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Re: High Page Fault

 
NDurand
Occasional Advisor

High Page Fault

Hi,
We actually encounter a problem on rp4460 with hp ux 11.23. The problem is the huge number of page fault wich, i think, is the cause of high cpu usage.

I'm not sure the memory is really under pressure as free mem is around 4GB/32GB.

I've noticed that the problem increases with the number of processes, but i don't know why.

Thanks for your help
MEMORY REPORT Users= 10
Event Current Cumulative Current Rate Cum Rate High Rate
--------------------------------------------------------------------------------
Page Faults 4988k 1010559029 959273.0 620393.5 1177835.9
Page In 47 1686 9.0 1.0 1291.3
Page Out 0 0 0.0 0.0 0.0
KB Paged In 188kb 6.6mb 36.1 4.1 465.3
KB Paged Out 0kb 0kb 0.0 0.0 0.0
Reactivations 0 0 0.0 0.0 0.0
Deactivations 0 0 0.0 0.0 0.0
KB Deactivated 0kb 0kb 0.0 0.0 0.0
VM Reads 1 201 0.1 0.1 24.0
VM Writes 0 636 0.0 0.3 10.0
19 REPLIES 19
Venkatesh BL
Honored Contributor

Re: High Page Fault

Is this causing a problem?

When new processes are run, the pages the process touches, have to be faulted on demand. This would be indicated as Page In.
NDurand
Occasional Advisor

Re: High Page Fault

Thanks for your reply,

Yes it is a problem as the cpu is always 100% used and 90% by system .

Rgds

Re: High Page Fault

Not necessarily a problem... do you fully understand what page faults are?

It would be interesting to see if you also have high values in the at (address translation) column in a vmstat output.

If I run  a new process for the first time on a system, the text pages (program code) of that process will not be in physcial memory so this will generate a page fault, and then (as the process has not been run before) a page-in from physical disk (i.e. loading the program code into physical memory).

When the process terminates, we don't need to write out for the static program code, so we don't get a page out from that and the pages will stay in physical memory (although not referenced by any processes) unless there is memory pressure.

Now if I run the same process again - the kernel virtual memory subsystem will know that those pages are already in physical memory and instead of having to do another page in, it can just do an address translation (which you can see from vmstat)

The upshot of all this is that you have a lot of processes that run and then terminate very often you can see very high page faults without a high number of page-ins, as these are just address translations.

There may be other reasons too (I'm not sure if memory mapped files play in this space as well), but we'll need a VM wizard such as Don Morris to answer that sort of question (and probably correct a few errors in my explanation above as well!)

HTH

Duncan


I am an HPE Employee
Accept or Kudo
Dennis Handly
Acclaimed Contributor

Re: High Page Fault

>the huge number of page fault which, I think, is the cause of high CPU usage.

As mentioned by Venkatesh, there are no page outs. Typically lots of paging wouldn't cause high CPU usage but lots of I/O.
NDurand
Occasional Advisor

Re: High Page Fault

Thank you Denis and Duncan for your clear explanation.



Here's an output of vmstat, as you can see cpu is really occuped:
procs memory page faults cpu
r b w avm free re at pi po fr de sr in sy cs us sy id
12 2 0 7676486 816524 220 29 3 0 0 0 4 4028 261890 3660 12 74 15
12 2 0 7676486 816447 176 29 0 0 0 0 0 3104 85448 3531 7 93 0
12 2 0 7676486 816510 152 25 0 0 0 0 0 3084 71618 3488 8 92 0
16 2 0 7680128 815360 163 27 0 0 0 0 0 3032 69571 3348 8 92 0
16 2 0 7680128 815592 344 61 0 0 0 0 0 3244 89772 3689 12 88 0
NDurand
Occasional Advisor

Re: High Page Fault

here an output of sar:
HP-UX su0374 B.11.23 U ia64 09/15/09

12:13:42 %usr %sys %wio %idle
12:13:45 11 89 0 0
12:13:48 7 93 0 0
12:13:51 10 90 0 0
12:13:54 8 92 0 0

Can you confirm high %sys is a normal beahviour ?

Thanks in advance

Re: High Page Fault

Well you certainly don't seem to have a lot og page-ins going on there, but then neither do you appear to have a lot of address translations...

A %sys of 90% is not particularly normal - what application is running on this host? Can you identify waht processes are using all the system CPU time? (from your original output it looks like you have glance)

HTH

Duncan

I am an HPE Employee
Accept or Kudo
Venkatesh BL
Honored Contributor

Re: High Page Fault

Use top or glance to find the cpu usage of the processes.
Hakki Aydin Ucar
Honored Contributor

Re: High Page Fault

most of CPU is in use by system process(es)
it is slightly normal %90 percent CPU ( my customer server sometime > %90 CPU but not all day long!)
I wanted to state, if this situation happens temporarily not prolonged it is not too bad. But if this happens due to bottlenecks OR got stuck processes then need to be solved.

you can check it lsof OR fuser tools..