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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Cache prefetching</span></span>
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<p><b>Cache prefetching</b> is a technique used by computer processors to boost execution performance by fetching instructions or data from their original storage in slower memory to a faster local memory before it is actually needed (hence the term 'prefetch').<sup id="cite_ref-:3_1-0" class="reference"><a href="#cite_note-:3-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> Most modern computer processors have fast and local <a href="CPU_cache" title="CPU cache">cache memory</a> in which prefetched data is held until it is required. The source for the prefetch operation is usually <a href="Computer_data_storage#Primary_storage" title="Computer data storage">main memory</a>. Because of their design, accessing cache memories is typically much faster than accessing <a href="Main_memory" class="mw-redirect" title="Main memory">main memory</a>, so prefetching data and then accessing it from caches is usually many orders of magnitude faster than accessing it directly from main memory. Prefetching can be done with non-blocking <a href="Cache_control_instruction" title="Cache control instruction">cache control instructions</a>.
</p>
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<div class="mw-heading mw-heading2"><h2 id="Data_vs._instruction_cache_prefetching">Data vs. instruction cache prefetching</h2></div>
<p>Cache prefetching can either fetch data or instructions into cache.
</p>
<ul><li><b>Data prefetching</b> fetches data before it is needed. Because data access patterns show less regularity than instruction patterns, accurate data prefetching is generally more challenging than instruction prefetching.</li>
<li><b>Instruction prefetching</b> fetches instructions before they need to be executed. The first mainstream microprocessors to use some form of instruction prefetch were the <a href="Intel" title="Intel">Intel</a> <a href="8086" class="mw-redirect" title="8086">8086</a> (six bytes) and the <a href="Motorola" title="Motorola">Motorola</a> <a href="68000" class="mw-redirect" title="68000">68000</a> (four bytes). In recent years, all high-performance processors use prefetching techniques.</li></ul>
<div class="mw-heading mw-heading2"><h2 id="Hardware_vs._software_cache_prefetching">Hardware vs. software cache prefetching</h2></div>
<p>Cache prefetching can be accomplished either by hardware or by software.<sup id="cite_ref-:2_3-0" class="reference"><a href="#cite_note-:2-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup>
</p>
<ul><li><b>Hardware based prefetching</b> is typically accomplished by having a dedicated hardware mechanism in the processor that watches the stream of instructions or data being requested by the executing program, recognizes the next few elements that the program might need based on this stream and prefetches into the processor's cache.<sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup></li>
<li><b>Software based prefetching</b> is typically accomplished by having the compiler analyze the code and insert additional "prefetch" instructions in the program during compilation itself.<sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup></li></ul>
<div class="mw-heading mw-heading2"><h2 id="Methods_of_hardware_prefetching">Methods of hardware prefetching</h2></div>
<div class="mw-heading mw-heading3"><h3 id="Stream_buffers">Stream buffers</h3></div>
<ul><li>Stream buffers were developed based on the concept of "one block lookahead (OBL) scheme" proposed by <a href="Alan_Jay_Smith" title="Alan Jay Smith">Alan Jay Smith</a>.<sup id="cite_ref-:3_1-1" class="reference"><a href="#cite_note-:3-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup></li>
<li>Stream <a href="Data_buffer" title="Data buffer">buffers</a> are one of the most common hardware based prefetching techniques in use. This technique was originally proposed by <a href="Norman_Jouppi" title="Norman Jouppi">Norman Jouppi</a> in 1990<sup id="cite_ref-:1_6-0" class="reference"><a href="#cite_note-:1-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> and many variations of this method have been developed since.<sup id="cite_ref-7" class="reference"><a href="#cite_note-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-8" class="reference"><a href="#cite_note-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-grannaes_9-0" class="reference"><a href="#cite_note-grannaes-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup> The basic idea is that the <a href="Cache_miss" class="mw-redirect" title="Cache miss">cache miss</a> address (and <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle k}">
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<annotation encoding="application/x-tex">{\displaystyle k}</annotation>
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</math></span><img src="./c3c9a2c7b599b37105512c5d570edc034056dd40.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:1.211ex; height:2.176ex;" alt="{\displaystyle k}" loading="lazy"></span> subsequent addresses) are fetched into a separate buffer of depth <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle k}">
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<annotation encoding="application/x-tex">{\displaystyle k}</annotation>
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</math></span><img src="./c3c9a2c7b599b37105512c5d570edc034056dd40.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:1.211ex; height:2.176ex;" alt="{\displaystyle k}" loading="lazy"></span>. This buffer is called a stream buffer and is separate from the cache. The processor then consumes data/instructions from the stream buffer if the address associated with the prefetched blocks match the requested address generated by the program executing on the processor. The figure below illustrates this setup:</li></ul>
<ul><li>Whenever the prefetch mechanism detects a miss on a memory block, say A, it allocates a stream to begin prefetching successive blocks from the missed block onward. If the stream buffer can hold 4 blocks, then the processor would prefetch A+1, A+2, A+3, A+4 and hold those in the allocated stream buffer. If the processor consumes A+1 next, then it shall be moved "up" from the stream buffer to the processor's cache. The first entry of the stream buffer would now be A+2 and so on. This pattern of prefetching successive blocks is called <b>Sequential Prefetching</b>. It is mainly used when contiguous locations are to be prefetched. For example, it is used when prefetching instructions.</li>
<li>This mechanism can be scaled up by adding multiple such 'stream buffers' - each of which would maintain a separate prefetch stream.<sup id="cite_ref-10" class="reference"><a href="#cite_note-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup> For each new miss, there would be a new stream buffer allocated and it would operate in a similar way as described above.</li>
<li>The ideal depth of the stream buffer is something that is subject to experimentation against various benchmarks<sup id="cite_ref-:1_6-2" class="reference"><a href="#cite_note-:1-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> and depends on the rest of the <a href="Microarchitecture" title="Microarchitecture">microarchitecture</a> involved.<sup id="cite_ref-11" class="reference"><a href="#cite_note-11"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup></li></ul>
<div class="mw-heading mw-heading3"><h3 id="Strided_prefetching">Strided prefetching</h3></div>
<p>This type of prefetching monitors the delta between the addresses of the memory accesses and looks for patterns within it.
</p>
<div class="mw-heading mw-heading4"><h4 id="Regular_strides">Regular strides</h4></div>
<p>In this pattern, consecutive memory accesses are made to blocks that are <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle s}">
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</math></span><img src="./01d131dfd7673938b947072a13a9744fe997e632.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:1.09ex; height:1.676ex;" alt="{\displaystyle s}" loading="lazy"></span> addresses apart.<sup id="cite_ref-:2_3-1" class="reference"><a href="#cite_note-:2-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-12" class="reference"><a href="#cite_note-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup> In this case, the prefetcher calculates the <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle s}">
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<mi>s</mi>
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<annotation encoding="application/x-tex">{\displaystyle s}</annotation>
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</math></span><img src="./01d131dfd7673938b947072a13a9744fe997e632.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:1.09ex; height:1.676ex;" alt="{\displaystyle s}" loading="lazy"></span> and uses it to compute the memory address for prefetching. E.g.: If the <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle s}">
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<mi>s</mi>
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<annotation encoding="application/x-tex">{\displaystyle s}</annotation>
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</math></span><img src="./01d131dfd7673938b947072a13a9744fe997e632.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:1.09ex; height:1.676ex;" alt="{\displaystyle s}" loading="lazy"></span> is 4, the address to be prefetched would A+4.
</p>
<div class="mw-heading mw-heading4"><h4 id="Irregular_spatial_strides">Irregular spatial strides</h4></div>
<p>In this case, the delta between the addresses of consecutive memory accesses is variable but still follows a pattern. Some prefetchers designs<sup id="cite_ref-grannaes_9-1" class="reference"><a href="#cite_note-grannaes-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-13" class="reference"><a href="#cite_note-13"><span class="cite-bracket">[</span>13<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-14" class="reference"><a href="#cite_note-14"><span class="cite-bracket">[</span>14<span class="cite-bracket">]</span></a></sup> exploit this property to predict and prefetch for future accesses.
</p>
<div class="mw-heading mw-heading4"><h4 id="Irregular_temporal_prefetching">Irregular temporal prefetching</h4></div>
<p>This class of prefetchers look for memory access streams that repeat over time.<sup id="cite_ref-15" class="reference"><a href="#cite_note-15"><span class="cite-bracket">[</span>15<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-16" class="reference"><a href="#cite_note-16"><span class="cite-bracket">[</span>16<span class="cite-bracket">]</span></a></sup> E.g. In this stream of memory accesses: N, A, B, C, E, G, H, A, B, C, I, J, K, A, B, C, L, M, N, O, A, B, C, ...; the stream A, B, C is repeating over time. Other design variation have tried to provide more efficient, performant implementations.<sup id="cite_ref-17" class="reference"><a href="#cite_note-17"><span class="cite-bracket">[</span>17<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-18" class="reference"><a href="#cite_note-18"><span class="cite-bracket">[</span>18<span class="cite-bracket">]</span></a></sup>
</p>
<div class="mw-heading mw-heading3"><h3 id="Collaborative_prefetching">Collaborative prefetching</h3></div>
<p>Computer applications generate a variety of access patterns. The processor and memory subsystem architectures used to execute these applications further disambiguate the memory access patterns they generate. Hence, the effectiveness and efficiency of prefetching schemes often depend on the application and the architectures used to execute them.<sup id="cite_ref-19" class="reference"><a href="#cite_note-19"><span class="cite-bracket">[</span>19<span class="cite-bracket">]</span></a></sup> Recent research<sup id="cite_ref-20" class="reference"><a href="#cite_note-20"><span class="cite-bracket">[</span>20<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-21" class="reference"><a href="#cite_note-21"><span class="cite-bracket">[</span>21<span class="cite-bracket">]</span></a></sup> has focused on building collaborative mechanisms to synergistically use multiple prefetching schemes for better prefetching coverage and accuracy.
</p>
<div class="mw-heading mw-heading2"><h2 id="Methods_of_software_prefetching">Methods of software prefetching</h2></div>
<div class="mw-heading mw-heading3"><h3 id="Compiler_directed_prefetching">Compiler directed prefetching</h3></div>
<p>Compiler directed prefetching is widely used within loops with a large number of iterations. In this technique, the compiler predicts future cache misses and inserts a prefetch instruction based on the <a href="Cache_Hierarchy" class="mw-redirect" title="Cache Hierarchy">miss penalty</a> and execution time of the instructions.
</p><p>These prefetches are non-blocking memory operations, i.e. these memory accesses do not interfere with actual memory accesses. They do not change the state of the processor or cause page faults.
</p><p>One main advantage of software prefetching is that it reduces the number of compulsory cache misses.<sup id="cite_ref-:2_3-2" class="reference"><a href="#cite_note-:2-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup>
</p><p>The following example shows how a prefetch instruction would be added into code to improve <a href="Cache_performance_measurement_and_metric" title="Cache performance measurement and metric">cache performance</a>.
</p><p>
Consider a for loop as shown below:</p><div class="mw-highlight mw-highlight-lang-c++ mw-content-ltr" dir="ltr"><pre><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">i</span><span class="o">=</span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o"><</span><span class="mi">1024</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o">++</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">array1</span><span class="p">[</span><span class="n">i</span><span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">2</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">array1</span><span class="p">[</span><span class="n">i</span><span class="p">];</span>
<span class="p">}</span>
</pre></div><p>At each iteration, the i<sup>th</sup> element of the array "array1" is accessed. Therefore, the system can prefetch the elements that are going to be accessed in future iterations by inserting a "prefetch" instruction as shown below:</p><div class="mw-highlight mw-highlight-lang-c++ mw-content-ltr" dir="ltr"><pre><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">i</span><span class="o">=</span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o"><</span><span class="mi">1024</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o">++</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">prefetch</span><span class="w"> </span><span class="p">(</span><span class="n">array1</span><span class="w"> </span><span class="p">[</span><span class="n">i</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">k</span><span class="p">]);</span>
<span class="w"> </span><span class="n">array1</span><span class="p">[</span><span class="n">i</span><span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">2</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">array1</span><span class="p">[</span><span class="n">i</span><span class="p">];</span>
<span class="p">}</span>
</pre></div><p>Here, the prefetch stride, <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle k}">
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</math></span><img src="./c3c9a2c7b599b37105512c5d570edc034056dd40.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.338ex; width:1.211ex; height:2.176ex;" alt="{\displaystyle k}" loading="lazy"></span> depends on two factors, the cache miss penalty and the time it takes to execute a single iteration of the <i><b>for</b></i> loop. For instance, if one iteration of the loop takes 7 cycles to execute, and the cache miss penalty is 49 cycles then there should be <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle k=49/7=7}">
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</math></span><img src="./aa88fa7dd8441e4c706dbc4096286780d2bf0648.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:13.22ex; height:2.843ex;" alt="{\displaystyle k=49/7=7}" loading="lazy"></span> - which means that the system should prefetch 7 elements ahead. With the first iteration, i will be 0, so the system prefetches the 7th element. Now, with this arrangement, the first 7 accesses (i=0->6) will still be misses (under the simplifying assumption that each element of array1 is in a separate cache line of its own).
</p><div class="mw-heading mw-heading2"><h2 id="Comparison_of_hardware_and_software_prefetching">Comparison of hardware and software prefetching</h2></div>
<ul><li>While software prefetching requires programmer or <a href="Compiler" title="Compiler">compiler</a> intervention, hardware prefetching requires special hardware mechanisms.<sup id="cite_ref-:2_3-3" class="reference"><a href="#cite_note-:2-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup></li>
<li>Software prefetching works well only with loops where there is regular array access as the programmer has to hand code the prefetch instructions, whereas hardware prefetchers work dynamically based on the program's behavior at <a href="Run_time_(program_lifecycle_phase)" class="mw-redirect" title="Run time (program lifecycle phase)">runtime</a>.<sup id="cite_ref-:2_3-4" class="reference"><a href="#cite_note-:2-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup></li>
<li>Hardware prefetching also has less CPU overhead when compared to software prefetching.<sup id="cite_ref-22" class="reference"><a href="#cite_note-22"><span class="cite-bracket">[</span>22<span class="cite-bracket">]</span></a></sup> However, software prefetching can mitigate certain constraints of hardware prefetching, leading to improvements in performance.<sup id="cite_ref-23" class="reference"><a href="#cite_note-23"><span class="cite-bracket">[</span>23<span class="cite-bracket">]</span></a></sup></li></ul>
<div class="mw-heading mw-heading2"><h2 id="Metrics_of_cache_prefetching">Metrics of cache prefetching</h2></div>
<p>There are three main metrics to judge cache prefetching<sup id="cite_ref-:2_3-5" class="reference"><a href="#cite_note-:2-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup>
</p>
<div class="mw-heading mw-heading3"><h3 id="Coverage">Coverage</h3></div>
<p>Coverage is the fraction of total misses that are eliminated because of prefetching, i.e.
</p><p><span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle Coverage={\frac {\text{Cache Misses eliminated by Prefetching}}{\text{Total Cache Misses}}}}">
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<mi>v</mi>
<mi>e</mi>
<mi>r</mi>
<mi>a</mi>
<mi>g</mi>
<mi>e</mi>
<mo>=</mo>
<mrow class="MJX-TeXAtom-ORD">
<mfrac>
<mtext>Cache Misses eliminated by Prefetching</mtext>
<mtext>Total Cache Misses</mtext>
</mfrac>
</mrow>
</mstyle>
</mrow>
<annotation encoding="application/x-tex">{\displaystyle Coverage={\frac {\text{Cache Misses eliminated by Prefetching}}{\text{Total Cache Misses}}}}</annotation>
</semantics>
</math></span><img src="./a5f32a3620e4eccd6bcb47a25692e42468672f4a.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -2.005ex; width:53.313ex; height:5.509ex;" alt="{\displaystyle Coverage={\frac {\text{Cache Misses eliminated by Prefetching}}{\text{Total Cache Misses}}}}" loading="lazy"></span>,
</p><p>where, <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle {\text{Total Cache Misses}}=({\text{Cache misses eliminated by prefetching}})+({\text{Cache misses not eliminated by prefetching}})}">
<semantics>
<mrow class="MJX-TeXAtom-ORD">
<mstyle displaystyle="true" scriptlevel="0">
<mrow class="MJX-TeXAtom-ORD">
<mtext>Total Cache Misses</mtext>
</mrow>
<mo>=</mo>
<mo stretchy="false">(</mo>
<mrow class="MJX-TeXAtom-ORD">
<mtext>Cache misses eliminated by prefetching</mtext>
</mrow>
<mo stretchy="false">)</mo>
<mo>+</mo>
<mo stretchy="false">(</mo>
<mrow class="MJX-TeXAtom-ORD">
<mtext>Cache misses not eliminated by prefetching</mtext>
</mrow>
<mo stretchy="false">)</mo>
</mstyle>
</mrow>
<annotation encoding="application/x-tex">{\displaystyle {\text{Total Cache Misses}}=({\text{Cache misses eliminated by prefetching}})+({\text{Cache misses not eliminated by prefetching}})}</annotation>
</semantics>
</math></span><img src="./7b7338d1a6ad16ddaa3d71f2e42c7497fe72ec80.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:111.59ex; height:2.843ex;" alt="{\displaystyle {\text{Total Cache Misses}}=({\text{Cache misses eliminated by prefetching}})+({\text{Cache misses not eliminated by prefetching}})}" loading="lazy"></span>
</p>
<div class="mw-heading mw-heading3"><h3 id="Accuracy">Accuracy</h3></div>
<p>Accuracy is the fraction of total prefetches that were useful - i.e. the ratio of the number of memory addresses prefetched were actually referenced by the program to the total prefetches done.
</p><p><span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle {\text{Prefetch Accuracy}}={\frac {\text{Cache Misses eliminated by prefetching}}{({\text{Useless Cache Prefetches}})+({\text{Cache Misses eliminated by prefetching}})}}}">
<semantics>
<mrow class="MJX-TeXAtom-ORD">
<mstyle displaystyle="true" scriptlevel="0">
<mrow class="MJX-TeXAtom-ORD">
<mtext>Prefetch Accuracy</mtext>
</mrow>
<mo>=</mo>
<mrow class="MJX-TeXAtom-ORD">
<mfrac>
<mtext>Cache Misses eliminated by prefetching</mtext>
<mrow>
<mo stretchy="false">(</mo>
<mrow class="MJX-TeXAtom-ORD">
<mtext>Useless Cache Prefetches</mtext>
</mrow>
<mo stretchy="false">)</mo>
<mo>+</mo>
<mo stretchy="false">(</mo>
<mrow class="MJX-TeXAtom-ORD">
<mtext>Cache Misses eliminated by prefetching</mtext>
</mrow>
<mo stretchy="false">)</mo>
</mrow>
</mfrac>
</mrow>
</mstyle>
</mrow>
<annotation encoding="application/x-tex">{\displaystyle {\text{Prefetch Accuracy}}={\frac {\text{Cache Misses eliminated by prefetching}}{({\text{Useless Cache Prefetches}})+({\text{Cache Misses eliminated by prefetching}})}}}</annotation>
</semantics>
</math></span><img src="./577ef350aef993c3cad07cb7d2e245f839eaf1c8.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -2.671ex; width:93.426ex; height:6.176ex;" alt="{\displaystyle {\text{Prefetch Accuracy}}={\frac {\text{Cache Misses eliminated by prefetching}}{({\text{Useless Cache Prefetches}})+({\text{Cache Misses eliminated by prefetching}})}}}" loading="lazy"></span>
</p><p>While it appears that having perfect accuracy might imply that there are no misses, this is not the case. The prefetches themselves might result in new misses if the prefetched blocks are placed directly into the cache. Although these may be a small fraction of the total number of misses observed without any prefetching, this is a non-zero number of misses.
</p>
<div class="mw-heading mw-heading3"><h3 id="Timeliness">Timeliness</h3></div>
<p>The qualitative definition of timeliness is how early a block is prefetched versus when it is actually referenced. An example to further explain timeliness is as follows:
</p><p>Consider a for loop where each iteration takes 3 cycles to execute and the 'prefetch' operation takes 12 cycles. This implies that for the prefetched data to be useful, the system must start the prefetch <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle 12/3=4}">
<semantics>
<mrow class="MJX-TeXAtom-ORD">
<mstyle displaystyle="true" scriptlevel="0">
<mn>12</mn>
<mrow class="MJX-TeXAtom-ORD">
<mo>/</mo>
</mrow>
<mn>3</mn>
<mo>=</mo>
<mn>4</mn>
</mstyle>
</mrow>
<annotation encoding="application/x-tex">{\displaystyle 12/3=4}</annotation>
</semantics>
</math></span><img src="./d0825e3b1d5cb1ab9622d17aadf3e93018873867.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.838ex; width:8.911ex; height:2.843ex;" alt="{\displaystyle 12/3=4}" loading="lazy"></span> iterations prior to its usage to maintain timeliness.
</p>
<div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2></div>
<ul><li><a href="Prefetch_input_queue" title="Prefetch input queue">Prefetch input queue</a></li>
<li><a href="Link_prefetching" title="Link prefetching">Link prefetching</a></li>
<li><a href="Prefetcher" title="Prefetcher">Prefetcher</a></li>
<li><a href="Cache_control_instruction" title="Cache control instruction">Cache control instruction</a></li></ul>
<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>
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