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| -rw-r--r-- | Documentation/timers/NO_HZ.txt | 44 |
1 files changed, 34 insertions, 10 deletions
diff --git a/Documentation/timers/NO_HZ.txt b/Documentation/timers/NO_HZ.txt index 88697584242b..cca122f25120 100644 --- a/Documentation/timers/NO_HZ.txt +++ b/Documentation/timers/NO_HZ.txt @@ -24,8 +24,8 @@ There are three main ways of managing scheduling-clock interrupts workloads, you will normally -not- want this option. These three cases are described in the following three sections, followed -by a third section on RCU-specific considerations and a fourth and final -section listing known issues. +by a third section on RCU-specific considerations, a fourth section +discussing testing, and a fifth and final section listing known issues. NEVER OMIT SCHEDULING-CLOCK TICKS @@ -121,14 +121,15 @@ boot parameter specifies the adaptive-ticks CPUs. For example, "nohz_full=1,6-8" says that CPUs 1, 6, 7, and 8 are to be adaptive-ticks CPUs. Note that you are prohibited from marking all of the CPUs as adaptive-tick CPUs: At least one non-adaptive-tick CPU must remain -online to handle timekeeping tasks in order to ensure that system calls -like gettimeofday() returns accurate values on adaptive-tick CPUs. -(This is not an issue for CONFIG_NO_HZ_IDLE=y because there are no -running user processes to observe slight drifts in clock rate.) -Therefore, the boot CPU is prohibited from entering adaptive-ticks -mode. Specifying a "nohz_full=" mask that includes the boot CPU will -result in a boot-time error message, and the boot CPU will be removed -from the mask. +online to handle timekeeping tasks in order to ensure that system +calls like gettimeofday() returns accurate values on adaptive-tick CPUs. +(This is not an issue for CONFIG_NO_HZ_IDLE=y because there are no running +user processes to observe slight drifts in clock rate.) Therefore, the +boot CPU is prohibited from entering adaptive-ticks mode. Specifying a +"nohz_full=" mask that includes the boot CPU will result in a boot-time +error message, and the boot CPU will be removed from the mask. Note that +this means that your system must have at least two CPUs in order for +CONFIG_NO_HZ_FULL=y to do anything for you. Alternatively, the CONFIG_NO_HZ_FULL_ALL=y Kconfig parameter specifies that all CPUs other than the boot CPU are adaptive-ticks CPUs. This @@ -232,6 +233,29 @@ scheduler will decide where to run them, which might or might not be where you want them to run. +TESTING + +So you enable all the OS-jitter features described in this document, +but do not see any change in your workload's behavior. Is this because +your workload isn't affected that much by OS jitter, or is it because +something else is in the way? This section helps answer this question +by providing a simple OS-jitter test suite, which is available on branch +master of the following git archive: + +git://git.kernel.org/pub/scm/linux/kernel/git/frederic/dynticks-testing.git + +Clone this archive and follow the instructions in the README file. +This test procedure will produce a trace that will allow you to evaluate +whether or not you have succeeded in removing OS jitter from your system. +If this trace shows that you have removed OS jitter as much as is +possible, then you can conclude that your workload is not all that +sensitive to OS jitter. + +Note: this test requires that your system have at least two CPUs. +We do not currently have a good way to remove OS jitter from single-CPU +systems. + + KNOWN ISSUES o Dyntick-idle slows transitions to and from idle slightly. |