The Impact Of A Tickless Kernel
This will be short and sweet. In this article we will be looking at the impact of CONFIG_NO_HZ/Dynamic Ticks, which will be found in the Linux 2.6.21 kernel. The option has been available as a patch for quite a while, but not until Linux 2.6.21-rc1 had it been merged into the upstream kernel. When enabled, there will only be timer ticks when they are needed. The end-user benefit is cooler-running processors and increased power savings. We have investigated this change with a notebook and desktop computer.
CONFIG_NO_HZ/Dynamic Ticks is supported by SMP systems, but at present the only architecture supported is x86. Support for PowerPC, x86_64, and ARM is under development. For today's testing we had monitored the power consumption of the system using a SeaSonic PowerAngel. We were monitoring the Intel Xeon E5320 temperatures using LM_Sensors, but had found these numbers to be slightly inaccurate which led to their removal from this article. The power was monitored while the system was idling and again during full load caused by compiling the Linux kernel and running glxgears. We had tested the kernel with both CONFIG_NO_HZ (Tickless System (Dynamic Ticks)) and CONFIG_HIGH_RES_TIMERS enabled and then disabled.
The main system we had used was a Tyan Tempest i5000XT motherboard with dual Intel Clovertown E5320 processors and eight sticks of 512MB DDR2 FB-DIMM RAM. We had used this system due to the number of cores and processing power to see how well the tickless kernel would perform. We had also reproduced the same tests using a Lenovo ThinkPad R52 with an Intel Pentium M 750, 2GB of DDR2, and ATI Mobility Radeon X300. On both systems, Fedora 7 Rawhide was used with the Linux 2.6.21-rc1 vanilla kernel.
From these tests we had found the CONFIG_NO_HZ option to cause very little difference on the two test systems used. The Wattage had barely dropped during idle and load with this kernel option. Granted for Intel processors there is already Enhanced Intel SpeedStep Technology (EIST) and Cool 'n' Quiet for AMD processors to reduce the heat output, improve the thermal performance, and drive down your energy bills. However, to reiterate tickless kernels are not brand-specific but at this time it is only compatible with x86 architecture (and also SMP/non-SMP supportive). For those running their systems 24/7 a tickless kernel could end up being an advantage. Likewise, for embedded systems and laptops this ultimately should be of benefit. Outside of the thermal and power advantages, a tickless kernel should improve KVM performance. Our investigating of CONFIG_NO_HZ has just begun and we will report back once we have proceeded with additional tests.
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