This chapter provides an overview of systems performance when running Linux on Power (LoP).
AIX administrators will be happy to know that the nmoncommand works great with Linux. SystemTap, which conducts performanceanalysis by analyzing a running kernel, also runs on the platform. Twoother popular tools, iostat and sar, are also available on Linux systems. While tools aren't the focus here, it's nice to know these options are available.
For our first monitoring example, let's inspect some basic Linux configuration files. The /procfile system is one you should use frequently, much more so than on AIXboxes because the information is simply more valuable in Linux. A lotof kernel and process information resides here, in the form ofconfiguration files. One such file is cpuinfo:
[root@172_29_140_173 proc]# more cpuinfo
processor : 0cpu : POWER5 (gr)clock : 1654.344000MHzrevision : 2.1 (pvr 003a 02001)
It's easy to see from this file that you're using a POWER5 system.
Next, we can look at the release text file to determine the operating system level:
[root@172_29_140_173 etc]# more redhat-release
Red Hat Enterprise Linux Server release 5.2 (Tikanga)
One of my favorite Linux commands is top.This command provides realtime information quickly, in acharacter-based display, including the processes that are consuming themost CPU time.
Another useful command is free. It reports the total amount of free and used physical and swap memory:
[root@172_29_140_173 etc]# free
total used free shared buffers cached
Mem: 2073856 2057536 16320 0 440832 1389760
-/+ buffers/cache: 226944 1846912
Swap: 0 0 0
Still, my favorite of all Unix/Linux performance commands is vmstat. I love this old standby because, unlike other tools, vmstat provides quick-and-dirty information about all subsystems. Nothing fancy here:
[root@172_29_140_173 etc]# vmstat 1
procs -----------memory---------- --swap-- ---io--- --system-- -----cpu------
r b swpd free buff cache si so bi bo in cs us sy id wa st
0 0 16256 440832 1389760 0 0 1 1 23 11 0 1 98 0 0 0
0 0 16384 440832 1389760 0 0 0 0 524 30 0 1 99 0 0 0
0 0 16384 440832 1389760 0 0 0 0 536 16 0 1 99 0 0 0
0 0 16320 440832 1389760 0 0 0 0 588 25 0 1 99 0 0 0
0 0 16320 440832 1389760 0 0 0 0 628 12 0 1 99 0 0 0
0 0 16320 440832 1389760 0 0 0 24 633 29 0 1 99 0 0 0
0 0 16320 440832 1389760 0 0 0 0 578 18 0 1 99 0 0 0
You will find that vmstat output on Linux differs a bit from what you see on AIX systems. Here's a quick description of what each field means:
free — Amount of idle memory
buff — Amount of memory used as buffers
cache — Amount of memory used as cache
Someadministrators don't take full advantage of the PowerVM capabilities ofLinux. If you administer LoP the same way you do on x86 boxes, you'redoing yourself and your organization a major disservice. Somecapabilities available on POWER systems include the following.
Froma CPU perspective, SMT is an important feature. It lets you maximizethe use of instruction sets and in some cases increase CPU performanceby 30 percent. SMT enables these improvements by supportingmultithreading, a capability that's part of PowerVM and the POWERarchitecture. Multithreading enables two separate instruction streamsto run concurrently on the same physical processor, with each threadappearing to run on its own independent logical processor. This featureis enabled by default.
Throughthe POWER architecture, you can create Linux and AIX partitions. Whencreating Linux partitions, you can "uncap" them, which means that thepartitions will receive unused CPU cycles from the shared processorpool over and above their entitled capacity. Other than the number ofcycles left in that shared processor pool, the only limitation is thenumber of virtual processors configured for the profile. I recommenduncapping partitions whenever possible to maximize all available CPUresources and increase performance. From a CPU perspective, I'd alsotake advantage of the capability to add CPU horsepower on the flythrough a dynamic LPAR (DLPAR) operation.
In Linux, the sysctlcommand changes kernel parameters. Be advised that the method you useto change parameters may depend on your distribution; for example, youcan use the Powertweak tool with Novell SUSE Linux, but it isn'tavailable with Red Hat. Because we're using Red Hat here, sysctl is the choice. Let's change some parameters.
One parameter that's changed frequently is SHMMAX,which is used to define the maximum size (in bytes) for a shared memorysegment. In Oracle, you should set this value large enough for thelargest System Global Area (SGA) size. Let's examine the defaultparameter:
# sysctl kernel.shmmax
kernel.shmmax = 268435456
In this case, the limit is set to 256 MB. Let's change this to 1 GB. To do so, use the vi command to display the /etc/sysctl.conf file. This is where you edit the value:
# Controls the maximum shared segment size, in bytes
kernel.shmmax = 107374182
When you view the file again using sysctl, you can see the change:
# sysctl kernel.shmmax
kernel.shmmax = 107374182
To make the parameter work without a reboot, issue the sysctl command with the -p parameter.
On the memory side, parameters worth examining include SEMMSL, which controls the maximum number of semaphores per semaphore set; SEMMNI, which controls the maximum number of semaphore sets on the entire Linux system; and SEMMNS, which controls the maximum number of semaphores (no semaphore sets) on the entire Linux system.
Another important parameter is vm.nr.hugepages. The background here is that the POWER architecture supports page sizes of 4 KB and 16 MB. The default vm.nr.hugepages setting for LoP is 4 KB, which is too small for larger databases. To enable large pages, you need to change this parameter.
Let's first view the hugepage parameter — in this case, by looking at the proc/meminfo file.
# grep -i hugepages /proc/meminfo
HugePages_Total: 0
HugePages_Free: 0
HugePages_Rsvd: 0
Hugepagesize: 16384 kB
Now, let's allocate 130 large pages to support an SGA of approximately 2 GB:
# sysctl -w vm.nr_hugepages=130
Then view the meminfo file again:
[root@172_29_140_173 ~]# grep -i hugepages /proc/meminfo
HugePages_Total: 41
HugePages_Free: 41
HugePages_Rsvd: 0
Hugepagesize: 16384 kB
You can see that the system is already starting to use the hugepage parameter.
Entirebooks are dedicated to Linux performance tuning. Check your applicationrecommendations to learn how kernel parameters should be configured inyour environment.
Linux starts many daemons that usually aren't needed, including autofs, cups, nfslock, sendmail, and xfs. You should turn off anything that isn't explicitly required. You can accomplish this in several ways, but the chkconfig command is probably the best method.
As an example, let's shut down cups:
<[root@172_29_140_173 ~]# chkconfig --del cups
Now, let's make sure it's not around anymore:
# chkconfig --list
service cups supports chkconfig, but is not referenced in any runlevel (run
'chkconfig --add cups')
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