10.2 Idle server still uses 2 to 3 of system

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10.2 Idle server still uses 2 to 3% of system

An idle server still requires some amount of storage and processor resource, as it constantly polls for new connections. The cost of this seems to be about 2 to 3% of a z800 processor. This may be a characteristic of Domino, as discussed in 11.8.2, "Idle servers" on page 277. However, it also may be due to the Linux timer, which pops 100 times per second.

There is a patch for the Linux kernel 2.4.7 that changes the timing method from the regular 100 hz interrupts to on-demand timer interrupts. This saves some CPU cycles in an idle Linux image. The drawback is that a busy Linux image will do a small amount of additional work every time the switch between user mode and kernel mode is done. In short, if you are running many images under VM, the on-demand timers are useful but you probably do not want to use them running under an LPAR. The patch can be found at:

• http://www-124.ibm.com/developerworks/oss/linux390/index.shtml

The patch introduces a configuration option CONFIG_NO_HZ_TIMER that lets you choose between the old and the new behavior. Note that you will need different lcs, qdio, qeth, and z90crypt modules if you use the no hz timer option.

In earlier versions of the timer patch, kernels that wer built with the patch applied but with CONFIG_NO_HZ_TIMER switched off were still incompatible with the regular OCO modules (and also with the timer-patch OCO modules).

This problem is fixed in this version of the patch. If CONFIG_NO_HZ_TIMER is switched off, the k" rnel is now compatible with the regular OCO modules.

Note 

The "order 2 allocation relief" patch for 64-bit is not compatible with the "on-demand timer" patch.

The value of the timer patch status is in /proc/sys/kernel/hz_timer. These are the values:

    1 - run free at 100hz    0 - sets the timer for on-demand switching for running under VM 

To enable the timer patch, issue the following command and put it in /etc/rc.d/boot.local:

    echo "0" >/proc/sys/kernel/hz_timer 

This patch is also discussed in 7.3. "The Linux timer patch" of Linux on IBM eServer zSeries and S/390: Performance Measurement and Tuning, SG24-6926.

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