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>CPU Load Measurements</TITLE
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CLASS="CHAPTER"
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><H1
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><A
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NAME="CPULOAD-FUNCTIONS">Chapter 51. CPU Load Measurements</H1
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><DIV
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CLASS="SECT1"
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><H1
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CLASS="SECT1"
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><A
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NAME="SERVICES-CPULOAD-API">CPU Load API</H1
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><P
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>The package allows the CPU load to be estimated. The measurement code
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must first be calibrated to the target it is running on. Once this
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has been performed the measurement process can be started. This is a
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continuous process, so always providing the most up to data
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measurements. The process can be stopped at any time if required. Once
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the process is active, the results can be retrieved.</P
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><P
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>Note that if the target/processor performs any power saving actions,
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such as reducing the clock speed, or halting until the next interrupt
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etc, these will interfere with the CPU load measurement. Under these
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conditions the measurement results are undefined. The synthetic target
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is one such system. See the implementation details at the foot of this
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page for further information. </P
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><P
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>SMP systems are not supported, only uniprocessor system.</P
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><P
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> The API for load measuring functions can be
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found in the file <TT
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CLASS="FILENAME"
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>cyg/cpuload/cpuload.h</TT
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>.</P
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><DIV
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CLASS="SECT2"
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><H2
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CLASS="SECT2"
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><A
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NAME="SERVICES-CPULOAD-API-CYG-CPULOAD-CALIBRATE">cyg_cpuload_calibrate</H2
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><P
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>This function is used to calibrate the cpu load measurement code. It
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makes a measurement to determine the CPU properties while idle.</P
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><TABLE
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WIDTH="70%"
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><TR
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><TD
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><PRE
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CLASS="PROGRAMLISTING"
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>void cyg_cpuload_calibrate(cyg_uint32 *calibration);</PRE
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></TD
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></TR
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></TABLE
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><P
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>The function returns the calibration value at the location pointed to
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by <TT
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CLASS="PARAMETER"
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><I
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>calibration</I
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></TT
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>.</P
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><P
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>This function is quite unusual. For it to work correctly a few
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conditions must be met. The function makes use of the two highest
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thread priorities. No other threads must be using these priorities
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while the function is being used. The kernel scheduler must be started
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and not disabled. The function takes 100ms to complete during which
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time no other threads will be run.</P
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></DIV
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><DIV
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CLASS="SECT2"
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><H2
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CLASS="SECT2"
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><A
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NAME="SERVICES-CPULOAD-API-CYG-CPULOAD-CREATE">cyg_cpuload_create</H2
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><P
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>This function starts the CPU load measurments.</P
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><TABLE
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BORDER="5"
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BGCOLOR="#E0E0F0"
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WIDTH="70%"
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><TR
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><PRE
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CLASS="PROGRAMLISTING"
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>void cyg_cpuload_create(cyg_cpuload_t *cpuload,
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cyg_uint32 calibrate,
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cyg_handle_t *handle);</PRE
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></TD
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></TR
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></TABLE
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><P
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>The measurement process is started and a handle to it is returned in
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<TT
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CLASS="PARAMETER"
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><I
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>*handle</I
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></TT
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>. This handle is used to access the
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results and the stop the measurement process. </P
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></DIV
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><DIV
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CLASS="SECT2"
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><H2
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CLASS="SECT2"
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><A
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NAME="SERVICES-CPULOAD-API-CYG-CPULOAD-DELETE">cyg_cpuload_delete</H2
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><P
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>This function stops the measurement process. </P
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><TABLE
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BORDER="5"
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BGCOLOR="#E0E0F0"
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WIDTH="70%"
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><TR
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><TD
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><PRE
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CLASS="PROGRAMLISTING"
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>void cyg_cpuload_delete(cyg_handle_t handle);</PRE
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></TD
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></TR
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></TABLE
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><P
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><TT
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CLASS="PARAMETER"
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><I
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>handle</I
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></TT
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> should be the value returned by the create function.</P
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></DIV
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><DIV
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CLASS="SECT2"
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><H2
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CLASS="SECT2"
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><A
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NAME="SERVICES-CPULOAD-API-CYG-CPULOAD-GET">cyg_cpuload_get</H2
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><P
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>This function returns the latest measurements.</P
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><TABLE
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BORDER="5"
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BGCOLOR="#E0E0F0"
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WIDTH="70%"
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><TR
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><TD
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><PRE
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CLASS="PROGRAMLISTING"
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>void cyg_cpuload_get(cyg_handle_t handle,
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cyg_uint32 *average_point1s,
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cyg_uint32 *average_1s,
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cyg_uint32 *average_10s);</PRE
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></TD
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></TR
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></TABLE
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><P
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><TT
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CLASS="PARAMETER"
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><I
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>handle</I
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></TT
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> should be the value returned by the
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create function. The load measurements for the last 100ms, 1s and 10s
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are returned in
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<TT
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CLASS="PARAMETER"
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><I
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>*average_point1s</I
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></TT
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>,<TT
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CLASS="PARAMETER"
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><I
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>*average_1s</I
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></TT
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>
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and <TT
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CLASS="PARAMETER"
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><I
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>*average_10s</I
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></TT
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> respectively.</P
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></DIV
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><DIV
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CLASS="SECT2"
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><H2
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CLASS="SECT2"
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><A
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NAME="SERVICES-CPULOAD-API-IMPLEMENTATION">Implementation details</H2
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><P
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>This section gives a few details of how the measurements are
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made. This should help to understand what the results mean.</P
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><P
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>When there are no other threads runnable, eCos will execute the idle
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thread. This thread is always runnable and uses the lowest thread
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priority. The idle thread does little. It is an endless loop which
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increments the variable, <TT
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CLASS="LITERAL"
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>idle_thread_loops</TT
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> and
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executes the macro <TT
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CLASS="LITERAL"
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>HAL_IDLE_THREAD_ACTION</TT
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>. The cpu
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load measurement code makes use of the variable. It periodically
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examines the value of the variable and sees how much it has
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changed. The idler the system, the more it will have incremented. From
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this it is simple to determine the load of the system.</P
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><P
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>The function <TT
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CLASS="LITERAL"
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>cyg_cpuload_calibrate</TT
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> executes the
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idle thread for 100ms to determine how much
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<TT
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CLASS="LITERAL"
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>idle_thread_loops</TT
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> is incremented on a system idle
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for 100ms. <TT
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CLASS="LITERAL"
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>cyg_cpuload_create</TT
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> starts an alarm which
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every 100ms calls an alarm function. This function looks at the
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difference in <TT
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CLASS="LITERAL"
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>idle_thread_loops</TT
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> since the last
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invocation of the alarm function and so calculated how idle or busy
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the system has been. The structure <TT
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CLASS="LITERAL"
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>cyg_cpuload</TT
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> is
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updated during the alarm functions with the new results. The 100ms
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result is simply the result from the last measurement period. A simple
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filter is used to average the load over a period of time, namely 1s
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and 10s. Due to rounding errors, the 1s and 10s value will probably
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never reach 100% on a fully loaded system, but 99% is often seen.</P
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><P
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>As stated above, clever power management code will interfere with
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these measurements. The basic assumption is that the idle thread will
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be executed un-hindered and under the same conditions as when the
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calibration function was executed. If the CPU clock rate is reduced,
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the idle thread counter will be incremented less and so the CPU load
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measurements will give values too high. If the CPU is halted entirely,
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100% cpu load will be measured.</P
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></DIV
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></TD
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| 363 |
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></TR
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| 364 |
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><TR
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| 365 |
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><TD
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| 366 |
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WIDTH="33%"
|
| 367 |
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ALIGN="left"
|
| 368 |
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VALIGN="top"
|
| 369 |
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|
>CPU load measurements</TD
|
| 370 |
|
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><TD
|
| 371 |
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WIDTH="34%"
|
| 372 |
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ALIGN="center"
|
| 373 |
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VALIGN="top"
|
| 374 |
|
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><A
|
| 375 |
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HREF="services-cpuload.html"
|
| 376 |
|
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ACCESSKEY="U"
|
| 377 |
|
|
>Up</A
|
| 378 |
|
|
></TD
|
| 379 |
|
|
><TD
|
| 380 |
|
|
WIDTH="33%"
|
| 381 |
|
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ALIGN="right"
|
| 382 |
|
|
VALIGN="top"
|
| 383 |
|
|
>Application profiling</TD
|
| 384 |
|
|
></TR
|
| 385 |
|
|
></TABLE
|
| 386 |
|
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></DIV
|
| 387 |
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></BODY
|
| 388 |
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></HTML
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| 389 |
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>
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