Newbie & LPAR Post: 302493607

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Operating Systems AIX Newbie & LPAR Post 302493607 by zaxxon on Thursday 3rd of February 2011 10:38:22 AM

02-03-2011

Registered User

I had a similar question about the monitoring of CPU usage on logical CPUs. I was trying to sort out some nmon reports and came to no conclusion. So I asked Nigel Griffiths (coder of nmon) in the IBM developerWorks forum about it:

https://www.ibm.com/developerworks/f...557461�

Quote:

...
I agree the logical CPU stats are largely meaningless for shared CPU partitions because the logical CPUs don't really exist as we think of them (they are sharing in unpredictable ways the underlying physical CPUs and certainly don't have fixed percentages of CPU time like 50:50) and especially when uncapped as its reported as 100% busy when its really say 5 times the entitlement which you could call 500% !!

Pay attention to the Physical CPU use (also called CPU used).
...

---------- Post updated at 04:38 PM ---------- Previous update was at 02:17 PM ----------

In addition to my previous somewhat general post:

Your assumption sounds correct to me, if the process is single threaded. If it is multithreaded and SMT is enabled, it should be dispatched by different logical CPUs and so take advantage being able to run multiple threads on multiple logical CPUs.
You can check dispatching with mpstat -s 1 for example.
There is also a "Virtualization best practices" Redbook - maybe this has information about it.

A single threaded process will most probably not take any advantage of SMT or having more than 1 virtual CPU available, if it is running mainly alone on the system. It can only be handled by one virtual CPU at a time. Though a second virtual CPU can meanwhile handle other operating system tasks etc. In that case turn of SMT and monitor it's performance.

For tests you could try out nstress.

You might want to check out what is best for your environment/application.
To check out if your application is multithreaded, check it out with svmon -P| grep -p Pid. There will be a Y in the column for Mthrd.

In our environments we have SMT turned on, because on every LPAR there is usually running single and multithreaded processes and we had no need to turn SMT yet off.

Last edited by zaxxon; 02-03-2011 at 11:42 AM.. Reason: changed "virtual" to "logical"

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LEARN ABOUT CENTOS

chcpu

CHCPU(8)							   User Manuals 							  CHCPU(8)

NAME

       chcpu - configure CPUs

SYNOPSIS

       chcpu [-e] [-d] [-c] [-g] cpu-list
       chcpu [-p] mode
       chcpu -r
       chcpu [-h] [-V]

DESCRIPTION

       chcpu  can  modify  the state of CPUs.  It can enable or disable CPUs, scan for new CPUs, change the CPU dispatching mode of the underlying
       hypervisor, and request CPUs from the hypervisor (configure) or return CPUs to the hypervisor (deconfigure).

       Some options have a cpu-list argument.  Use this argument to specify a comma-separated list of CPUs.  The list can contain  individual  CPU
       addresses  or ranges of addresses.  For example, 0,5,7,9-11 makes the command applicable to the CPUs with the addresses 0, 5, 7, 9, 10, and
       11.

OPTIONS

       -r, --rescan
	      Trigger a rescan of CPUs.  Use this option on systems that do not automatically detect newly attached CPUs.  The Linux  kernel  then
	      recognizes the new CPUs.

       -c, --configure cpu-list
	      Configure  all specified CPUs.  Configuring a CPU means that the hypervisor takes a CPU from the CPU pool and assigns it to the vir-
	      tual hardware on which your kernel runs.

       -e, --enable cpu-list
	      Enable all specified CPUs.  Enabling a CPU means that the kernel sets it online.	A CPU must be configured, see -c, before it can be
	      enabled.

       -p, --dispatch mode
	      Set  the	CPU  dispatching mode (polarization).  This option has an effect only if your hardware architecture and hypervisor support
	      CPU polarization.  Available modes are:

		     horizontal  The workload is spread across all available CPUs.
		     vertical	 The workload is concentrated on few CPUs.

       -d, --disable cpu-list
	      Disable all specified CPUs.  Disabling a CPU means that the kernel sets it offline.

       -g, --deconfigure cpu-list
	      Deconfigure all specified CPUs.  Deconfiguring a CPU means that the hypervisor removes the CPU from the virtual  hardware  on  which
	      the Linux instance runs and returns it to the CPU pool.  A CPU must be offline, see -d, before it can be deconfigured.

       -h, --help
	      Display help information and exit.

       -V, --version
	      Display version information and exit.

AUTHOR

       Heiko Carstens <heiko.carstens@de.ibm.com>

COPYRIGHT

       Copyright IBM Corp. 2011

SEE ALSO

       lscpu(1)

AVAILABILITY

       The  chcpu command is part of the util-linux package and is available from Linux Kernel Archive <ftp://ftp.kernel.org/pub/linux/utils/util-
       linux/>.

Linux								     June 2012								  CHCPU(8)