lib0046.html

Memory Management—Algorithms and implementation in C/C++ Introduction Chapter 1 - Memory Manag

    <span style="background-color:d9d9d9">for(i=0;i&lt;lastAlloc;i++)</span>
    <span style="background-color:d9d9d9">{</span>
        <span style="background-color:d9d9d9">if(indices[i].free==TRUE)</span>
        <span style="background-color:d9d9d9">{</span>
            <span style="background-color:d9d9d9">indices[i].free=FALSE;</span>
            <span style="background-color:d9d9d9">lastAlloc = i;</span>
            <span style="background-color:d9d9d9">return(&amp;indices[i]);</span>
        <span style="background-color:d9d9d9">}</span>
    <span style="background-color:d9d9d9">}</span>
    <span style="background-color:d9d9d9">return(NULL);</span>

<span style="background-color:d9d9d9">}/*end alloc-----------------------------------------------*/</span>

<span style="background-color:d9d9d9">void SubAllocator::release(struct Indices *addr)</span>
<span style="background-color:d9d9d9">{</span>
    <span style="background-color:d9d9d9">//sanity check</span>
    <span style="background-color:d9d9d9">if((addr&gt;=&amp;indices[0])&amp;&amp;(addr&lt;=&amp;indices[size-1]))</span>
    <span style="background-color:d9d9d9">{</span>
        <span style="background-color:d9d9d9">(*addr).free=TRUE;</span>
    <span style="background-color:d9d9d9">}</span>
    <span style="background-color:d9d9d9">else</span>
    <span style="background-color:d9d9d9">{</span>
        <span style="background-color:d9d9d9">printf("SubAllocator::release():");</span>
        <span style="background-color:d9d9d9">printf("release failed, address out of bounds\n");</span>
    <span style="background-color:d9d9d9">}</span>
    <span style="background-color:d9d9d9">return;</span>

<span style="background-color:d9d9d9">}/*end release-----------------------------------------------*/</span>

<span style="background-color:d9d9d9">void SubAllocator::printList()</span>
<span style="background-color:d9d9d9">{</span>
    <span style="background-color:d9d9d9">U4 i;</span>
    <span style="background-color:d9d9d9">for(i=0;i&lt;size;i++)</span>
    <span style="background-color:d9d9d9">{</span>
        <span style="background-color:d9d9d9">if(indices[i].free==FALSE)</span>
        <span style="background-color:d9d9d9">{</span>
            <span style="background-color:d9d9d9">printf("indices[%lu] ",i);</span>
            <span style="background-color:d9d9d9">printf("[%lu, ",indices[i].index1);</span>
            <span style="background-color:d9d9d9">printf("%lu,",indices[i].index2);<a name="644"></a><a name="IDX-347"></a></span>
            <span style="background-color:d9d9d9">printf("%lu]\n",indices[i].index3);</span>
        <span style="background-color:d9d9d9">}</span>
        <span style="background-color:d9d9d9">else</span>
        <span style="background-color:d9d9d9">{</span>
            <span style="background-color:d9d9d9">printf("indices[%lu]=FREE\n",i);</span>
        <span style="background-color:d9d9d9">}</span>
    <span style="background-color:d9d9d9">}</span>
    <span style="background-color:d9d9d9">return;</span>

<span style="background-color:d9d9d9">}/*end printList-----------------------------------------------*/</span>

<span style="background-color:d9d9d9">void main()</span>
<span style="background-color:d9d9d9">{</span>
    <span style="background-color:d9d9d9">U4 ticks1,ticks2;</span>
    <span style="background-color:d9d9d9">U4 nAllocations=1024;</span>
    <span style="background-color:d9d9d9">U4 i;</span>
    <span style="background-color:d9d9d9">SubAllocator *ptr;</span>
    <span style="background-color:d9d9d9">struct Indices **addr;</span>

    <span style="background-color:d9d9d9">ptr = new SubAllocator(nAllocations);</span>
    <span style="background-color:d9d9d9">addr = (struct Indices**)malloc(nAllocations*sizeof(struct</span>
            <span style="background-color:d9d9d9">Indices*));</span>

    <span style="background-color:d9d9d9">ticks1 = GetTickCount();</span>

    <span style="background-color:d9d9d9">for(i=0;i&lt;nAllocations;i++)</span>
    <span style="background-color:d9d9d9">{</span>
        <span style="background-color:d9d9d9">addr[i] = (*ptr).alloc();</span>
        <span style="background-color:d9d9d9">if(addr[i]==NULL)</span>
        <span style="background-color:d9d9d9">{</span>
            <span style="background-color:d9d9d9">printf("addr[%lu]==NULL\n",i);</span>
            <span style="background-color:d9d9d9">exit(1);</span>
        <span style="background-color:d9d9d9">}</span>
    <span style="background-color:d9d9d9">}</span>

    <span style="background-color:d9d9d9">for(i=0;i&lt;nAllocations;i++)</span>
    <span style="background-color:d9d9d9">{</span>
        <span style="background-color:d9d9d9">(*ptr).release(addr[i]);</span>
    <span style="background-color:d9d9d9">}</span>

    <span style="background-color:d9d9d9">ticks2 = GetTickCount();</span>

    <span style="background-color:d9d9d9">delete(ptr);</span>
    <span style="background-color:d9d9d9">free(addr);</span>

    <span style="background-color:d9d9d9">printf("msecs=%lu\n",ticks2-ticks1);</span>

    <span style="background-color:d9d9d9">return;</span>

<span style="background-color:d9d9d9">}/*end main-----------------------------------------------*/</span><a name="645"></a><a name="IDX-348"></a>
</pre>
</div>
<p class="para">When this application is executed, the following output is produced:</p>
<div class="informalexample">
<pre class="literallayout">
   msecs=0
</pre>
</div>
<p class="para">The allocation and release of 1,024 <span class="fixed">Indices</span> structures took <i class="emphasis">less than a millisecond</i>. This is obviously much faster than anything we have looked at so far.</p>
<p class="last-para">The moral of this story: If you have predictable application behavior, you can tailor a memory manager to exploit that predictability and derive significant performance gains.</p>
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