The Tick Rate: HZ
The frequency of the system timer (the tick rate) is programmed on system boot based on a static preprocessor define, HZ. The value of HZ differs for each supported architecture. In fact, on some supported architectures, it even differs between machine types.
The kernel defines the value in <asm/param.h>. The tick rate has a frequency of HZ hertz and a period of 1/HZ seconds. For example, in include/asm-i386/param.h, the i386 architecture defines:
#define HZ 1000 /* internal kernel time frequency */
Therefore, the timer interrupt on i386 has a frequency of 1000HZ and occurs 1,000 times per second (every one-thousandth of a second, which is every millisecond). Most other architectures have a tick rate of 100. Table 10.1 is a complete listing of the supported architectures and their defined tick rates.
When writing kernel code, never assume that HZ has any given value. This is not a common mistake these days because so many architectures have varying tick rates. In the past, however, Alpha was the only architecture with a tick rate not equal to 100Hz, and it was common to see code incorrectly hard-code the value 100 when the HZ value should have been used. Examples of using HZ in kernel code are shown later.
The frequency of the timer interrupt is rather important. As you already saw, the timer interrupt performs a lot of work. Indeed, the kernel's entire notion of time derives from the periodicity of the system timer. Picking the right value, like a successful relationship, is all about compromise.
The Ideal HZ Value
Starting with the initial version of Linux, the i386 architecture has had a timer interrupt frequency of 100 Hz. During the 2.5 development series, however, the frequency was raised to 1000 Hz and was (as such things are) controversial. Because so much of the system is dependent on the timer interrupt, changing its frequency has a reasonable impact on the system. Of course, there are pros and cons to larger versus smaller HZ values.
Increasing the tick rate means the timer interrupt runs more frequently. Consequently, the work it performs occurs more often. This has the following benefits:
The resolution increases by the same factor as the tick rate increases. For example, the granularity of timers with HZ=100 is 10 milliseconds. In other words, all periodic events occur on the timer interrupt's 10 millisecond boundary and no finer precision is guaranteed. With HZ=1000, however, resolution is 1 millisecondten times finer. Although kernel code can create timers with 1-millisecond resolution, there is no guarantee the precision afforded with HZ=100 is sufficient to execute the timer on anything better than 10-millisecond intervals.
Likewise, accuracy improves in the same manner. Assuming the kernel starts timers at random times, the average timer is off by half the period of the timer interrupt because timers might expire at any time, but are executed only on occurrences of the timer interrupt. For example, with HZ=100, the average event occurs +/ 5 milliseconds off from the desired time. Thus, error is 5 milliseconds on average. With HZ=1000, the average error drops to 0.5 millisecondsa tenfold improvement.
Some of the most readily noticeable performance benefits come from the improved precision of poll() and select() timeouts. The improvement might be quite large; an application that makes heavy use of these system calls might waste a great deal of time waiting for the timer interrupt, when, in fact, the timeout has actually expired. Remember, the average error (that is, potentially wasted time) is half the period of the timer interrupt.
Another benefit of a higher tick rate is the greater accuracy in process preemption, which results in decreased scheduling latency. Recall from Chapter 4 that the timer interrupt is responsible for decrementing the running process's timeslice count. When the count reaches zero, need_resched is set and the kernel runs the scheduler as soon as possible. Now assume a given process is running and has 2 milliseconds of its timeslice remaining. In 2 milliseconds, the scheduler should preempt the running process and begin executing a new process. Unfortunately, this event does not occur until the next timer interrupt, which might not be in 2 milliseconds. In fact, at worst the next timer interrupt might be 1/HZ of a second away! With HZ=100, a process can get nearly ten extra milliseconds to run. Of course, this all balances out and fairness is preserved, because all tasks receive the same imprecision in schedulingbut that is not the issue. The problem stems from the latency created by the delayed preemption. If the to-be-scheduled task had something time sensitive to do, such as refill an audio buffer, the delay might not be acceptable. Increasing the tick rate to 1000Hz lowers the worst-case scheduling overrun to just 1 millisecond, and the average-case overrun to just 0.5 milliseconds.
Now, there must be some downside to increasing the tick rate or it would have been 1000Hz (or even higher) to start. Indeed, there is one large issue: A higher tick rate implies more frequent timer interrupts, which implies higher overhead, because the processor must spend more time executing the timer interrupt handler. The higher the tick rate, the more time the processor spends executing the timer interrupt. This adds up to not just less processor time available for other work, but also a more frequent thrashing of the processor's cache. The issue of the overhead's impact is debatable. A move from HZ=100 to HZ=1000 clearly brings with it ten times greater overhead. However, how substantial is the overhead to begin with? The final agreement is that, at least on modern systems, HZ=1000 does not create unacceptable overhead and the move to a 1000Hz timer has not hurt performance too much. Nevertheless, it is possible in 2.6 to compile the kernel with a different value for HZ.