exp-dhat/docs/dh-manual.xml

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<chapter id="dh-manual"
         xreflabel="DHAT: a dynamic heap analysis tool">
  <title>DHAT: a dynamic heap analysis tool</title>

<para>To use this tool, you must specify
<option>--tool=exp-dhat</option> on the Valgrind
command line.</para>


<sect1 id="dh-manual.overview" xreflabel="Overview">
<title>Overview</title>

<para>DHAT is a tool for examining how programs use their heap
allocations.</para>

<para>It tracks the allocated blocks, and inspects every memory access
to find which block, if any, it is to.  The following data is
collected and presented per allocation point (allocation
stack):</para>

<itemizedlist>
  <listitem><para>Total allocation (number of bytes and
  blocks)</para></listitem>

  <listitem><para>maximum live volume (number of bytes and
  blocks)</para></listitem>

  <listitem><para>average block lifetime (number of instructions
   between allocation and freeing)</para></listitem>

  <listitem><para>average number of reads and writes to each byte in
   the block ("access ratios")</para></listitem>

  <listitem><para>for allocation points which always allocate blocks
   only of one size, and that size is 4096 bytes or less: counts
   showing how often each byte offset inside the block is
   accessed.</para></listitem>
</itemizedlist>

<para>Using these statistics it is possible to identify allocation
points with the following characteristics:</para>

<itemizedlist>

  <listitem><para>potential process-lifetime leaks: blocks allocated
   by the point just accumulate, and are freed only at the end of the
   run.</para></listitem>

 <listitem><para>excessive turnover: points which chew through a lot
  of heap, even if it is not held onto for very long</para></listitem>

 <listitem><para>excessively transient: points which allocate very
 short lived blocks</para></listitem>

 <listitem><para>useless or underused allocations: blocks which are
  allocated but not completely filled in, or are filled in but not
  subsequently read.</para></listitem>

 <listitem><para>blocks with inefficient layout -- areas never
  accessed, or with hot fields scattered throughout the
  block.</para></listitem>
</itemizedlist>

<para>As with the Massif heap profiler, DHAT measures program progress
by counting instructions, and so presents all age/time related figures
as instruction counts.  This sounds a little odd at first, but it
makes runs repeatable in a way which is not possible if CPU time is
used.</para>

</sect1>


<sect1 id="dh-manual.understanding" xreflabel="Understanding DHAT's output">
<title>Understanding DHAT's output</title>


<para>DHAT provides a lot of useful information on dynamic heap usage.
Most of the art of using it is in interpretation of the resulting
numbers.  That is best illustrated via a set of examples.</para>


<sect2>
<title>Interpreting the max-live, tot-alloc and deaths fields</title>

<sect3><title>A simple example</title></sect3>

<screen><![CDATA[
   ======== SUMMARY STATISTICS ========

   guest_insns:  1,045,339,534
   [...]
   max-live:    63,490 in 984 blocks
   tot-alloc:   1,904,700 in 29,520 blocks (avg size 64.52)
   deaths:      29,520, at avg age 22,227,424
   acc-ratios:  6.37 rd, 1.14 wr  (12,141,526 b-read, 2,174,460 b-written)
      at 0x4C275B8: malloc (vg_replace_malloc.c:236)
      by 0x40350E: tcc_malloc (tinycc.c:6712)
      by 0x404580: tok_alloc_new (tinycc.c:7151)
      by 0x40870A: next_nomacro1 (tinycc.c:9305)
]]></screen>

<para>Over the entire run of the program, this stack (allocation
point) allocated 29,520 blocks in total, containing 1,904,700 bytes in
total.  By looking at the max-live data, we see that not many blocks
were simultaneously live, though: at the peak, there were 63,490
allocated bytes in 984 blocks.  This tells us that the program is
steadily freeing such blocks as it runs, rather than hanging on to all
of them until the end and freeing them all.</para>

<para>The deaths entry tells us that 29,520 blocks allocated by this stack
died (were freed) during the run of the program.  Since 29,520 is
also the number of blocks allocated in total, that tells us that
all allocated blocks were freed by the end of the program.</para>

<para>It also tells us that the average age at death was 22,227,424
instructions.  From the summary statistics we see that the program ran
for 1,045,339,534 instructions, and so the average age at death is
about 2% of the program's total run time.</para>

<sect3><title>Example of a potential process-lifetime leak</title></sect3>

<para>This next example (from a different program than the above)
shows a potential process lifetime leak.  A process lifetime leak
occurs when a program keeps allocating data, but only frees the
data just before it exits.  Hence the program's heap grows constantly
in size, yet Memcheck reports no leak, because the program has
freed up everything at exit.  This is particularly a hazard for
long running programs.</para>

<screen><![CDATA[
   ======== SUMMARY STATISTICS ========

   guest_insns:  418,901,537
   [...]
   max-live:    32,512 in 254 blocks
   tot-alloc:   32,512 in 254 blocks (avg size 128.00)
   deaths:      254, at avg age 300,467,389
   acc-ratios:  0.26 rd, 0.20 wr  (8,756 b-read, 6,604 b-written)
      at 0x4C275B8: malloc (vg_replace_malloc.c:236)
      by 0x4C27632: realloc (vg_replace_malloc.c:525)
      by 0x56FF41D: QtFontStyle::pixelSize(unsigned short, bool) (qfontdatabase.cpp:269)
      by 0x5700D69: loadFontConfig() (qfontdatabase_x11.cpp:1146)
]]></screen>

<para>There are two tell-tale signs that this might be a
process-lifetime leak.  Firstly, the max-live and tot-alloc numbers
are identical.  The only way that can happen is if these blocks are
all allocated and then all deallocated.</para>

<para>Secondly, the average age at death (300 million insns) is 71% of
the total program lifetime (419 million insns), hence this is not a
transient allocation-free spike -- rather, it is spread out over a
large part of the entire run.  One interpretation is, roughly, that
all 254 blocks were allocated in the first half of the run, held onto
for the second half, and then freed just before exit.</para>

</sect2>


<sect2>
<title>Interpreting the acc-ratios fields</title>


<sect3><title>A fairly harmless allocation point record</title></sect3>

<screen><![CDATA[
   max-live:    49,398 in 808 blocks
   tot-alloc:   1,481,940 in 24,240 blocks (avg size 61.13)
   deaths:      24,240, at avg age 34,611,026
   acc-ratios:  2.13 rd, 0.91 wr  (3,166,650 b-read, 1,358,820 b-written)
      at 0x4C275B8: malloc (vg_replace_malloc.c:236)
      by 0x40350E: tcc_malloc (tinycc.c:6712)
      by 0x404580: tok_alloc_new (tinycc.c:7151)
      by 0x4046C4: tok_alloc (tinycc.c:7190)
]]></screen>

<para>The acc-ratios field tells us that each byte in the blocks
allocated here is read an average of 2.13 times before the block is
deallocated.  Given that the blocks have an average age at death of
34,611,026, that's one read per block per approximately every 15
million instructions.  So from that standpoint the blocks aren't
"working" very hard.</para>

<para>More interesting is the write ratio: each byte is written an
average of 0.91 times.  This tells us that some parts of the allocated
blocks are never written, at least 9% on average.  To completely
initialise the block would require writing each byte at least once,
and that would give a write ratio of 1.0.  The fact that some block
areas are evidently unused might point to data alignment holes or
other layout inefficiencies.</para>

<para>Well, at least all the blocks are freed (24,240 allocations,
24,240 deaths).</para>

<para>If all the blocks had been the same size, DHAT would also show
the access counts by block offset, so we could see where exactly these
unused areas are.  However, that isn't the case: the blocks have
varying sizes, so DHAT can't perform such an analysis.  We can see
that they must have varying sizes since the average block size, 61.13,
isn't a whole number.</para>


<sect3><title>A more suspicious looking example</title></sect3>

<screen><![CDATA[
   max-live:    180,224 in 22 blocks
   tot-alloc:   180,224 in 22 blocks (avg size 8192.00)
   deaths:      none (none of these blocks were freed)
   acc-ratios:  0.00 rd, 0.00 wr  (0 b-read, 0 b-written)
      at 0x4C275B8: malloc (vg_replace_malloc.c:236)
      by 0x40350E: tcc_malloc (tinycc.c:6712)
      by 0x40369C: __sym_malloc (tinycc.c:6787)
      by 0x403711: sym_malloc (tinycc.c:6805)
]]></screen>

<para>Here, both the read and write access ratios are zero.  Hence
this point is allocating blocks which are never used, neither read nor
written.  Indeed, they are also not freed ("deaths: none") and are
simply leaked.  So, here is 180k of completely useless allocation that
could be removed.</para>

<para>Re-running with Memcheck does indeed report the same leak.  What
DHAT can tell us, that Memcheck can't, is that not only are the blocks
leaked, they are also never used.</para>

<sect3><title>Another suspicious example</title></sect3>

<para>Here's one where blocks are allocated, written to,
but never read from.  We see this immediately from the zero read
access ratio.  They do get freed, though:</para>

<screen><![CDATA[
   max-live:    54 in 3 blocks
   tot-alloc:   1,620 in 90 blocks (avg size 18.00)
   deaths:      90, at avg age 34,558,236
   acc-ratios:  0.00 rd, 1.11 wr  (0 b-read, 1,800 b-written)
      at 0x4C275B8: malloc (vg_replace_malloc.c:236)
      by 0x40350E: tcc_malloc (tinycc.c:6712)
      by 0x4035BD: tcc_strdup (tinycc.c:6750)
      by 0x41FEBB: tcc_add_sysinclude_path (tinycc.c:20931)
]]></screen>

<para>In the previous two examples, it is easy to see blocks that are
never written to, or never read from, or some combination of both.
Unfortunately, in C++ code, the situation is less clear.  That's
because an object's constructor will write to the underlying block,
and its destructor will read from it.  So the block's read and write
ratios will be non-zero even if the object, once constructed, is never
used, but only eventually destructed.</para>

<para>Really, what we want is to measure only memory accesses in
between the end of an object's construction and the start of its
destruction.  Unfortunately I do not know of a reliable way to
determine when those transitions are made.</para>


</sect2>

<sect2>
<title>Interpreting "Aggregated access counts by offset" data</title>

<para>For allocation points that always allocate blocks of the same
size, and which are 4096 bytes or smaller, DHAT counts accesses
per offset, for example:</para>

<screen><![CDATA[
   max-live:    317,408 in 5,668 blocks
   tot-alloc:   317,408 in 5,668 blocks (avg size 56.00)
   deaths:      5,668, at avg age 622,890,597
   acc-ratios:  1.03 rd, 1.28 wr  (327,642 b-read, 408,172 b-written)
      at 0x4C275B8: malloc (vg_replace_malloc.c:236)
      by 0x5440C16: QDesignerPropertySheetPrivate::ensureInfo (qhash.h:515)
      by 0x544350B: QDesignerPropertySheet::setVisible (qdesigner_propertysh...)
      by 0x5446232: QDesignerPropertySheet::QDesignerPropertySheet (qdesigne...)

   Aggregated access counts by offset:

   [   0]  28782 28782 28782 28782 28782 28782 28782 28782
   [   8]  20638 20638 20638 20638 0 0 0 0
   [  16]  22738 22738 22738 22738 22738 22738 22738 22738
   [  24]  6013 6013 6013 6013 6013 6013 6013 6013
   [  32]  18883 18883 18883 37422 0 0 0 0
   [  36]  5668 11915 5668 5668 11336 11336 11336 11336
   [  48]  6166 6166 6166 6166 0 0 0 0
]]></screen>

<para>This is fairly typical, for C++ code running on a 64-bit
platform.  Here, we have aggregated access statistics for 5668 blocks,
all of size 56 bytes.  Each byte has been accessed at least 5668
times, except for offsets 12--15, 36--39 and 52--55.  These are likely
to be alignment holes.</para>

<para>Careful interpretation of the numbers reveals useful information.
Groups of N consecutive identical numbers that begin at an N-aligned
offset, for N being 2, 4 or 8, are likely to indicate an N-byte object
in the structure at that point.  For example, the first 32 bytes of
this object are likely to have the layout</para>

<screen><![CDATA[
   [0 ]  64-bit type
   [8 ]  32-bit type
   [12]  32-bit alignment hole
   [16]  64-bit type
   [24]  64-bit type
]]></screen>

<para>As a counterexample, it's also clear that, whatever is at offset 32,
it is not a 32-bit value.  That's because the last number of the group
(37422) is not the same as the first three (18883 18883 18883).</para>

<para>This example leads one to enquire (by reading the source code)
whether the zeroes at 12--15 and 52--55 are alignment holes, and
whether 48--51 is indeed a 32-bit type.  If so, it might be possible
to place what's at 48--51 at 12--15 instead, which would reduce
the object size from 56 to 48 bytes.</para>

<para>Bear in mind that the above inferences are all only "maybes".  That's
because they are based on dynamic data, not static analysis of the
object layout.  For example, the zeroes might not be alignment
holes, but rather just parts of the structure which were not used
at all for this particular run.  Experience shows that's unlikely
to be the case, but it could happen.</para>

</sect2>

</sect1>


<sect1 id="dh-manual.options" xreflabel="DHAT Command-line Options">
<title>DHAT Command-line Options</title>

<para>DHAT-specific command-line options are:</para>

<!-- start of xi:include in the manpage -->
<variablelist id="dh.opts.list">

  <varlistentry id="opt.show-top-n" xreflabel="--show-top-n">
    <term>
      <option><![CDATA[--show-top-n=<number>
      [default: 10] ]]></option>
    </term>
    <listitem>
      <para>At the end of the run, DHAT sorts the accumulated
       allocation points according to some metric, and shows the
       highest scoring entries.  <varname>--show-top-n</varname>
       controls how many entries are shown.  The default of 10 is
       quite small.  For realistic applications you will probably need
       to set it much higher, at least several hundred.</para>
    </listitem>
  </varlistentry>

  <varlistentry id="opt.sort-by" xreflabel="--sort-by=string">
    <term>
      <option><![CDATA[--sort-by=<string> [default: max-bytes-live] ]]></option>
    </term>
    <listitem>
      <para>At the end of the run, DHAT sorts the accumulated
       allocation points according to some metric, and shows the
       highest scoring entries.  <varname>--sort-by</varname>
       selects the metric used for sorting:</para>
      <para><varname>max-bytes-live   </varname>  maximum live bytes [default]</para>
      <para><varname>tot-bytes-allocd </varname>  bytes allocates in total (turnover)</para>
      <para><varname>max-blocks-live  </varname>  maximum live blocks</para>
      <para><varname>tot-blocks-allocd </varname> blocks allocated in total (turnover)</para>
      <para>This controls the order in which allocation points are
       displayed.  You can choose to look at allocation points with
       the highest number of live bytes, or the highest total byte turnover, or
       by the highest number of live blocks, or the highest total block
       turnover.  These give usefully different pictures of program behaviour.
       For example, sorting by maximum live blocks tends to show up allocation
       points creating large numbers of small objects.</para>
    </listitem>
  </varlistentry>

</variablelist>

<para>One important point to note is that each allocation stack counts
as a separate allocation point.  Because stacks by default have 12
frames, this tends to spread data out over multiple allocation points.
You may want to use the flag --num-callers=4 or some such small
number, to reduce the spreading.</para>

<!-- end of xi:include in the manpage -->

</sect1>

</chapter>