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Date: Mon, 20 May 2013 07:50:00 -0700 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: linux-kernel@...r.kernel.org Cc: mingo@...e.hu, laijs@...fujitsu.com, dipankar@...ibm.com, akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com, edumazet@...gle.com, darren@...art.com, fweisbec@...il.com, sbw@....edu, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> Subject: [PATCH tip/core/rcu 1/2] nohz_full: Update based on Sedat Dilek review From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> Make it more clear that there are three options, and give hints as to which of the three is most likely to be useful in different situations. Reported-by: Sedat Dilek <sedat.dilek@...il.com> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com> --- Documentation/timers/NO_HZ.txt | 58 +++++++++++++++++++++++++++++++++++------- 1 file changed, 49 insertions(+), 9 deletions(-) diff --git a/Documentation/timers/NO_HZ.txt b/Documentation/timers/NO_HZ.txt index 5b53220..d5323e0 100644 --- a/Documentation/timers/NO_HZ.txt +++ b/Documentation/timers/NO_HZ.txt @@ -7,21 +7,59 @@ efficiency and reducing OS jitter. Reducing OS jitter is important for some types of computationally intensive high-performance computing (HPC) applications and for real-time applications. -There are two main contexts in which the number of scheduling-clock -interrupts can be reduced compared to the old-school approach of sending -a scheduling-clock interrupt to all CPUs every jiffy whether they need -it or not (CONFIG_HZ_PERIODIC=y or CONFIG_NO_HZ=n for older kernels): +There are three main ways of managing scheduling-clock interrupts +(also known as "scheduling-clock ticks" or simply "ticks"): -1. Idle CPUs (CONFIG_NO_HZ_IDLE=y or CONFIG_NO_HZ=y for older kernels). +1. Never omit scheduling-clock ticks (CONFIG_HZ_PERIODIC=y or + CONFIG_NO_HZ=n for older kernels). You normally will -not- + want to choose this option. -2. CPUs having only one runnable task (CONFIG_NO_HZ_FULL=y). +2. Omit scheduling-clock ticks on idle CPUs (CONFIG_NO_HZ_IDLE=y or + CONFIG_NO_HZ=y for older kernels). This is the most common + approach, and should be the default. -These two cases are described in the following two sections, followed +3. Omit scheduling-clock ticks on CPUs that are either idle or that + have only one runnable task (CONFIG_NO_HZ_FULL=y). Unless you + are running realtime applications or certain types of HPC + 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. -IDLE CPUs +NEVER OMIT SCHEDULING-CLOCK TICKS + +Very old versions of Linux from the 1990s and the very early 2000s +are incapable of omitting scheduling-clock ticks. It turns out that +there are some situations where this old-school approach is still the +right approach, for example, in heavy workloads with lots of tasks +that use short bursts of CPU, where there are very frequent idle +periods, but where these idle periods are also quite short (tens or +hundreds of microseconds). For these types of workloads, scheduling +clock interrupts will normally be delivered any way because there +will frequently be multiple runnable tasks per CPU. In these cases, +attempting to turn off the scheduling clock interrupt will have no effect +other than increasing the overhead of switching to and from idle and +transitioning between user and kernel execution. + +This mode of operation can be selected using CONFIG_HZ_PERIODIC=y (or +CONFIG_NO_HZ=n for older kernels). + +However, if you are instead running a light workload with long idle +periods, failing to omit scheduling-clock interrupts will result in +excessive power consumption. This is especially bad on battery-powered +devices, where it results in extremely short battery lifetimes. If you +are running light workloads, you should therefore read the following +section. + +In addition, if you are running either a real-time workload or an HPC +workload with short iterations, the scheduling-clock interrupts can +degrade your applications performance. If this describes your workload, +you should read the following two sections. + + +OMIT SCHEDULING-CLOCK TICKS FOR IDLE CPUs If a CPU is idle, there is little point in sending it a scheduling-clock interrupt. After all, the primary purpose of a scheduling-clock interrupt @@ -59,10 +97,12 @@ By default, CONFIG_NO_HZ_IDLE=y kernels boot with "nohz=on", enabling dyntick-idle mode. -CPUs WITH ONLY ONE RUNNABLE TASK +OMIT SCHEDULING-CLOCK TICKS FOR CPUs WITH ONLY ONE RUNNABLE TASK If a CPU has only one runnable task, there is little point in sending it a scheduling-clock interrupt because there is no other task to switch to. +Note that omitting scheduling-clock ticks for CPUs with only one runnable +task implies also omitting them for idle CPUs. The CONFIG_NO_HZ_FULL=y Kconfig option causes the kernel to avoid sending scheduling-clock interrupts to CPUs with a single runnable task, -- 1.8.1.5 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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