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<i class="icon"><svg><use href="#svg-toc"></use></svg></i>
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</label>
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<section id="optimizing-gnome-applications">
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<h1>Optimizing GNOME Applications<a class="headerlink" href="#optimizing-gnome-applications" title="Permalink to this headline">#</a></h1>
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<section id="what-are-we-optimizing">
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<h2>What are we optimizing?<a class="headerlink" href="#what-are-we-optimizing" title="Permalink to this headline">#</a></h2>
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<p>When we optimize for GNOME the first thing to remember is this: we are not
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trying to make the program better, we are trying to make the person using the
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computer happier.</p>
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<p>Better programs make people happier, but there are some improvements that will
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make them a lot happier than others: Responsiveness, start-up time, easy to
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access commands and not having the computer go into swap the moment more than
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two programs are open.</p>
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<p>Traditional optimization tackles concepts like CPU use, code size, the number of
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mouse clicks and the memory use of the program. This second list has been chosen
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to correlate with the first list, however there is an important difference: the
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person using GNOME doesn’t care about the second list, but they care a lot about
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the first list. When optimizing GNOME programs we will reduce CPU use, memory
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use and all those things, but these are the means to the end, not the final
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goal. We are optimizing for people.</p>
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</section>
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<section id="optimization">
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<h2>Optimization<a class="headerlink" href="#optimization" title="Permalink to this headline">#</a></h2>
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<p>The previous section omitted one important qualifier: to optimize something it
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has to be measurable. You can’t measure happiness. However, you can measure
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start-up time so you can tell if you have improved it. Happiness will then,
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hopefully, follow.</p>
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<p>Optimization is the process of measurement, refinement and re-measurement. So
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the first thing you must do is find a way to measure what you are optimizing.
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Ideally this measurement is a single number, for example: the time taken to
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perform a task. This is your benchmark, it is the only way to tell if you are
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winning or losing. There is a big difference between a program that should be
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fast and a program that is fast.</p>
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<p>Once you have a basic benchmark you need to find out why your code is not doing
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as well as it should. It is tempting to do this by inspection: just looking at
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the code and trying to spot something that looks like it needs improvement. You
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will invariably be wrong. Using a profiler to get a detailed break-down of what
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your program really does is the only way to be sure.</p>
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<p>Usually the problem is isolated to small sections of code. Pick the worst place
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and concentrate on that first. Once that is done, rerun the profiler and repeat.
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As you proceed the gains made at each step will get less and less, at some point
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you will have to decide that the results are good enough. If your efforts are
|
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only extracting 10% improvements then you are well past the point where you
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should have stopped.</p>
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<p>Don’t forget the big picture. For example, rather than just trying to speed up a
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piece of code, ask yourself if it needs to be run at all. Could it be combined
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with another piece of code? Can the results of previous calculations be saved
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and reused? It won’t even need to be optimized if it is in a place where the
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user is never going to notice it. Worse still, the code may already be optimized
|
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and is doing the heavy calculations now to avoid doing them later. Code does not
|
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run in isolation and neither does the optimization process.</p>
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</section>
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<section id="hints">
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<h2>Hints<a class="headerlink" href="#hints" title="Permalink to this headline">#</a></h2>
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<section id="the-fundamentals">
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<h3>The fundamentals<a class="headerlink" href="#the-fundamentals" title="Permalink to this headline">#</a></h3>
|
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<p>1. Re-run your benchmark after every change you make to the code and keep a log
|
||
of everything you change and how it affects the benchmark. This lets you undo
|
||
mistakes and also helps you not to repeat mistakes.</p>
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<p>2. Make sure your code is correct and bug-free before optimizing it. Check that
|
||
it remains correct and bug-free after optimization.</p>
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<ol class="arabic simple" start="3">
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<li><p>Optimize at the high level before optimizing the details.</p></li>
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</ol>
|
||
<p>4. Use the right algorithm. The classic text-book example is using quick-sort
|
||
instead of bubble-sort. There are many others, some save memory, some save CPU.
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||
Also, see what shortcuts you can make: you can do quicker than quick-sort if you
|
||
are prepared to make some compromises.</p>
|
||
<p>5. Optimization is a trade-off. Caching results speeds up calculations, but
|
||
increases memory use. Saving data to disk saves memory, but costs time when it
|
||
is loaded back from disk.</p>
|
||
<p>6. Make sure you choose a wide variety of inputs to optimize against. If you
|
||
don’t it is easy to end up with a piece of code carefully optimized for one file
|
||
and no others.</p>
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<p>7. Avoid expensive operations: Multiple small disk reads. Using up lots of
|
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memory so disk swapping becomes necessary. Avoid anything that writes or reads
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from the hard disk unnecessarily. The network is slow too. Also avoid graphics
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||
operations that need a response from the X server.</p>
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</section>
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<section id="traps-for-the-unwary">
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<h3>Traps for the unwary<a class="headerlink" href="#traps-for-the-unwary" title="Permalink to this headline">#</a></h3>
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||
<p>1. Beware of side effects. There can often be strange interactions between
|
||
different sections of code, a speed-up in one part can slow another part down.</p>
|
||
<p>2. When timing code, even on a quiet system, events outside the program add
|
||
noise to the timing results. Average over multiple runs. If the code is very
|
||
short, timer resolution is also a problem. In this case measure the time the
|
||
computer takes to run the code 100 or 1000 times. If the times you are recording
|
||
are longer than a few seconds, you should be OK.</p>
|
||
<p>3. It is very easy to be misled by the profiler. There are stories of people
|
||
optimizing the operating system idle-loop because that is where it spent all its
|
||
time! Don’t optimize code that does nothing the user cares about.</p>
|
||
<p>4. Remember the resources on the X server. Your program’s memory usage doesn’t
|
||
include the pixmaps that are stored in the X server’s process, but they are
|
||
still using up memory. Use xrestop to see what resources your program is using.</p>
|
||
</section>
|
||
<section id="low-level-hints">
|
||
<h3>Low level hints<a class="headerlink" href="#low-level-hints" title="Permalink to this headline">#</a></h3>
|
||
<p>1. When optimizing memory use, be wary of the difference between peak usage and
|
||
average memory usage. Some memory is almost always allocated, this is usually
|
||
bad. Some is only briefly allocated, this may be quite acceptable. Tools like
|
||
massif use the concept of space-time, the product of memory used and the
|
||
duration it was allocated for, instead.</p>
|
||
<p>2. Time simplified bits of code that do only the things you know are essential,
|
||
this gives an absolute lower limit on the time your code will take. For example,
|
||
when optimizing a loop time the empty loop. If that is still too long no amount
|
||
of micro-optimization will help and you will have to change your design. Make
|
||
sure the compiler doesn’t optimize away your empty loop.</p>
|
||
<p>3. Move code out from inside loops. A slightly more complicated piece of code
|
||
that is executed once is far quicker than a simple piece of code executed a
|
||
thousand times. Avoid calling slow code often.</p>
|
||
<p>4. Give the compiler as many hints as possible. Use the const keyword. Use
|
||
<code class="docutils literal notranslate"><span class="pre">G_INLINE_FUNC</span></code> for short, frequently called, functions. Look up
|
||
<code class="docutils literal notranslate"><span class="pre">G_GNUC_PURE</span></code>, <code class="docutils literal notranslate"><span class="pre">G_LIKELY</span></code> and the other GLib miscellaneous macros. Use the
|
||
macros instead of compiler-specific keywords to ensure portability.</p>
|
||
<p>5. Don’t use assembly language. It is not portable and, while it may be fast on
|
||
one processor, it is not even guaranteed to be fast on every processor that
|
||
supports that architecture.</p>
|
||
<p>6. Don’t rewrite an existing library routine unless you are sure it is
|
||
unnecessarily slow. Many CPU-intensive library routines have already been
|
||
optimized. Conversely, some library routines are slow, especially ones that make
|
||
system calls to the operating system.</p>
|
||
<p>7. Minimize the number of libraries you link to. The fewer libraries to link in,
|
||
the faster the program starts. This can be a difficult thing to do with GNOME.</p>
|
||
</section>
|
||
<section id="high-level-tricks">
|
||
<h3>High level tricks<a class="headerlink" href="#high-level-tricks" title="Permalink to this headline">#</a></h3>
|
||
<p>1. Take advantage of concurrency. This doesn’t just mean using multiple
|
||
processors, it also means taking advantage of the time the user spends thinking
|
||
about what they are going to do next to perform some calculations in
|
||
anticipation. Do calculations while waiting for data to be loaded off disk. Take
|
||
advantage of multiple resources, use them all at once.</p>
|
||
<p>2. Cheat. The user only has to think that the computer is fast, it doesn’t
|
||
matter whether it actually is or not. It is the time between the command and the
|
||
answer that is important, it doesn’t matter if the response is pre-calculated,
|
||
cached, or will in fact be worked out later at a more convenient time, as long
|
||
as the user gets what they expect.</p>
|
||
<p>3. Do things in the idle loop. It is easier to program than using full
|
||
multi-threading but still gets things done out of the users eye. Be careful
|
||
though, if you spend too long in the idle loop your program will become
|
||
sluggish. So regularly give control back to the main loop.</p>
|
||
<p>4. If all else fails, tell the user that the code is going to be slow and put up
|
||
a progress bar. They won’t be as happy as if you had just presented the results,
|
||
but they will at least know the program hasn’t crashed and they can go get a cup
|
||
of coffee.</p>
|
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</section>
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Contents
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<div class="toc-tree">
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<ul>
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<li><a class="reference internal" href="#">Optimizing GNOME Applications</a><ul>
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<li><a class="reference internal" href="#what-are-we-optimizing">What are we optimizing?</a></li>
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<li><a class="reference internal" href="#optimization">Optimization</a></li>
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<li><a class="reference internal" href="#hints">Hints</a><ul>
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<li><a class="reference internal" href="#the-fundamentals">The fundamentals</a></li>
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<li><a class="reference internal" href="#traps-for-the-unwary">Traps for the unwary</a></li>
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<li><a class="reference internal" href="#low-level-hints">Low level hints</a></li>
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<li><a class="reference internal" href="#high-level-tricks">High level tricks</a></li>
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