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><H1
><A
NAME="PROFILE-FUNCTIONS">Chapter 52. Profiling functions</H1
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="SERVICES-PROFILE-API">API</H1
><P
>In order for profile data to be gathered for an application, the
program has to initiate the process.
Once started, execution histogram data will be collected in a
dynamic memory buffer.
This data can be uploaded to a host using <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>TFTP</I
></SPAN
>.
A side effect of the upload of the data is that the histogram
is reset.
This is useful, especially for high resolution histograms, since
the histogram data are collected as 16-bit counters which can be quickly
saturated.
For example, if the histogram is being collected at a rate of 10,000
samples per second, a hot spot in the program could saturate after
only 6.5 seconds.</P
><P
> The API for the application profiling functions can be
found in the file <TT
CLASS="FILENAME"
><cyg/profile/profile.h></TT
>.</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="SERVICES-PROFILE-API-PROFILE-ON">profile_on</H2
><P
>This function is used to initiate the gathering of the
runtime execution histogram data.</P
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><PRE
CLASS="PROGRAMLISTING"
>void profile_on(void *start, void *end, int bucket_size, int resolution);</PRE
></TD
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><P
>Calling this function will initiate execution profiling.
An execution histogram is collected at the rate of
<TT
CLASS="PARAMETER"
><I
>resolution</I
></TT
> times per second.
The area between <TT
CLASS="PARAMETER"
><I
>start</I
></TT
> and <TT
CLASS="PARAMETER"
><I
>end</I
></TT
>
will be divided up into a number of buckets, each representing
<TT
CLASS="PARAMETER"
><I
>bucket_size</I
></TT
>
program bytes in length. Using statistical sampling (via a high speed timer), when
the program counter is found to be within the range
<TT
CLASS="PARAMETER"
><I
>start</I
></TT
>..<TT
CLASS="PARAMETER"
><I
>end</I
></TT
>, the appropriate
bucket (histogram entry) will be incremented.</P
><P
>The choice of <TT
CLASS="PARAMETER"
><I
>resolution</I
></TT
> and <TT
CLASS="PARAMETER"
><I
>bucket_size</I
></TT
>
control how large the data gathered will be, as well as how much overhead is
encumbered for gathering the histogram.
Smaller values for <TT
CLASS="PARAMETER"
><I
>bucket_size</I
></TT
> will garner better
results (<TT
CLASS="FUNCTION"
>gprof</TT
> can more closely align the data with
actual function names) at the expense of a larger data buffer.</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>NOTE: </B
>The value of <TT
CLASS="PARAMETER"
><I
>bucket_size</I
></TT
> will be rounded up to a power of two.</P
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