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<div class="section" id="debugging-with-the-memory-pool-system">
<span id="guide-debug"></span><span id="index-0"></span><h1>4. Debugging with the Memory Pool System<a class="headerlink" href="#debugging-with-the-memory-pool-system" title="Permalink to this headline"></a></h1>
<p>Memory management errors are some of the most stubborn and difficult
to track down, because the effect so often appears at a distant point
in the program that is seemingly unrelated to the cause, and by the
time the error is revealed, the information needed to reconstruct the
cause has long vanished. Immediately after an <a class="reference internal" href="../glossary/o.html#term-overwriting-error"><em class="xref std std-term">overwriting
error</em></a>, the block that overran its bounds is fine, and the block that
was overwritten may not be visited for a long time. A failure to
<a class="reference internal" href="../glossary/f.html#term-fix"><em class="xref std std-term">fix</em></a> a <a class="reference internal" href="../glossary/r.html#term-reference"><em class="xref std std-term">reference</em></a> does not necessarily cause the object
pointed to by the missed reference to die immediately: there may be
other references to that object, or a garbage collection may be
delayed. And even if it does die, the space it occupies may not be
re-allocated for some time.</p>
<div class="section" id="general-debugging-advice">
<span id="guide-debug-advice"></span><h2>4.1. General debugging advice<a class="headerlink" href="#general-debugging-advice" title="Permalink to this headline"></a></h2>
<ol class="arabic">
<li><p class="first">Compile with debugging information turned on (<tt class="docutils literal"><span class="pre">-g</span></tt> on the GCC or
Clang command line).</p>
</li>
<li><p id="index-1">Build the <a class="reference internal" href="../glossary/c.html#term-cool"><em class="xref std std-term">cool</em></a> <a class="reference internal" href="../glossary/v.html#term-variety"><em class="xref std std-term">variety</em></a> of the MPS (by defining the
preprocessor constant <tt class="docutils literal"><span class="pre">CONFIG_VAR_COOL</span></tt>, for example by setting
<tt class="docutils literal"><span class="pre">-DCONFIG_VAR_COOL</span></tt> on the GCC or Clang command line). This
variety contains many internal consistency checks (including such
checks on the <a class="reference internal" href="../glossary/c.html#term-critical-path"><em class="xref std std-term">critical path</em></a>, which make it too slow for use
in production), and can generate profiling output in the form of
the <a class="reference internal" href="../glossary/t.html#term-telemetry-stream"><em class="xref std std-term">telemetry stream</em></a>.</p>
</li>
<li><p id="index-2">Prepare a reproducible test case if possible. The MPS may be
<a class="reference internal" href="../glossary/a.html#term-asynchronous-garbage-collector"><em class="xref std std-term">asynchronous</em></a>, but it is
deterministic, so in single-threaded applications you should be
able to get consistent results. (But you need to beware of <a class="reference external" href="http://en.wikipedia.org/wiki/Address_space_layout_randomization">address
space layout randomization</a>: if you perform computation based on
the addresses of objects, for example, hashing objects by their
address, then ASLR will cause your hash tables to be laid out
differently on each run, which may affect the order of memory
management operations.)</p>
<p>A fact that assists with reproducibility is that the more
frequently the collector runs, the sooner and more reliably errors
are discovered. So if you have a bug that&#8217;s hard to reproduce, or
which manifests itself in different ways on different runs, you may
be able to provoke it more reliably, or get a more consistent
result, by having a mode for testing in which you run frequent
collections (by calling <a class="reference internal" href="../topic/arena.html#mps_arena_collect" title="mps_arena_collect"><tt class="xref c c-func docutils literal"><span class="pre">mps_arena_collect()</span></tt></a> followed by
<a class="reference internal" href="../topic/arena.html#mps_arena_release" title="mps_arena_release"><tt class="xref c c-func docutils literal"><span class="pre">mps_arena_release()</span></tt></a>), perhaps as frequently as every
allocation.</p>
</li>
<li><p id="index-3">Run your test case inside the debugger. Use <tt class="docutils literal"><span class="pre">assert</span></tt> and
<tt class="docutils literal"><span class="pre">abort</span></tt> in your error handler (rather than <tt class="docutils literal"><span class="pre">exit</span></tt>) so that you
can enter the debugger with the contents of the control stack
available for inspection.</p>
<p>You may need to make sure that the debugger isn&#8217;t entered on
<a class="reference internal" href="../glossary/b.html#term-barrier-1"><em class="xref std std-term">barrier<sup>(1)</sup></em></a> hits (because the MPS uses barriers to protect
parts of memory, and barrier hits are common and expected).</p>
<p>If you are using GDB on OS X, run these commands:</p>
<div class="highlight-none"><div class="highlight"><pre>set dont-handle-bad-access 1
handle SIGBUS pass nostop noprint
</pre></div>
</div>
<p>If you are using GDB on Linux, run this command:</p>
<div class="highlight-none"><div class="highlight"><pre>handle SIGXFSZ pass nostop noprint
</pre></div>
</div>
<p>(On either operating system, you can add these commands to your
<tt class="docutils literal"><span class="pre">.gdbinit</span></tt> if you always want them to be run.)</p>
</li>
</ol>
</div>
<div class="section" id="example-underscanning">
<span id="guide-debug-underscanning"></span><span id="index-4"></span><h2>4.2. Example: underscanning<a class="headerlink" href="#example-underscanning" title="Permalink to this headline"></a></h2>
<p>An easy mistake to make is to omit to <a class="reference internal" href="../glossary/f.html#term-fix"><em class="xref std std-term">fix</em></a> a <a class="reference internal" href="../glossary/r.html#term-reference"><em class="xref std std-term">reference</em></a>
when <a class="reference internal" href="../glossary/s.html#term-scan"><em class="xref std std-term">scanning</em></a> a <a class="reference internal" href="../glossary/f.html#term-formatted-object"><em class="xref std std-term">formatted object</em></a>. For example,
in the Scheme interpreter&#8217;s <a class="reference internal" href="lang.html#guide-lang-scan"><em>scan method</em></a>, I
might have forgotten to fix the first element of a pair:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="k">case</span> <span class="n">TYPE_PAIR</span>:
<span class="hll"> <span class="cm">/* oops, forgot: FIX(CAR(obj)); */</span>
</span> <span class="n">FIX</span><span class="p">(</span><span class="n">CDR</span><span class="p">(</span><span class="n">obj</span><span class="p">));</span>
<span class="n">base</span> <span class="o">=</span> <span class="p">(</span><span class="kt">char</span> <span class="o">*</span><span class="p">)</span><span class="n">base</span> <span class="o">+</span> <span class="n">ALIGN</span><span class="p">(</span><span class="k">sizeof</span><span class="p">(</span><span class="n">pair_s</span><span class="p">));</span>
<span class="k">break</span><span class="p">;</span>
</pre></div>
</div>
<p>This means that as far as the MPS is concerned, the first element of
the pair is <a class="reference internal" href="../glossary/u.html#term-unreachable"><em class="xref std std-term">unreachable</em></a> and so <a class="reference internal" href="../glossary/d.html#term-dead"><em class="xref std std-term">dead</em></a>, so after
collecting the region of memory containing this object, the space will
be reused for other objects. So <tt class="docutils literal"><span class="pre">CAR(obj)</span></tt> might end up pointing to
the start of a valid object (but the wrong one), or to the middle of a
valid object, or to an unused region of memory, or into an MPS
internal control structure.</p>
<p>The reproducible test case is simple. Run a garbage collection by
calling <tt class="docutils literal"><span class="pre">(gc)</span></tt> and then evaluate any expression:</p>
<div class="highlight-none"><div class="highlight"><pre>$ gdb ./scheme
GNU gdb 6.3.50-20050815 (Apple version gdb-1820) (Sat Jun 16 02:40:11 UTC 2012)
(gdb) run
Starting program: example/scheme/scheme
Reading symbols for shared libraries +............................. done
MPS Toy Scheme Example
7944, 0&gt; (gc)
Collection started.
Why: Client requests: immediate full collection.
Clock: 11357
Collection finished.
live 1888
condemned 7968
not_condemned 0
clock: 12008
7968, 1&gt; foo
Assertion failed: (TYPE(frame) == TYPE_PAIR), function lookup_in_frame, file scheme.c, line 1065.
Program received signal SIGABRT, Aborted.
0x00007fff91aeed46 in __kill ()
</pre></div>
</div>
<p>What&#8217;s going on?</p>
<div class="highlight-none"><div class="highlight"><pre>(gdb) backtrace
#0 0x00007fff91aeed46 in __kill ()
#1 0x00007fff90509df0 in abort ()
#2 0x00007fff9050ae2a in __assert_rtn ()
#3 0x0000000100003f55 in lookup_in_frame (frame=0x1003fa7d0, symbol=0x1003faf20) at scheme.c:1066
#4 0x0000000100003ea6 in lookup (env=0x1003fb130, symbol=0x1003faf20) at scheme.c:1087
#5 0x000000010000341f in eval (env=0x1003fb130, op_env=0x1003fb148, exp=0x1003faf20) at scheme.c:1135
#6 0x000000010000261b in start (p=0x0, s=0) at scheme.c:3204
#7 0x0000000100011ded in ProtTramp (resultReturn=0x7fff5fbff7d0, f=0x100002130 &lt;start&gt;, p=0x0, s=0) at protix.c:132
#8 0x0000000100001ef7 in main (argc=1, argv=0x7fff5fbff830) at scheme.c:3314
(gdb) frame 4
#4 0x0000000100003ea6 in lookup (env=0x1003fb130, symbol=0x1003faf20) at scheme.c:1087
1086 binding = lookup_in_frame(CAR(env), symbol);
(gdb) print (char *)symbol-&gt;symbol.string
$1 = 0x1003faf30 &quot;foo&quot;
</pre></div>
</div>
<p>The backtrace shows that the interpreter is in the middle of looking
up the symbol <tt class="docutils literal"><span class="pre">foo</span></tt> in the environment. The Scheme interpreter
implements the environment as a list of <em>frames</em>, each of which is a
list of <em>bindings</em>, each binding being a pair of a symbol and its
value, as shown here:</p>
<div class="figure align-center">
<img alt="Diagram: The environment data structure in the Scheme interpreter." src="../_images/scheme-env.svg" /><p class="caption">The environment data structure in the Scheme interpreter.</p>
</div>
<p>In this case, because the evaluation is taking place at top level,
there is only one frame in the environment (the global frame). And
it&#8217;s this frame that&#8217;s corrupt:</p>
<div class="highlight-none"><div class="highlight"><pre>(gdb) frame 3
#3 0x0000000100003f55 in lookup_in_frame (frame=0x1003fa7d0, symbol=0x1003faf20) at scheme.c:1066
1066 assert(TYPE(frame) == TYPE_PAIR);
(gdb) list
1061 */
1062
1063 static obj_t lookup_in_frame(obj_t frame, obj_t symbol)
1064 {
1065 while(frame != obj_empty) {
<span class="hll">1066 assert(TYPE(frame) == TYPE_PAIR);
</span>1067 assert(TYPE(CAR(frame)) == TYPE_PAIR);
1068 assert(TYPE(CAAR(frame)) == TYPE_SYMBOL);
1069 if(CAAR(frame) == symbol)
1070 return CAR(frame);
(gdb) print frame-&gt;type.type
$2 = 13
</pre></div>
</div>
<p>The number 13 is the value <tt class="docutils literal"><span class="pre">TYPE_PAD</span></tt>. So instead of the expected
pair, <tt class="docutils literal"><span class="pre">frame</span></tt> points to a <a class="reference internal" href="../glossary/p.html#term-padding-object"><em class="xref std std-term">padding object</em></a>.</p>
<p>You might guess at this point that the frame had not been fixed, and
since you know that the frame is referenced by the <tt class="docutils literal"><span class="pre">car</span></tt> of the
first pair in the environment, that&#8217;s the suspect reference. But in a
more complex situation this might not yet be clear. In such a
situation it can be useful to look at the sequence of events leading
up to the detection of the error. See <a class="reference internal" href="../topic/telemetry.html#topic-telemetry"><em>Telemetry</em></a>.</p>
</div>
<div class="section" id="example-allocating-with-wrong-size">
<span id="guide-debug-size"></span><span id="index-5"></span><h2>4.3. Example: allocating with wrong size<a class="headerlink" href="#example-allocating-with-wrong-size" title="Permalink to this headline"></a></h2>
<p>Here&#8217;s another kind of mistake: an off-by-one error in <tt class="docutils literal"><span class="pre">make_string</span></tt>
leading to the allocation of string objects with the wrong size:</p>
<div class="highlight-c"><div class="highlight"><pre><span class="k">static</span> <span class="n">obj_t</span> <span class="nf">make_string</span><span class="p">(</span><span class="kt">size_t</span> <span class="n">length</span><span class="p">,</span> <span class="kt">char</span> <span class="o">*</span><span class="n">string</span><span class="p">)</span>
<span class="p">{</span>
<span class="n">obj_t</span> <span class="n">obj</span><span class="p">;</span>
<span class="n">mps_addr_t</span> <span class="n">addr</span><span class="p">;</span>
<span class="hll"> <span class="kt">size_t</span> <span class="n">size</span> <span class="o">=</span> <span class="n">ALIGN</span><span class="p">(</span><span class="n">offsetof</span><span class="p">(</span><span class="n">string_s</span><span class="p">,</span> <span class="n">string</span><span class="p">)</span> <span class="o">+</span> <span class="n">length</span><span class="cm">/* oops, forgot: +1 */</span><span class="p">);</span>
</span> <span class="k">do</span> <span class="p">{</span>
<span class="n">mps_res_t</span> <span class="n">res</span> <span class="o">=</span> <span class="n">mps_reserve</span><span class="p">(</span><span class="o">&amp;</span><span class="n">addr</span><span class="p">,</span> <span class="n">obj_ap</span><span class="p">,</span> <span class="n">size</span><span class="p">);</span>
<span class="k">if</span> <span class="p">(</span><span class="n">res</span> <span class="o">!=</span> <span class="n">MPS_RES_OK</span><span class="p">)</span> <span class="n">error</span><span class="p">(</span><span class="s">&quot;out of memory in make_string&quot;</span><span class="p">);</span>
<span class="n">obj</span> <span class="o">=</span> <span class="n">addr</span><span class="p">;</span>
<span class="n">obj</span><span class="o">-&gt;</span><span class="n">string</span><span class="p">.</span><span class="n">type</span> <span class="o">=</span> <span class="n">TYPE_STRING</span><span class="p">;</span>
<span class="n">obj</span><span class="o">-&gt;</span><span class="n">string</span><span class="p">.</span><span class="n">length</span> <span class="o">=</span> <span class="n">length</span><span class="p">;</span>
<span class="k">if</span> <span class="p">(</span><span class="n">string</span><span class="p">)</span> <span class="n">memcpy</span><span class="p">(</span><span class="n">obj</span><span class="o">-&gt;</span><span class="n">string</span><span class="p">.</span><span class="n">string</span><span class="p">,</span> <span class="n">string</span><span class="p">,</span> <span class="n">length</span><span class="o">+</span><span class="mi">1</span><span class="p">);</span>
<span class="k">else</span> <span class="n">memset</span><span class="p">(</span><span class="n">obj</span><span class="o">-&gt;</span><span class="n">string</span><span class="p">.</span><span class="n">string</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="n">length</span><span class="o">+</span><span class="mi">1</span><span class="p">);</span>
<span class="p">}</span> <span class="k">while</span><span class="p">(</span><span class="o">!</span><span class="n">mps_commit</span><span class="p">(</span><span class="n">obj_ap</span><span class="p">,</span> <span class="n">addr</span><span class="p">,</span> <span class="n">size</span><span class="p">));</span>
<span class="n">total</span> <span class="o">+=</span> <span class="n">size</span><span class="p">;</span>
<span class="k">return</span> <span class="n">obj</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Here&#8217;s a test case that exercises this bug:</p>
<div class="highlight-scheme"><div class="highlight"><pre><span class="p">(</span><span class="k">define </span><span class="p">(</span><span class="nf">church</span> <span class="nv">n</span> <span class="nv">f</span> <span class="nv">a</span><span class="p">)</span> <span class="p">(</span><span class="k">if </span><span class="p">(</span><span class="nb">eqv? </span><span class="nv">n</span> <span class="mi">0</span><span class="p">)</span> <span class="nv">a</span> <span class="p">(</span><span class="nf">church</span> <span class="p">(</span><span class="nb">- </span><span class="nv">n</span> <span class="mi">1</span><span class="p">)</span> <span class="nv">f</span> <span class="p">(</span><span class="nf">f</span> <span class="nv">a</span><span class="p">))))</span>
<span class="p">(</span><span class="nf">church</span> <span class="mi">1000</span> <span class="p">(</span><span class="k">lambda </span><span class="p">(</span><span class="nf">s</span><span class="p">)</span> <span class="p">(</span><span class="nb">string-append </span><span class="nv">s</span> <span class="s">&quot;x&quot;</span><span class="p">))</span> <span class="s">&quot;&quot;</span><span class="p">)</span>
</pre></div>
</div>
<p>And here&#8217;s how it shows up in the debugger:</p>
<div class="highlight-none"><div class="highlight"><pre>$ gdb ./scheme
GNU gdb 6.3.50-20050815 (Apple version gdb-1820) (Sat Jun 16 02:40:11 UTC 2012)
[...]
(gdb) run &lt; test.scm
Starting program: example/scheme/scheme &lt; test.scm
Reading symbols for shared libraries +............................. done
MPS Toy Scheme Example
[...]
9960, 0&gt; church
Assertion failed: (0), function obj_skip, file scheme.c, line 2940.
10816, 0&gt;
Program received signal SIGABRT, Aborted.
0x00007fff91aeed46 in __kill ()
(gdb) backtrace
#0 0x00007fff91aeed46 in __kill ()
#1 0x00007fff90509df0 in abort ()
#2 0x00007fff9050ae2a in __assert_rtn ()
#3 0x00000001000014e3 in obj_skip (base=0x1003f9b88) at scheme.c:2940
#4 0x0000000100068050 in amcScanNailedOnce (totalReturn=0x7fff5fbfef2c, moreReturn=0x7fff5fbfef28, ss=0x7fff5fbff0a0, pool=0x1003fe278, seg=0x1003fe928, amc=0x1003fe278) at poolamc.c:1485
#5 0x0000000100067ca1 in amcScanNailed (totalReturn=0x7fff5fbff174, ss=0x7fff5fbff0a0, pool=0x1003fe278, seg=0x1003fe928, amc=0x1003fe278) at poolamc.c:1522
#6 0x000000010006631f in AMCScan (totalReturn=0x7fff5fbff174, ss=0x7fff5fbff0a0, pool=0x1003fe278, seg=0x1003fe928) at poolamc.c:1595
#7 0x000000010002686d in PoolScan (totalReturn=0x7fff5fbff174, ss=0x7fff5fbff0a0, pool=0x1003fe278, seg=0x1003fe928) at pool.c:405
#8 0x0000000100074106 in traceScanSegRes (ts=1, rank=1, arena=0x10012a000, seg=0x1003fe928) at trace.c:1162
#9 0x000000010002b399 in traceScanSeg (ts=1, rank=1, arena=0x10012a000, seg=0x1003fe928) at trace.c:1222
#10 0x000000010002d020 in TraceQuantum (trace=0x10012a5a0) at trace.c:1833
#11 0x000000010001f2d2 in TracePoll (globals=0x10012a000) at trace.c:1981
#12 0x000000010000d75f in ArenaPoll (globals=0x10012a000) at global.c:684
#13 0x000000010000ea40 in mps_ap_fill (p_o=0x7fff5fbff3e0, mps_ap=0x1003fe820, size=208) at mpsi.c:961
#14 0x000000010000447d in make_string (length=190, string=0x0) at scheme.c:468
#15 0x0000000100008ca2 in entry_string_append (env=0x1003cbe38, op_env=0x1003cbe50, operator=0x1003fad48, operands=0x1003f9af8) at scheme.c:2562
#16 0x0000000100002fe4 in eval (env=0x1003cbe38, op_env=0x1003cbe50, exp=0x1003f9ae0) at scheme.c:1159
#17 0x0000000100005ff5 in entry_interpret (env=0x1003cb958, op_env=0x1003cb970, operator=0x1003f99d8, operands=0x1003f9948) at scheme.c:1340
#18 0x0000000100002fe4 in eval (env=0x1003cb958, op_env=0x1003cb970, exp=0x1003f9878) at scheme.c:1159
#19 0x000000010000206b in start (p=0x0, s=0) at scheme.c:3213
#20 0x000000010001287d in ProtTramp (resultReturn=0x7fff5fbff7a0, f=0x100001b80 &lt;start&gt;, p=0x0, s=0) at protix.c:132
#21 0x0000000100001947 in main (argc=1, argv=0x7fff5fbff808) at scheme.c:3314
(gdb) frame 3
#3 0x00000001000014e3 in obj_skip (base=0x1003f9b88) at scheme.c:2940
2940 assert(0);
(gdb) list
2935 break;
2936 case TYPE_PAD1:
2937 base = (char *)base + ALIGN(sizeof(pad1_s));
2938 break;
2939 default:
2940 assert(0);
<span class="hll">2941 fprintf(stderr, &quot;Unexpected object on the heap\n&quot;);
</span>2942 abort();
2943 return NULL;
2944 }
</pre></div>
</div>
<p>The object being skipped is corrupt:</p>
<div class="highlight-none"><div class="highlight"><pre>(gdb) print obj-&gt;type.type
$1 = 4168560
</pre></div>
</div>
<p>What happened to it? It&#8217;s often helpful in these situations to have a
look at nearby memory.</p>
<div class="highlight-none"><div class="highlight"><pre>(gdb) x/20g obj
0x1003f9b88: 0x00000001003f9b70 0x00000001003fb000
0x1003f9b98: 0x0000000000000000 0x00000001003f9c90
0x1003f9ba8: 0x00000001003fb130 0x0000000000000000
0x1003f9bb8: 0x00000001003fb000 0x00000001003fb148
0x1003f9bc8: 0x0000000000000000 0x00000001003f9730
0x1003f9bd8: 0x00000001003f9a58 0x0000000000000000
0x1003f9be8: 0x00000001003f9bc8 0x00000001003fb000
0x1003f9bf8: 0x0000000000000000 0x00000001003fb0a0
0x1003f9c08: 0x00000001003f9b40 0x0000000000000004
0x1003f9c18: 0x000000010007b14a 0x0000000100005e30
</pre></div>
</div>
<p>You can see that this is a block containing mostly pairs (which have
tag 0 and consist of three words), though you can see an operator
(with tag 4) near the bottom. But what&#8217;s that at the start of the
block, where <tt class="docutils literal"><span class="pre">obj</span></tt>&#8216;s tag should be? It looks like a pointer. So
what&#8217;s in the memory just below <tt class="docutils literal"><span class="pre">obj</span></tt>? Let&#8217;s look at the previous
few words:</p>
<div class="highlight-none"><div class="highlight"><pre>(gdb) x/10g (mps_word_t*)obj-8
0x1003f9b48: 0x00000001003f9ae0 0x00000001003fb000
0x1003f9b58: 0x0000000000000000 0x00000001003f9a80
0x1003f9b68: 0x00000001003f9b80 0x0000000000000005
0x1003f9b78: 0x0000000000000000 0x0000000000000000
0x1003f9b88: 0x00000001003f9b70 0x00000001003fb000
</pre></div>
</div>
<p>Yes: there&#8217;s a pair (with tag 0) at <tt class="docutils literal"><span class="pre">0x1003f9b80</span></tt>. So it looks as
though the previous object was allocated with one size, but skipped
with a different size. The previous object being the string (with tag
5) at <tt class="docutils literal"><span class="pre">0x1003f9b70</span></tt> which has length 0 and so is three words long as
far as <tt class="docutils literal"><span class="pre">obj_skip</span></tt> is concerned:</p>
<div class="highlight-none"><div class="highlight"><pre>(gdb) print obj_skip(0x1003f9b70)
$2 = (mps_addr_t) 0x1003f9b88
</pre></div>
</div>
<p>but the next object (the pair) was clearly allocated at
<tt class="docutils literal"><span class="pre">0x1003f9b80</span></tt> (overwriting the last word of the string), so the
string must have been allocated with a size of only two words. This
should be enough evidence to track down the cause.</p>
</div>
<div class="section" id="what-next">
<h2>4.4. What next?<a class="headerlink" href="#what-next" title="Permalink to this headline"></a></h2>
<p>If you tracked down all your bugs, then the next step is the chapter
<a class="reference internal" href="perf.html#guide-perf"><em>Tuning the Memory Pool System for performance</em></a>.</p>
<p>But if you&#8217;re still struggling, please <a class="reference internal" href="../contact.html#contact"><em>contact us</em></a> and
see if we can help.</p>
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<ul>
<li><a class="reference internal" href="#">4. Debugging with the Memory Pool System</a><ul>
<li><a class="reference internal" href="#general-debugging-advice">4.1. General debugging advice</a></li>
<li><a class="reference internal" href="#example-underscanning">4.2. Example: underscanning</a></li>
<li><a class="reference internal" href="#example-allocating-with-wrong-size">4.3. Example: allocating with wrong size</a></li>
<li><a class="reference internal" href="#what-next">4.4. What next?</a></li>
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<h4>Issues</h4>
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