topas - computational memory 95% : Any Impact? Post: 302492877

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Operating Systems AIX topas - computational memory 95% : Any Impact? Post 302492877 by zxmaus on Tuesday 1st of February 2011 07:39:59 PM

02-01-2011

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We do not know which AIX version you are running, we do not know your vmo settings, we do not know what is running on your server, so its a little hard to tell what may or may not cause your computational memory utilization and if it is a leak or normal growth. Since AIX has a dynamic kernel, the memory utilization grows over time which is normal, and many workloads are preallocating memory - Websphere, Oracle, Sybase, DB2, SAS just to name a few.

Depending on your AIX version and tunable settings you may start paging out computational content - which usually slows down your corresponding application as AIX is paging in that case entire processes to disk - it stops the process, moves it - and than continues using it.

When you are that high in comp memory, that means you have literally no room in memory for non-comp (file caching) what is as well vital for many applications. If they are requesting memory, than the system starts thrashing (excessive scanning / freeing memory) what may cause a huge amount of cpu overhead and slowdown of your box - this can go that far that no other processing happens on the corresponding server.

Since you are already starting to page (2% paging space used) you are at risk that when you filling your paging space, your system might at first randomly kill processes to survive and later either hang with no login possible or crash and restart itself to free up memory.

What you need to do is monitor your system very closely - like if the amount of comp memory in use is growing and if your paging space utilization is growing.

IBM recommends to have comp memory around 75% (and from a performance perspective I can only confirm this - my boxes - DB servers - perform best when they are somewhere between 66 and 80%) - depnding on the criticality of your applications, buying a few gig of memory may be cheaper than risking a crash.

Hope that helps
kind regards
zxmaus

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LEARN ABOUT MINIX

chmem

CHMEM(1)						      General Commands Manual							  CHMEM(1)

NAME

       chmem - change memory allocation

SYNOPSIS

       chmem [+] [-] [=] amount file

EXAMPLES

       chmem =50000 a.out  # Give a.out 50K of stack space

       chmem -4000 a.out   # Reduce the stack space by 4000 bytes

       chmem +1000 file1   # Increase each stack by 1000 bytes

DESCRIPTION

       When  a	program is loaded into memory, it is allocated enough memory for the text and data+bss segments, plus an area for the stack.  Data
       segment growth using malloc , brk , or sbrk eats up stack space from the low end.  The amount of stack space to allocate is derived from  a
       field  in  the executable program's file header.  If the combined stack and data segment growth exceeds the stack space allocated, the pro-
       gram will be terminated.

       It is therefore important to set the amount of stack space carefully.  If too little is provided, the program may crash.  If  too  much	is
       provided,  memory  will	be  wasted, and fewer programs will be able to fit in memory and run simultaneously.  MINIX does not swap, so that
       when memory is full, subsequent attempts to fork will fail.  The compiler sets the stack space to the largest possible value (for the Intel
       CPUs, 64K - text - data).  For many programs, this value is far too large.  Nonrecursive programs that do not call brk , sbrk , or malloc ,
       and do not have any local arrays usually do not need more than 8K of stack space.

       The chmem command changes the value of the header field that determines the stack allocation, and thus indirectly the total memory required
       to run the program.  The = option sets the stack size to a specific value; the + and - options increment and decrement the current value by
       the indicated amount.  The old and new stack sizes are printed.

SEE ALSO

       install(1), brk(2).

																	  CHMEM(1)