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POHow can a usage counter in Solaris 10 /proc filesystem decrease?
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  1. POHow can a usage counter in Solaris 10 /proc filesystem decrease?
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    copied!<p>I'm trying to determine the CPU utilization of specific LWPs in specific processes in Solaris 10 using data from the /proc filesystem. The problem I have is that sometimes a utilization counter <em>decreases</em>.</p> <p>Here's the gist of it:</p> <pre><code>// we'll be reading from the file named /proc/&lt;pid&gt;/lwp/&lt;lwpid&gt;/lwpusage std::stringstream filename; filename &lt;&lt; "/proc/" &lt;&lt; pid &lt;&lt; "/lwp/" &lt;&lt; lwpid &lt;&lt; "/lwpusage"; int fd = open(filename.str().c_str(), O_RDONLY); // error checking while(1) { prusage_t usage; ssize_t readResult = pread(usage_fd, &amp;usage, sizeof(prusage_t), 0); // error checking std::cout &lt;&lt; "sec=" &lt;&lt; usage.pr_stime.tv_sec &lt;&lt; "nsec=" &lt;&lt; usage.pr_stime.tv_nsec &lt;&lt; std::endl; // wait } close(fd); </code></pre> <p>The number of nanoseconds reported in the prusage_t struct are derived from timestamps recorded each time an LWP changes state. This feature is called <a href="http://blogs.oracle.com/eschrock/entry/microstate_accounting_in_solaris_10" rel="nofollow noreferrer">microstate accounting</a>. Sounds good, but every so often the "system call cpu time" counter <em>decreases</em> roughly 1-10 milliseconds.</p> <p>Update: its not just the "system call cpu time" counter, I've since seen other counters decreasing as well.</p> <p>Another curiosity is that it always seems to be exactly one sample that's bogus - never two near each other. All the <em>other</em> samples are monotonically increasing at the expected rate. This seems to rule out the possibility that the counter is somehow reset in the kernel. </p> <p>Any clues as to what's going on here?</p> <pre><code>&gt; uname -a SunOS cdc-build-sol10u7 5.10 Generic_139556-08 i86pc i386 i86pc </code></pre>
 

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