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nohz_full: Add testing information to documentation 

This commit adds information about testing nohz_full, and also emphasizes
the fact that you need a multi-CPU system to get any benefit from nohz_full.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
This commit is contained in:
Paul E. McKenney 2013-06-18 11:15:21 -07:00
parent feed66ed26
commit 8bdf7a252b
1 changed files with 34 additions and 10 deletions
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44
Documentation/timers/NO_HZ.txt
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@ -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.