#!/usr/bin/perl

# This software code is made available "AS IS" without warranties of any        
# kind.  You may copy, display, modify and redistribute the software            
# code either by itself or as incorporated into your code; provided that        
# you do not remove any proprietary notices.  Your use of this software         
# code is at your own risk and you waive any claim against the author
# with respect to your use of this software code. 
# (c) 2006 Gregory Bell
#

# usage:
# ionice <some command, with arguments>

# What is it:
# A poor man's attempt at the real "ionice" which isn't supported
# until 2.6.13 (the boxes I use are based on RHEL 4)
# The theory is to run processes which are known to be IO-intensive and
# low-priority (such as a daily backup) under 'ionice' to make them
# good neighbors to the rest of the system in terms of IO usage.
# It tries to prevent IO starvation of other processes, which may be 
# more important to have fast response time.

# The name of this utility (ionice) is unfortunately a bit of a misnomer.
# There's no way to flag a process as having a different "io priority"
# than others, which would be the ideal solution.
# Instead, this is more of a "blunt instrument" approach to the problem.

# Theory of operation:
# Launch a coprocess and when system iowait gets too high, 
# suspend the coprocess for a while.
# When the iowait gets back to an acceptable level, re-activate copro.

# Configurable things:
$MAX_ALLOWED_IOWAIT_PERCENT = 50;	# of whole system
$MIN_BACKOFF_SECONDS = 0.5;		# how often do we poll the IO
$BACKOFF_MULTIPLIER = 1.5;		# higher value = greater back-off

use POSIX ":sys_wait_h";
use Time::HiRes qw(sleep);

#initialize the bins (values accumulate over time)
my ($lastio, $lasttotal) = ReadProcStat();

sub ReadProcStat
{
	open(PROCSTAT, "</proc/stat") or die;
	$_ = <PROCSTAT>;
	close(PROCSTAT);
	my @words = split(/\s+/);
	my $alltimes = 0;
	my $i = 0;
	for($i = 1; $i < 8; $i++)
	{
		$alltimes = $alltimes + $words[$i];
	}
	my $iowait = $words[5] + 0;
	return ($iowait, $alltimes);
}


# coprocess GO!
$copid = fork();
if(0 == $copid)
{
	# this is the child process
	# become the copro
	exec(@ARGV);
}

my $backoff = $MIN_BACKOFF_SECONDS;
my $suspended = 0;
while(0 == waitpid($copid, &WNOHANG))
{
	my ($currentio, $currenttotal) = ReadProcStat();
	my $deltaio = $currentio - $lastio;
	my $deltatotal = $currenttotal - $lasttotal;
	($lastio, $lasttotal) = ($currentio, $currenttotal);
	# don't divide by zero
	if($deltatotal == 0)
	{
		$deltatotal = 1;
	}
	my $iowait = ($deltaio / $deltatotal) * 100;

	if($MAX_ALLOWED_IOWAIT_PERCENT < $iowait)
	{
		if(0 == $suspended)
		{
			$suspended = 1;
			kill('STOP', $copid);
		}
		else
		{
			# still bad; slow down checking
			$backoff = $backoff * $BACKOFF_MULTIPLIER;
		}
	}
	elsif($MAX_ALLOWED_IOWAIT_PERCENT >= $iowait  and  1 == $suspended)
	{
		$suspended = 0;
		$backoff = $MIN_BACKOFF_SECONDS;
		kill('CONT', $copid);
	}
	sleep($backoff);
}