffies;
kernel_stat.h:struct kernel_stat {
kernel_stat.h: unsigned int per_cpu_user[NR_CPUS],
kernel_stat.h: per_cpu_nice[NR_CPUS],
kernel_stat.h: per_cpu_system[NR_CPUS];... and therefor flow over after around 500 days, on SMP systems the summary calculations in the proc-fs:
proc_misc.c: unsigned int sum = 0, user = 0, nice = 0, system = 0;
... already flow over after around 500 / NR_CPUS days, this means, on a 2-CPU system after around 250 days, on a 4-CPU system after around 125 days and so on. [Yes, I know, it's some 2^(32 - round_up(log2(NR_CPUS))) calculation in real :)]
Resulting in - at least - permanent warnings in procps utilities.
Since it should not be a big problem to fix this, to at least reduce the problem back to the 500 days jiffies-overflow problem, I'd suggest to do so.
No need to mention, that 64bit jiffies and statistics on all platforms at all would be great :)
Yes, I know the performance implications resulting in this for 32bit platforms.
Btw... Could anybody please explain me the problems to expect while a jiffies overflow? Would a kernel possibly survive this at all and if, what's the chance to? :)
PS: Please CC: me in replies, because I'm not on the list.
regards,
Mario
Signature · 1 line
We are the Bore. Resistance is futile. You will be bored.
Quoted 3 lines · Mario 'BitKoenig' Holbe
Btw... Could anybody please explain me the problems to expect while a jiffies overflow? Would a kernel possibly survive this at all and if, what's the chance to? :)
The kernel uses time_before/time_after functions which know about the wrapping of time so it should all go ok
On Fri, 15 Nov 2002, Mario 'BitKoenig' Holbe wrote:
[system idle time will overflow after about 497/NR_CPUS days]
Quoted 7 lines
Since it should not be a big problem to fix this, to at least reduce the problem back to the 500 days jiffies-overflow problem, I'd suggest to do so.
No need to mention, that 64bit jiffies and statistics on all platforms at all would be great :)
2.5 has 64 bit jiffies (but not (yet?) 64 bit statistics).
A patch for 2.4 that fixes the overflow in proc_stat as well as introducing 63 bit jiffies is at
http://www.physik3.uni-rostock.de/tim/kernel/2.4/jiffies64-20.patch.gz
Quoted 4 lines
Btw... Could anybody please explain me the problems to expect while a jiffies overflow? Would a kernel possibly survive this at all and if, what's the chance to? :)
"ps" will report processes started before the jiffies wrap as being started in the future, but this won't do any harm.
Tim