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    <p>Don't worry! Your confusion is nothing but expected. Linux timer interrupts are very confusing and have had a long and quite exciting history.</p> <p><strong>CLK_TCK</strong></p> <p>Linux has no <code>sysconf</code> system call and glibc is just returning the constant value 100. Sorry.</p> <p><strong>HZ</strong> &lt;-- what you probably want</p> <p>When configuring your kernel you can choose a timer frequency of either 100Hz, 250Hz, 300Hz or 1000Hz. All of these are supported, and although 1000Hz is the default it's not always the best.</p> <p>People will generally choose a high value when they value latency (a desktop or a webserver) and a low value when they value throughput (HPC).</p> <p><strong>CONFIG_HIGH_RES_TIMERS</strong></p> <p>This has nothing to do with timer interrupts, it's just a mechanism that allows you to have higher resolution timers. This basically means that timeouts on calls like <code>select</code> can be more accurate than 1/HZ seconds.</p> <p><strong>divider</strong></p> <p>This command line option is a patch provided by Red Hat. You can probably use this (if you're using Red Hat or CentOS), but I'd be careful. It's caused lots of bugs and you should probably just recompile with a different Hz value.</p> <p><strong>CONFIG_NO_HZ</strong></p> <p>This really doesn't do much, it's for power saving and it means that the ticks will stop (or at least become less frequent) when nothing is executing. This is probably already enabled on your kernel. It doesn't make any difference when at least one task is runnable.</p> <p>Frederic Weisbecker actually has a patch pending which generalizes this to cases where only a single task is running, but it's a little way off yet.</p>
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    1. COIf I do `cat /proc/interrupts` 2 times , I see the `Local timer interrupts` increasing between the first and second invocation. But the one labelled `IO-APIC-edge timer` remains the same between 2 invocations. So are the `Local timer interrupts` running on each CPU responsible for incrementing the global timer variable `jiffies` which basically record the number of ticks since boot time?? Also `IRQ 0` correspond to timer interrupt in the 1st line of `cat /proc/interrupts` ... so I guess I have a `tickless system` By the way very clear explanation... thanks
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    2. CODon't worry about the I/O APIC, the local timer interrupts actually come from the Local APIC, which is an entirely different thing. What you see as IRQ 0 is the I/O APIC timer, which is probably unused (except maybe during boot). You're correct in saying that the local timer interrupts increment jiffies, however.
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    3. COHey i read abt timer from this book called professional linux kernel architecture and a bit from understanding the linux kernel but I clearly dont understand the process yet... I have to try and modify the code which updates jiffies in efficent way ... Can you recommend a good source where I can understand not just the theory but also the code in kernel which does the job .... Thanks
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