Monitoring Paging and Swapping


 
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Operating Systems Solaris Monitoring Paging and Swapping
# 29  
Old 12-22-2018
Quote:
Originally Posted by bakunin
No. What i meant was: a process gets (though whatever means, mmap(), shmget(), the OS or something else) space in the swap. This space will show up in swap -l as "used".
bakunin
Hi Bakunin,

Sorry for the late reply and thank you for your response. I think the above is not the behavior for Solaris as virtual memory reservation does not cause actual physical swap nor memory to be "used" . Hence it will not be shown in swap -l at all.

Quoted from Oracle metalink ->
p.s. They call virtual memory = virtual swap

Quote:
Virtual swap = Physical Memory + Fixed Disk swap

When a process calls the malloc()/sbrk() commands, only virtual swap is allocated. The operating system allocates the memory from physical disk-based swap first. If disk-based swap is exhausted or unconfigured, the reservation is allocated from physical memory. If both resources are exhausted then the malloc() call fails
ISM pages are locked by the kernel upon return from the shmat() system call. Swap reservation of ISM memory is allocated from physical memory, not physical disk-based swap.
When a process calls shmget(), it results in a reservation against physical disk-based swap. However, when a process calls shmat() next, swap deduction against physical disk is reversed and deduction is then performed against physical memory

As discussed, when measuring virtual swap available for reservation, consider monitoring vmstat ("swap" column) or swap -s (the "available" value). Free memory as reported by vmstat ("free" column) or swap usage as reported by swap -l ("free" column) is unrelated with virtual swap available for reservation. When allocating swap reservation from memory, there is no memory deducted and vmstat continues to show same amount of "free" memory. Similarly, when a swap reservation is made from physical disk-based swap, swap -l will continue to show the same amount of "free" swap.
I am not sure if the above is only applicable to Solaris though..

Regards
Noob

--- Post updated at 11:48 AM ---

Quote:
Originally Posted by jlliagre
Yes. The issue is the word "swap" has several meanings. When you say "swap allocated", you talk about virtual memory but when you say "swap used", that might still be virtual memory (swap -s) or on disk swap (swap -l).
Not that much. Generally, most of the paging activity won't be related to the on-disk swap area but just regular files read/write operations. Therefore, hi page-in/page-out counters do not imply there is a shortage in RAM, just some active disk I/Os. As I wrote, the relevant statistic about RAM shortage is the scan rate (sr).

Not an easy task, and depending on the method used, reading what is there would have the side effect to page it in, i.e. to put it back into RAM.
You can have detailed information about a process memory usage with the pmap -xs command.
The blocks related to swap are tagged "[anon]" for anonymous. That means the pages have no file back-end. The RSS column tells the number of pages that are currently in RAM while the KBytes column gives the size of the block.
Knowing these addresses, you can use the kernel modular debugger (mdb -k)to retrieve the mapping between virtual addresses returned by pmap and the corresponding physical addresses.

Yes, and I have already observed such puzzling at first sight situations in real life systems.

Hi Jlliagre,

Sorry for the late reply and thank you for the insight.

Regards,
Noob
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