# Power Management¶

HTCondor supports placing machines in low power states. A machine in the low power state is identified as being offline. Power setting decisions are based upon HTCondor configuration.

Power conservation is relevant when machines are not in heavy use, or when there are known periods of low activity within the pool.

## Entering a Low Power State¶

By default, HTCondor does not do power management. When desired, the ability to place a machine into a low power state is accomplished through configuration. This occurs when all slots on a machine agree that a low power state is desired.

A slot’s readiness to hibernate is determined by the evaluating the HIBERNATE configuration variable (see the condor_startd Configuration File Macros section) within the context of the slot. Readiness is evaluated at fixed intervals, as determined by the HIBERNATE_CHECK_INTERVAL configuration variable. A non-zero value of this variable enables the power management facility. It is an integer value representing seconds, and it need not be a small value. There is a trade off between the extra time not at a low power state and the unnecessary computation of readiness.

To put the machine in a low power state rapidly after it has become idle, consider checking each slot’s state frequently, as in the example configuration:

HIBERNATE_CHECK_INTERVAL = 20


This checks each slot’s readiness every 20 seconds. A more common value for frequency of checks is 300 (5 minutes). A value of 300 loses some degree of granularity, but it is more reasonable as machines are likely to be put in to a low power state after a few hours, rather than minutes.

A slot’s readiness or willingness to enter a low power state is determined by the HIBERNATE expression. Because this expression is evaluated in the context of each slot, and not on the machine as a whole, any one slot can veto a change of power state. The HIBERNATE expression may reference a wide array of variables. Possibilities include the change in power state if none of the slots are claimed, or if the slots are not in the Owner state.

Here is a concrete example. Assume that the START expression is not set to always be True. This permits an easy determination whether or not the machine is in an Unclaimed state through the use of an auxiliary macro called ShouldHibernate.

TimeToWait  = (2 * $(HOUR)) ShouldHibernate = ( (KeyboardIdle >$(StartIdleTime)) \
&& $(CPUIdle) \ && ($(StateTimer) > $(TimeToWait)) )  This macro evaluates to True if the following are all True: • The keyboard has been idle long enough. • The CPU is idle. • The slot has been Unclaimed for more than 2 hours. The sample HIBERNATE expression that enters the power state called “RAM”, if ShouldHibernate evaluates to True, and remains in its current state otherwise is HibernateState = "RAM" HIBERNATE = ifThenElse($(ShouldHibernate), \$(HibernateState), "NONE" )


If any slot returns “NONE”, that slot vetoes the decision to enter a low power state. Only when values returned by all slots are all non-zero is there a decision to enter a low power state. If all agree to enter the low power state, but differ in which state to enter, then the largest magnitude value is chosen.

## Returning From a Low Power State¶

The HTCondor command line tool condor_power may wake a machine from a low power state by sending a UDP Wake On LAN (WOL) packet. See the condor_power manual page.

To automatically call condor_power under specific conditions, condor_rooster may be used. The configuration options for condor_rooster are described in the condor_rooster Configuration File Macros section.

## Keeping a ClassAd for a Hibernating Machine¶

A pool’s condor_collector daemon can be configured to keep a persistent ClassAd entry for each machine, once it has entered hibernation. This is required by condor_rooster so that it can evaluate the UNHIBERNATE expression of the offline machines.

To do this, define a log file using the OFFLINE_LOG configuration variable. See the condor_startd Configuration File Macros section for the definition. An optional expiration time for each ClassAd can be specified with OFFLINE_EXPIRE_ADS_AFTER . The timing begins from the time the hibernating machine’s ClassAd enters the condor_collector daemon. See the condor_startd Configuration File Macros section for the definition.

## Linux Platform Details¶

Depending on the Linux distribution and version, there are three methods for controlling a machine’s power state. The methods:

1. pm-utils is a set of command line tools which can be used to detect and switch power states. In HTCondor, this is defined by the string “pm-utils”.

2. The directory in the virtual file system /sys/power contains virtual files that can be used to detect and set the power states. In HTCondor, this is defined by the string “/sys”.

3. The directory in the virtual file system /proc/acpi contains virtual files that can be used to detect and set the power states. In HTCondor, this is defined by the string “/proc”.

By default, the HTCondor attempts to detect the method to use in the order shown. The first method detected as usable on the system is chosen.

This ordered detection may be bypassed, to use a specified method instead by setting the configuration variable LINUX_HIBERNATION_METHOD with one of the defined strings. This variable is defined in the condor_startd Configuration File Macros section. If no usable methods are detected or the method specified by LINUX_HIBERNATION_METHOD is either not detected or invalid, hibernation is disabled.

The details of this selection process, and the final method selected can be logged via enabling D_FULLDEBUG in the relevant subsystem’s log configuration.

## Windows Platform Details¶

If after a suitable amount of time, a Windows machine has not entered the expected power state, then HTCondor is having difficulty exercising the operating system’s low power capabilities. While the cause will be specific to the machine’s hardware, it may also be due to improperly configured software. For hardware difficulties, the likely culprit is the configuration within the machine’s BIOS, for which HTCondor can offer little guidance. For operating system difficulties, the powercfg tool can be used to discover the available power states on the machine. The following command demonstrates how to list all of the supported power states of the machine:

> powercfg -A
The following sleep states are available on this system:
Standby (S3) Hibernate Hybrid Sleep
The following sleep states are not available on this system:
Standby (S1)
The system firmware does not support this standby state.
Standby (S2)
The system firmware does not support this standby state.


Note that the HIBERNATE expression is written in terms of the Sn state, where n is the value evaluated from the expression.

This tool can also be used to enable and disable other sleep states. This example turns hibernation on.

> powercfg -h on


If this tool is insufficient for configuring the machine in the manner required, the Power Options control panel application offers the full extent of the machine’s power management abilities. Windows 2000 and XP lack the powercfg program, so all configuration must be done via the Power Options control panel application.