319 lines
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<main>
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<h1 id="profiling-on-windows"><a class="header" href="#profiling-on-windows">Profiling on Windows</a></h1>
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<h2 id="introducing-wpr-and-wpa"><a class="header" href="#introducing-wpr-and-wpa">Introducing WPR and WPA</a></h2>
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<p>High-level performance analysis (including memory usage) can be performed with the Windows
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Performance Recorder (WPR) and Windows Performance Analyzer (WPA). As the names suggest, WPR is for
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recording system statistics (in the form of event trace log a.k.a. ETL files), while WPA is for
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analyzing these ETL files.</p>
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<p>WPR collects system wide statistics, so it won't just record things relevant to rustc but also
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everything else that's running on the machine. During analysis, we can filter to just the things we
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find interesting.</p>
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<p>These tools are quite powerful but also require a bit of learning
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before we can successfully profile the Rust compiler.</p>
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<p>Here we will explore how to use WPR and WPA for analyzing the Rust compiler as well as provide
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links to useful "profiles" (i.e., settings files that tweak the defaults for WPR and WPA) that are
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specifically designed to make analyzing rustc easier.</p>
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<h3 id="installing-wpr-and-wpa"><a class="header" href="#installing-wpr-and-wpa">Installing WPR and WPA</a></h3>
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<p>You can install WPR and WPA as part of the Windows Performance Toolkit which itself is an option as
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part of downloading the Windows Assessment and Deployment Kit (ADK). You can download the ADK
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installer <a href="https://go.microsoft.com/fwlink/?linkid=2086042">here</a>. Make sure to select the Windows
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Performance Toolkit (you don't need to select anything else).</p>
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<h2 id="recording"><a class="header" href="#recording">Recording</a></h2>
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<p>In order to perform system analysis, you'll first need to record your system with WPR. Open WPR and
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at the bottom of the window select the "profiles" of the things you want to record. For looking
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into memory usage of the rustc bootstrap process, we'll want to select the following items:</p>
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<ul>
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<li>CPU usage</li>
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<li>VirtualAlloc usage</li>
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</ul>
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<p>You might be tempted to record "Heap usage" as well, but this records every single heap allocation
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and can be very, very expensive. For high-level analysis, it might be best to leave that turned
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off.</p>
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<p>Now we need to get our setup ready to record. For memory usage analysis, it is best to record the
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stage 2 compiler build with a stage 1 compiler build with debug symbols. Having symbols in the
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compiler we're using to build rustc will aid our analysis greatly by allowing WPA to resolve Rust
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symbols correctly. Unfortunately, the stage 0 compiler does not have symbols turned on which is why
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we'll need to build a stage 1 compiler and then a stage 2 compiler ourselves.</p>
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<p>To do this, make sure you have set <code>debuginfo-level = 1</code> in your <code>bootstrap.toml</code> file. This tells
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rustc to generate debug information which includes stack frames when bootstrapping.</p>
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<p>Now you can build the stage 1 compiler: <code>x build --stage 1 -i library</code> or however
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else you want to build the stage 1 compiler.</p>
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<p>Now that the stage 1 compiler is built, we can record the stage 2 build. Go back to WPR, click the
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"start" button and build the stage 2 compiler (e.g., <code>x build --stage=2 -i library</code>).
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When this process finishes, stop the recording.</p>
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<p>Click the Save button and once that process is complete, click the "Open in WPA" button which
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appears.</p>
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<blockquote>
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<p>Note: The trace file is fairly large so it can take WPA some time to finish opening the file.</p>
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</blockquote>
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<h2 id="analysis"><a class="header" href="#analysis">Analysis</a></h2>
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<p>Now that our ETL file is open in WPA, we can analyze the results. First, we'll want to apply the
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pre-made "profile" which will put WPA into a state conducive to analyzing rustc bootstrap. Download
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the profile <a href="https://github.com/wesleywiser/rustc-bootstrap-wpa-analysis/releases/download/1/rustc.generic.wpaProfile">here</a>.
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Select the "Profiles" menu at the top, then "apply" and then choose the downloaded profile.</p>
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<p>You should see something resembling the following:</p>
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<p><img src="../img/wpa-initial-memory.png" alt="WPA with profile applied" /></p>
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<p>Next, we will need to tell WPA to load and process debug symbols so that it can properly demangle
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the Rust stack traces. To do this, click "Trace" and then choose "Load Symbols". This step can take
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a while.</p>
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<p>Once WPA has loaded symbols for rustc, we can expand the rustc.exe node and begin drilling down
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into the stack with the largest allocations.</p>
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<p>To do that, we'll expand the <code>[Root]</code> node in the "Commit Stack" column and continue expanding
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until we find interesting stack frames.</p>
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<blockquote>
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<p>Tip: After selecting the node you want to expand, press the right arrow key. This will expand the
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node and put the selection on the next largest node in the expanded set. You can continue pressing
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the right arrow key until you reach an interesting frame.</p>
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</blockquote>
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<p><img src="../img/wpa-stack.png" alt="WPA with expanded stack" /></p>
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<p>In this sample, you can see calls through codegen are allocating ~30gb of memory in total
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throughout this profile.</p>
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<h2 id="other-analysis-tabs"><a class="header" href="#other-analysis-tabs">Other Analysis Tabs</a></h2>
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<p>The profile also includes a few other tabs which can be helpful:</p>
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<ul>
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<li>System Configuration
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<ul>
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<li>General information about the system the capture was recorded on.</li>
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</ul>
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</li>
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<li>rustc Build Processes
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<ul>
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<li>A flat list of relevant processes such as rustc.exe, cargo.exe, link.exe etc.</li>
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<li>Each process lists its command line arguments.</li>
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<li>Useful for figuring out what a specific rustc process was working on.</li>
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</ul>
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</li>
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<li>rustc Build Process Tree
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<ul>
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<li>Timeline showing when processes started and exited.</li>
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</ul>
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</li>
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<li>rustc CPU Analysis
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<ul>
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<li>Contains charts preconfigured to show hotspots in rustc.</li>
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<li>These charts are designed to support analyzing where rustc is spending its time.</li>
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</ul>
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</li>
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<li>rustc Memory Analysis
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<ul>
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<li>Contains charts preconfigured to show where rustc is allocating memory.</li>
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</ul>
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</li>
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</ul>
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