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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>4.2.7. Periodically Print I/O Block Time</title><link rel="stylesheet" href="./Common_Content/css/default.css" type="text/css" /><meta name="generator" content="publican 1.6" /><meta name="package" content="Systemtap-SystemTap_Beginners_Guide-1.0-en-US-2.0-2" /><link rel="home" href="index.html" title="SystemTap Beginners Guide" /><link rel="up" href="mainsect-disk.html" title="4.2. Disk" /><link rel="prev" href="inodewatch2sect.html" title="4.2.6. Monitoring Changes to File Attributes" /><link rel="next" href="mainsect-profiling.html" title="4.3. Profiling" /></head><body class=""><p id="title"><a class="left" href="http://www.fedoraproject.org"><img src="Common_Content/images/image_left.png" alt="Product Site" /></a><a class="right" href="http://docs.fedoraproject.org"><img src="Common_Content/images/image_right.png" alt="Documentation Site" /></a></p><ul class="docnav"><li class="previous"><a accesskey="p" href="inodewatch2sect.html"><strong>Prev</strong></a></li><li class="next"><a accesskey="n" href="mainsect-profiling.html"><strong>Next</strong></a></li></ul><div xml:lang="en-US" class="section" title="4.2.7. Periodically Print I/O Block Time" lang="en-US"><div class="titlepage"><div><div><h3 class="title" id="ioblktimesect">4.2.7. Periodically Print I/O Block Time</h3></div></div></div><a id="id4352547" class="indexterm"></a><a id="id2748811" class="indexterm"></a><a id="id2973930" class="indexterm"></a><a id="id2751725" class="indexterm"></a><a id="id2765400" class="indexterm"></a><div class="para">
This section describes how to track the amount of time each block I/O requests spends waiting for completion. This is useful in determining whether there are too many outstanding block I/O operations at any given time.
</div><div class="formalpara"><h5 class="formalpara" id="ioblktime">ioblktime.stp</h5>
<pre class="programlisting">
#! /usr/bin/env stap
global req_time, etimes
probe ioblock.request
{
req_time[$bio] = gettimeofday_us()
}
probe ioblock.end
{
t = gettimeofday_us()
s = req_time[$bio]
delete req_time[$bio]
if (s) {
etimes[devname, bio_rw_str(rw)] <<< t - s
}
}
probe timer.s(10), end {
ansi_clear_screen()
printf("%10s %3s %10s %10s %10s\n",
"device", "rw", "total (us)", "count", "avg (us)")
foreach ([dev,rw] in etimes - limit 20) {
printf("%10s %3s %10d %10d %10d\n", dev, rw,
@sum(etimes[dev,rw]), @count(etimes[dev,rw]), @avg(etimes[dev,rw]))
}
delete etimes
}
</pre>
</div><div class="para">
<a class="xref" href="ioblktimesect.html#ioblktime" title="ioblktime.stp">ioblktime.stp</a> computes the average waiting time for block I/O per device, and prints a list every 10 seconds. As always, you can revise this refresh rate by editing the specified value in <code class="command">probe timer.s(10), end {</code>.
</div><div class="para">
In some cases, there can be too many outstanding block I/O operations, at which point the script can exceed the default number of <code class="command">MAXMAPENTRIES</code>. <code class="command">MAXMAPENTRIES</code> is the maximum number of rows in an array if the array size is not specified explicitly when declared. If the script exceeds the default <code class="command">MAXMAPENTRIES</code> value of 2048, run the script again with the <code class="command">stap</code> option <code class="command">-DMAXMAPENTRIES=10000</code>.
</div><div class="example" id="ioblktimeoutput"><div class="example-contents"><pre class="screen">
device rw total (us) count avg (us)
sda W 9659 6 1609
dm-0 W 20278 6 3379
dm-0 R 20524 5 4104
sda R 19277 5 3855
</pre></div><h6>Example 4.12. <a class="xref" href="ioblktimesect.html#ioblktime" title="ioblktime.stp">ioblktime.stp</a> Sample Output</h6></div><br class="example-break" /><div class="para">
<a class="xref" href="ioblktimesect.html#ioblktimeoutput" title="Example 4.12. ioblktime.stp Sample Output">Example 4.12, “ioblktime.stp Sample Output”</a> displays the device name, operations performed (<code class="command">rw</code>), total wait time of all operations (<code class="command">total(us)</code>), number of operations (<code class="command">count</code>), and average wait time for all those operations (<code class="command">avg (us)</code>). The times tallied by the script are in microseconds.
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