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James Gregory 2013-12-30 12:25:34 +11:00
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<P>As you can see, it&#146;s easy to be drawn into thinking you&#146;re saving cycles when you&#146;re not. You can only improve the performance of a specific bit of code by reducing the factor&#151;either instruction fetch time or execution time, or sometimes a mix of the two&#151;that&#146;s limiting the performance of that code.
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<P>In case you missed it in all the excitement, the variability of prefetching means that our method of testing performance by executing 1,000 instructions in a row by no means produces &#147;true&#148; instruction execution times, any more than the official execution times in the Intel manuals are &#147;true&#148; times. The fact of the matter is that a given instruction takes <I>at least</I> as long to execute as the time given for it in the Intel manuals, but may take as much as 4 cycles per byte longer, depending on the state of the prefetch queue when the preceding instruction ends.</P>
<TABLE WIDTH="100%"><TD VALIGN="TOP" ALIGN="LEFT" WIDTH="5%"><IMG SRC="images/04-02i.jpg"><TD VALIGN="TOP" ALIGN="LEFT" WIDTH="95%"><SMALL><I>The only true execution time for an instruction is a time measured in a certain context, and that time is meaningful only in that context.</I></SMALL>
<TABLE WIDTH="100%"><TD VALIGN="TOP" ALIGN="LEFT" WIDTH="5%"><IMG SRC="images/i.jpg"><TD VALIGN="TOP" ALIGN="LEFT" WIDTH="95%"><SMALL><I>The only true execution time for an instruction is a time measured in a certain context, and that time is meaningful only in that context.</I></SMALL>
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<P>What we <I>really</I> want is to know how long useful working code takes to run, not how long a single instruction takes, and the Zen timer gives us the tool we need to gather that information. Granted, it would be easier if we could just add up neatly documented instruction execution times&#151;but that&#146;s not going to happen. Without actually measuring the performance of a given code sequence, you simply don&#146;t know how fast it is. For crying out loud, even the people who <I>designed</I> the 8088 at Intel couldn&#146;t tell you exactly how quickly a given 8088 code sequence executes on the PC just by looking at it! Get used to the idea that execution times are only meaningful in context, learn the rules of thumb in this book, and use the Zen timer to measure your code.</P>
<H4 ALIGN="LEFT"><A NAME="Heading12"></A><FONT COLOR="#000077">Approximating Overall Execution Times</FONT></H4>