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Otherwise, it is possible to tune the notion of inactivity using
the DiskIO and Loadavg constraints.

=item The B<Freq> Constraint

The B<Freq> constraint specifies the maximum frequency of
invocation of a task. B<Freq> requires one parameter, the number
of seconds, that is the minimum difference between two
consecutive invocations. So, B<Freq, 86400> implies that the 
task should not be run more than once a day.  B<Freq> is usually 
coupled with another constraint (eg: B<Inactivity>) to
limit the frequency of opportunistic scheduling. For example:

    command = "emerge rsync"; constraint = Inactivity, 120; \ 
        constraint = Freq, 86400;

says, run "emerge rsync" once a day. However, if the system is
not inactive for more than one day, "emerge rsync" won't be run
till the system is inactive.

=item The B<Loadavg> Constraint

The B<Loadavg> constraint monitors the load average of the
system. It accepts two parameters: a load average threshold and
the amount of time the system load should linger at or below the
this threshold before the constraint is considered to be met.
The syntax is B<Loadavg, TIME, LOADAVG>. eg. B<Loadavg, 60, 0.02> 
is met when the system has a load average of 0.02 or lower 
for 60 consecutive seconds. Here's an example of usage:

    command = "cd /home/user/project/; cvs update; make all"; \ 
        constraint = Loadavg, 60, 0.0; constraint = Freq, 3600;

This task specifies invocation of CVS update under
B</home/user/project/> and B<make> if the load average is 0.0
for 60 seconds. The B<Freq> constraint additionally limits the
task to no more than once in 3600 seconds (one hour).

=item The B<DiskIO> Constraint

The B<DiskIO> constraint monitors the disk read/write activity
through the B<vmstat> program. It accepts three parameters: a
threshold for number of blocks read from disk, a threshold for
number of blocks written to disk and the amount of time for
which read/write linger at or below these thresholds. B<DiskIO>
syntax is B<DiskIO, TIME, BLOCKS WRITTEN, BLOCKS
READ>. eg. B<DiskIO, 60, 10, 15> is met when 10 or less blocks
READ>  are written to disk and 15 or less blocks are read from 
disk for 60 consecutive seconds. Here's an example of usage:

    command = "/usr/bin/updatedb"; constraint = DiskIO, 60, 0, 0; \
        constraint = Freq, 3600;

=item The B<Xscreensaver> Constraint

The B<Xscreensaver> constraints asynchronously monitors
Xscreensaver through the B<xscreensaver-command> program. It
accepts one parameter: the number of seconds for which
B<Xscreensaver> must be enabled before the constraint is met.
The syntax for the constraint is B<Xscreensaver, TIME>. eg.
B<Xscreensaver, 120> is met when B<Xscreensaver> is locked for
120 or more seconds. The constraint is considered to be met
until the screensaver is unlocked. Here's an example of usage:

    command = "emerge rsync"; Constraint = Xscreensaver, 120; \
        constraint = Freq, 86400;

=item The B<InXs> Constraint

The B<InXs> constraint (unrelated to the POP band) combines the
Xscreensaver and Inactivity constraints. It is met when
B<either> of the constraints is met. B<InXs> is useful for
scheduling high-load applications on end-user machines because
it's a good indicator of the user being away from the computer.
It accepts one parameter: the number of seconds for which either
of the constraints is enabled before B<InXs> is met. The syntax
for the constraint is B<InXs, TIME>. eg. B<InXs, 120>. Here's an
example of usage:

    command = "emerge rsync"; Constraint = InXs, 120; \
        constraint = Freq, 86400;

=item The B<Ping> Constraint

The B<Ping> constraint sends a ping to a specified address. It
accepts one parameter: IP address to be pinged. If the address
is not specified, C<4.2.2.1> is pinged -- an arbitrary address
on the Internet. B<Ping> syntax is B<Ping, IP>. The constraint
is considered to be met if the ping is successful. It should be
noted that the current version of the constraint uses the
external ping program to send a single ICMP packet to the IP.
The B<Ping> constraint is useful for tasks that need a
connection to the Internet but might be run on machines that
have intermittent connectivity to the Net.

=item The B<Concurrent> Constraint

The B<Concurrent> constraint monitors the number of running
processes of a specified application and enforces a specified
level of concurrency. The systax is B<Concurrent, APPLICATION,
MAX_PROCESSES>. As an example, B<Concurrent, perl, 5> will be
met if 4 or less B<perl>'s are running at the time of
evaluation.

=back

=head1 OTHER PARAMETERS

In addition to B<command> and B<constraint> keys, a chrontab
task entry can also have the following optional keys.

=over 4

=item notify

B<notify> is used to specify a recipient for an email
notification that is sent with the task is executed. This
notification contains information on timing and status of the
executed task. Syntax B<notify = admin@domain>.

=item only_once

B<only_once> is used to indicate that the task should be