How much more physical memory do I need? 1

[reclspeak]

I have a P660 with 16GB installed that swaps for an overnight batch process for 4 hours. Performance drops to a crawl.

nmon reports that at its peak, when physical memory is exhausted, "fsin" is occurring at 3470 (pages) per second. A page is 4096 bytes, so I am presuming the swap-in rate is 14MB per second.

But how do I figure how much swap is being employed, with a view to calculating how much physical RAM is required to upgrade the server with? I can see that the free amount of virtual paging space drops by just over 300MB compared to the normal (pre-allocated) amount shown but I'm not sure that is an useful way of determining how much more RAM is needed.

Obviously its a ball-of-string problem - give the server more physcial memory and processes will employ it - bt wat I am trying to determine is how much paging space is being called-upon to satisfy the memory shortfall.

Any pointers or strategy would be appreciated.


recl

 

[RMGBELGIUM]

what you could do is schedule a cronjob that runs every 5 minutes during your 4 hour batch, with the command "lsps -as" and send output to a logfile, and see how paging is evolving during the run of that batch job.Then you at least know what is happening at what time ( if paging is consumed slowly, or if there is a sudden major increase.)

rgds,

R.

 

[khalidaaa]

You can as well monitor the memory usage by using the vmstat avm column which shows how much virtual memory used for that period of time (virtual memory = actual memory (RAM installed) + paging)

So by identifying the virtual memory usage, you would be able to know how much physical memory to buy for your server!

Also try looking into memory tuning over here:

http://www.redbooks.ibm.com/abstracts/sg246039.html?Open

Regards,
Khalid

 

[RodKnowlton]

reclspeak, fsin is normal file system reads. AIX uses the same paging mechanism for regular file i/o and virtual memory.

pgsin and pgsout are the real paging numbers, in the virtual memory sense. Are they non-zero for significant amounts of time?

Like Khalid said, the avm column of vmstat is the key. Find its highest value over a typical work load, and if it's less than your physical memory (avm is reported in pages, so multiply it by 4096), you need to tune the VMM rather than add memory.

- Rod


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