TAO/docs/minimumTAO.html

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   5     <title>Minimum TAO</title>
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  16
  17 <strong>Minimum TAO</strong>
  18 <p>
  19
  20 In addition to our work on <A HREF="../../docs/ACE-subsets.html">ACE
  21 subsetting</A>, we have also been reducing the footprint of TAO.  We
  22 are pursuing two complementary strategies to reduce TAO's footprint:
  23
  24 <OL>
  25
  26 <LI> <B><EM>Implicit subsetting</EM></B>, e.g., by reducing
  27     dependencies in the TAO library so that programs need not link
  28     unused TAO components. <P>
  29
  30 <LI> <B><EM>Explicit subsetting</EM></B>, e.g., by supporting the <a
  31 href="http://www.omg.org/cgi-bin/doc?orbos/98-08-04.pdf"><em>minimumCORBA
  32 </em></a> specification to <a href =
  33 "http://www.dre.vanderbilt.edu/~schmidt/TAO.html">TAO</a>.  The <em>
  34 minimumCORBA </em> specification removes the following features from
  35 the <a href
  36 ="http://www.omg.org/technology/documents/formal/">CORBA</a>
  37 specification. <P>
  38 <ul>
  39 <li>Dynamic Skeleton Interface<br>
  40 <li>Dynamic Invocation Interface<br>
  41 <li>Dynamic Any<br>
  42 <li>Interceptors<br>
  43 <li>Interface Repository<br>
  44 <li>Advanced POA features<br>
  45 <li>CORBA/COM interworking<br>
  46 </ul>
  47 </OL>
  48
  49 The implicit subsetting of TAO requires no explicit application
  50 programmer intervention.  In contrast, to minimize the footprint of
  51 TAO explicitly, you must
  52 <OL>
  53 <LI> <a
  54 href="../../ACE-INSTALL.html">Configure</a> ACE to support only those
  55 components that are required by TAO and <P>
  56 <LI> <a href =
  57 "../../TAO/TAO-INSTALL.html">Configure</a> TAO to only support the
  58 components specified by the <em>minimumCORBA</em> specification.
  59 </OL>
  60
  61 The following two tables show the footprint reduction achievable via
  62 explicit subsetting.  Note that the IDL Compiler column refers to the
  63 code required to collaborate between the IDL compiler and the ORB, and
  64 not to the code for the IDL compiler itself.
  65
  66 <p><hr width=50% align=left> <p>
  67
  68 Note: All measurement are for ACE 5.0 and TAO 1.0 using egcs-2.91.60
  69 on SunOS5.7<P>
  70
  71 The <a
  72 href="../../ACE-INSTALL.html#flags">make
  73 flags</a> options used were: <P>
  74
  75 <code> debug=0 optimize=1 static_libs_only=1</code> <P>
  76
  77 These options translate into:<P>
  78 <UL>
  79 <LI> No debugging
  80 <LI> Optimization is set to -O2
  81 <LI> Static ACE and TAO libraries
  82 <LI> Use of RCS Ids is turned off
  83 </UL>
  84 <p>
  85
  86 To build a TAO static library, if shared libraries are the default,
  87 use <code>make static_libs_only=1</code> (make sure to do this for
  88 ACE, as well).  If you're using recent versions of GNU GCC, you can
  89 use the <A HREF="../../ACE-INSTALL.html#repo">-frepo</A> option, which
  90 typically reduces the footprint by another 25 percent. <P>
  91
  92 <HR>
  93
  94 <h3><a name="Status">ACE+TAO Subsetting Work in Progress</a></h3>
  95
  96 We've been tracking the footprint reduction of ACE+TAO periodically
  97 since April, 2000.  All the statistics are available <A
  98 HREF="http://www.dre.vanderbilt.edu/Stats/footprint.shtml">online</A>.  As the result of
  99 this prior work, we've identified various areas for improvement that
 100 we're now addressing.  For example, the following are the remaining
 101 areas for ACE subsetting:
 102
 103   <ul>
 104     <li><EM><B>Log_Msg decoupling</EM></B> --  We have a good start on this, but it needs work
 105       to finish.  It might be easiest to make an abstract
 106       base class, then have ACE_Log_Msg derive from it.  That way
 107       we could remove the exposure of all the #includes in Log_Msg.cpp
 108       to applications that don't need it.  Another alternative would
 109       be to disable compilation of Log_Msg.cpp when ACE_NLOGGING is
 110       enabled.<p>
 111
 112     <li><EM><B>Higher layer interdependencies</EM></B> -- We haven't
 113     exhaustively tested all possible combinations of subsets.  It's possible
 114       that there are interdependencies between some layers that
 115       we haven't yet identified and removed.<p>
 116
 117     <li><EM><B>Remove reliance on multiple inheritance</EM></B> -- Multiple
 118         inheriance is only used in a few
 119       places in ACE.  This
 120       isn't really a subsetting problem, but is necessary to
 121       fully take advantage of optimizations available on C++ compilers for
 122       embedded systems.<p>
 123   </ul>
 124
 125 We anticipate that these changes should reduce the default size of ACE
 126 by around 100-200 kbytes.<P>
 127
 128 As the effort to reduce TAO's footprint continues, we are planning
 129 several modifications for TAO that should reduce the footprint for
 130 both the full CORBA and minimum CORBA configurations by around 300-400
 131 Kbytes.  The list below contains an estimate of the impact of each one
 132 of these changes, along with the estimated effort to implement them.
 133 </P>
 134
 135 <P>
 136   <TABLE CELLPADDING=4 BORDER=4>
 137     <TR>
 138       <TD>Component</TD><TD>Impact</TD><TD>Effort</TD>
 139       <TD>Description</TD>
 140     </TR>
 141     <TR>
 142       <TD>ACE</TD><TD>14 Kb</TD><TD>4 weeks</TD>
 143         <TD>
 144   Implement a TAO-specific Reactor.
 145   ACE's reactor supports a number of features that TAO does not
 146   require.  Thus, a TAO-specific implementation is an important way to
 147   reduce the footprint.
 148         </TD>
 149     </TR>
 150     <TR>
 151       <TD>ACE</TD><TD>20 Kb</TD><TD>4 weeks</TD>
 152         <TD>
 153   Implement a TAO-specific Service Configurator.
 154   TAO uses the ACE Service Configurator to dynamically configure
 155   its strategies.  In many embedded applications the set of
 156   strategies are selected at design-time, on those platforms it
 157   would be appropriate to disable all the features to
 158   dynamically load components into the ORB.
 159         </TD>
 160     </TR>
 161     <TR>
 162       <TD>TAO</TD><TD>10-15 Kb</TD><TD>1-2 weeks</TD>
 163         <TD>
 164   Eliminate duplicate code due to instantiations of string ->
 165   pointer maps.  TAO uses several such maps, they could be replaced by a
 166   generic version, wrapped with a fully inlined (i.e. zero
 167   footprint) adapter for type-safety.
 168         </TD>
 169     </TR>
 170     <TR>
 171       <TD>TAO</TD><TD>3-10 Kb</TD><TD>1-2 weeks</TD>
 172         <TD>
 173   Make message buffering strategies optional.
 174   TAO supports policy extensions to control the outgoing oneway and AMI
 175   request buffers.  Those policies are not used by all
 176   applications.
 177         </TD>
 178     </TR>
 179     <TR>
 180       <TD>TAO</TD><TD>10 Kb</TD><TD>2 weeks</TD>
 181         <TD>
 182   Make support for multiple ORBs optional.
 183   TAO can support multiple ORBs in the same process, but most
 184   applications only require one.
 185         </TD>
 186     </TR>
 187     <TR>
 188       <TD>TAO</TD><TD>5 Kb</TD><TD>1 week</TD>
 189         <TD>
 190   Move the less common transport muxing and reply waiting
 191   strategies to an optional library.
 192         </TD>
 193     </TR>
 194     <TR>
 195       <TD>ACE+TAO</TD><TD>30 Kb</TD><TD>8 weeks</TD>
 196         <TD>
 197   Decouple ACE (and then TAO) from the
 198   <CODE>ACE_Thread_Manager</CODE> component.
 199   This component is only used in the thread-per-connection
 200   model, if we could decouple it in ACE then TAO could be
 201   modified to only link this component when that concurrency
 202   model is enabled.
 203         </TD>
 204     </TR>
 205     <TR>
 206       <TD>TAO</TD><TD>&gt;50 Kb</TD><TD>6 weeks</TD>
 207         <TD>
 208   Move <CODE>&lt;&lt;=</CODE> and <CODE>&gt;&gt;=</CODE>
 209   operators to separate files.
 210   Currently TAO includes nearly 500 such operators, moving them
 211   to separate files (grouped by component?) would eliminate them
 212   from most applications.
 213         </TD>
 214     </TR>
 215   </TABLE>
 216 </P>
 217
 218 <P>In parallel with the activities described above we are pursuing
 219 other avenues of research to find sources of rarely used or unused
 220 code in ACE+TAO, and to modify the software to eliminate such code.
 221 These activities include the following:</P>
 222
 223 <UL>
 224   <LI><P>
 225   Using profiling tools, such as gprof, Quantify and True Coverage
 226   to find unreachable code, or code only reachable in certain
 227   applications.
 228   </P></LI>
 229   <LI><P>
 230   The code TAO's IDL compiler generates for <CODE>CORBA::Any</CODE>
 231   operators is large, so we are evaluating designs that reduce the impact of the <CODE>CORBA::Any</CODE>
 232   type support.  TAO's IDL compiler already makes that support optional.
 233   However, for applications that require <CODE>Anys</CODE> it may be useful to separate that code
 234   in another file to reduce the size of generated stubs and skeletons,
 235   without losing the opportunity to use more dynamic CORBA invocation
 236   modes.
 237   </P></LI>
 238   <LI><P>
 239   The Notification Service currently depends on the Trading service to
 240   implement the Trader Constraint Language.  We are planning to break that
 241   dependency and factor the TCL parser into a smaller library shared
 242   by both services.
 243   </P></LI>
 244   <LI><P>
 245   TAO still contains features that are rarely or never used.
 246   Examples include the interfaces to query the well-known services and
 247   to dynamically discover the level of security support.  Those
 248   components should only be linked (dynamically) in applications that require them.
 249   </P></LI>
 250   <LI><P>
 251   The support for interceptors currently generated by the IDL compiler
 252   can be partially refactored into common ORB code.
 253   Moreover, we evaluating a new implementation of interceptors
 254   that can be configured dynamically, thereby eliminating the need for
 255   compile-time configuration flags.
 256   </P></LI>
 257   <LI><P>
 258   We are planning to provide compile-time flags to eliminate certain mandatory features
 259   in CORBA that are not used in all applications, such as IOR
 260   parsers (corbaloc, corbaname, etc.).
 261   </P></LI>
 262   <LI><P>
 263   Finally, we will perfom more code inspections to determine if template code can be
 264   refactored into base classes and thereby shared by many objects in
 265   the ACE+TAO implementations.
 266   </P></LI>
 267 </UL>
 268
 269 Although we do not yet have sufficient insights to know how much
 270 footprint reduction these activities will afford, we anticipate these
 271 enhancements could reduce the default footprint of TAO by another
 272 100-200 Kbytes.
 273
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