DIFFERENCES IN MEMORY UTILIZATION REPORTED BY
ITEM: RTA000034247
QUESTION:
The environment is AIX 3.2.3 E.
Please help me understand the differences between the way 'svmon -G'
and 'ps aux' reports memory utilization. Also, some explanation on
how to read and understand what is reported by 'svmon -Pa pid#' would
be greatly appreciated. We are trying to understand why a numerically
intensive application shows hardly any memory utilization with 'ps aus'
but 'svmon -G' shows almost all the real memory inuse.
What are we really looking at?
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A: In the output of the 'ps aux' command, the percent MEM column is
the percentage of real memory currently used by the process.
Specifically, MEM is the total size of the memory resident
portions of the code and data segments of the process, divided by the
size of real memory and multiplied by 100. Because multiple processes
can share segments, the sum of MEM values over all processes
may exceed 100.
The "inuse" column of the svmon -G report is a little confusing. The
reason that the inuse column shows almost all the real memory in use, is
that the Virtual Memory Manager will keep pages in memory that have
been accessed recently. Thus, frequently accessed pages will tend to
stay in memory longer, and logical file accesses to the corresponding
blocks can be satisfied without physical disk access. Then the numbers
in the inuse column may not represent how much memory is in use at that
specific time, but how much memory contains pages. If you wish to
determine how much memory is in use for a given workload, you can
simulate a small memory size using the rmss -c command. Then the pages
in memory that are not presently in use, but have been recently accessed
so are still in memory, will be forced to the free list. Then you can
reset the memory size to the physical size using rmss, and run a normal
workload to see how much memory the system will typically use. For
more information on this, please reference item CHQRH in ASKQ.
The output of the svmon -Pa pid command is interpreted as follows:
segid: The segment id of the process
Type: Either works, pers or clnt, meaning the segment is a
working, persistent or client segment.
Description: A description of the segment.
Inuse: The total number of pages in real memory from segments
that are used by the process.
NOTE: A segment may be used by multiple processes. Each page in
Real memory from such a segment is accounted for in the
Inuse field for each process that uses the segment. Thus,
the total of the Inuse fields over all active processes
may exceed the total number of pages in real memory.
Pin: Total number of pages pinned from segments that are used
by the process.
NOTE: A segment may be used by multiple processes. Each page
pinned from such a segment is accounted for in the Pin
field for each process that uses the segment. Thus, the
total of the Pin fields over all active processes may
exceed the total number of pages pinned.
Pgspace: Total number of pages on paging space by segments that
are used by the process.
NOTE: A segment may be used by multiple processes. Each page on
paging space from such a segment is accounted for in the
Pgspace field for each process that uses the segment. Thus,
the total of the Pgspace fields over all active processes
may exceed the total number of pages on paging space.
Address
Range: The address range of the segment.
There is a hardcopy documentation entitled "Performance Monitoring
and Tuning Guide" (SC23-2365-01) that you may wish to receive. The
above information was taken from this guide and it is very helpful
in understanding performance issues and using the performance tools.
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QUESTION:
What is the difference between MEM and RSS as reported by the 'ps aux' comman
We are running large NASTRAN jobs, and are surprised by the MEM being
so small, leading us to suspect that the NASTRAN jobs are paging more
than normal. Any guidance here?
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A: The RSS output reported from the ps aux command is the resident
set size of the process. Specifically, RSS is the total size of the
memory resident portions of the code and data segments of the process
expressed in KB. Because processes can share segments, the sum of RSS
over all processes may exceed the size of real memory.
The difference between RSS and percent MEM is that percent MEM reports
the percentage of the memory used by the process, and RSS is the number
of kilobytes of memory the process uses.
If you are worried about paging, I would use the vmstat command to
determine if the NASTRAN jobs are indeed paging. You will not be able
to determine the extent of paging through the output of the ps command.
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QUESTION:
We are still trying to understand why when NASTRAN is running, the
entries in the process table (as reported by ps aux) show virtually
nothing for both MEM and RSS, where other large numerically intensive
jobs such as DYNA3D, ABAQUS, etc., show significant resource usage
in the MEM and RSS column. Is 'ps' getting fooled? How do I
accurately determine how much real memory (RAM, not virtual) an
application is using during run time? We have a 256 MB RAM Model 580.
NASTRAN gives us the capability to allocate real memory to a job.
We are specifying 50,000,000 words x 4 MB/word (RS/6K arch) = 200 MB
real memory. There are no other applications running that would be
competing for memory, but we cannot confirm that we are using that
memory, but instead it looks like we are paging? This just doesn't
make sense. How do I get to the bottom of this?
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A: The command you need to view memory segments is svmon.
There are several forms:
svmon -P with show memory segments for a process.
svmon -Pau will list memory segments for the 10
biggest memory users.
svmon -Pag will list memory segments for the 10
biggest paging space users.
I left you a phone mail message. I'd rather continue this
by phone.
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This item was created from library item Q640605 CKNRV
Additional search words:
AUX AUXILIARY CKNRV DEC94 DIFFERENCES IX MAIN MEASURE MEMORY OP
OZIBM OZNEW PERFORMANCE PS REPORTED RISCPERF RISCSYSTEM SOFTWARE
STORAGE SVMON SYS TUNE UTILIZATION VS
WWQA: ITEM: RTA000034247 ITEM: RTA000034247
Dated: 03/1996 Category: RISCPERF
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