Add article on using WPA to profile rustc memory usage on Windows (#1074)

Document how to use WPA to profile rustc and what the normal workflow
should be for investigating bootstrap memory usage issues.

Co-authored-by: Ryan Levick <ryan.levick@gmail.com>

src/SUMMARY.md

@@ -23,6 +23,7 @@
 - [Debugging the Compiler](./compiler-debugging.md)
 - [Profiling the compiler](./profiling.md)
     - [with the linux perf tool](./profiling/with_perf.md)
+    - [with Windows Performance Analyzer](./profiling/wpa_profiling.md)
 - [crates.io Dependencies](./crates-io.md)
 
 

src/profiling.md

@@ -21,6 +21,10 @@ Depending on what you're trying to measure, there are several different approach
   eg. `cargo -Z timings build`.
   You can use this flag on the compiler itself with `CARGOFLAGS="-Z timings" ./x.py build`
 
+- If you want to profile memory usage, you can use various tools depending on what operating system
+  you are using.
+  - For Windows, read our [WPA guide](profiling/wpa_profiling.html).
+
 ## Optimizing rustc's bootstrap times with `cargo-llvm-lines`
 
 Using [cargo-llvm-lines](https://github.com/dtolnay/cargo-llvm-lines) you can count the

src/profiling/wpa_profiling.md

@@ -0,0 +1,108 @@
+# Profiling on Windows
+
+## Introducing WPR and WPA
+
+High-level performance analysis (including memory usage) can be performed with the Windows
+Performance Recorder (WPR) and Windows Performance Analyzer (WPA). As the names suggest, WPR is for
+recording system statistics (in the form of event trace log a.k.a. ETL files), while WPA is for
+analyzing these ETL files.
+
+WPR collects system wide statistics, so it won't just record things relevant to rustc but also
+everything else that's running on the machine. During analysis, we can filter to just the things we
+find interesting.
+
+These tools are quite powerful but also require a bit of learning
+before we can successfully profile the Rust compiler.
+
+Here we will explore how to use WPR and WPA for analyzing the Rust compiler as well as provide
+links to useful "profiles" (i.e., settings files that tweak the defaults for WPR and WPA) that are
+specifically designed to make analyzing rustc easier.
+
+### Installing WPR and WPA
+
+You can install WPR and WPA as part of the Windows Performance Toolkit which itself is an option as
+part of downloading the Windows Assessment and Deployment Kit (ADK). You can download the ADK
+installer [here](https://go.microsoft.com/fwlink/?linkid=2086042). Make sure to select the Windows
+Performance Toolkit (you don't need to select anything else).
+
+## Recording
+
+In order to perform system analysis, you'll first need to record your system with WPR. Open WPR and
+at the bottom of the window select the "profiles" of the things you want to record. For looking
+into memory usage of the rustc bootstrap process, we'll want to select the following items:
+
+* CPU usage
+* VirtualAlloc usage
+
+You might be tempted to record "Heap usage" as well, but this records every single heap allocation
+and can be very, very expensive. For high-level analysis, it might be best to leave that turned
+off.
+
+Now we need to get our setup ready to record. For memory usage analysis, it is best to record the
+stage 2 compiler build with a stage 1 compiler build with debug symbols. Having symbols in the
+compiler we're using to build rustc will aid our analysis greatly by allowing WPA to resolve Rust
+symbols correctly. Unfortunately, the stage 0 compiler does not have symbols turned on which is why
+we'll need to build a stage 1 compiler and then a stage 2 compiler ourselves.
+
+To do this, make sure you have set `debuginfo-level = 1` in your `config.toml` file. This tells
+rustc to generate debug information which includes stack frames when bootstrapping.
+
+Now you can build the stage 1 compiler: `python x.py build --stage 1 -i library/std` or however
+else you want to build the stage 1 compiler.
+
+Now that the stage 1 compiler is built, we can record the stage 2 build. Go back to WPR, click the
+"start" button and build the stage 2 compiler (e.g., `python x build --stage=2 -i library/std `).
+When this process finishes, stop the recording.
+
+Click the Save button and once that process is complete, click the "Open in WPA" button which
+appears.
+
+> Note: The trace file is fairly large so it can take WPA some time to finish opening the file.
+
+## Analysis
+
+Now that our ETL file is open in WPA, we can analyze the results. First, we'll want to apply the
+pre-made "profile" which will put WPA into a state conducive to analyzing rustc bootstrap. Download
+the profile [here](https://github.com/wesleywiser/rustc-bootstrap-wpa-analysis/releases/download/1/rustc.generic.wpaProfile).
+Select the "Profiles" menu at the top, then "apply" and then choose the downloaded profile.
+
+You should see something resembling the following:
+
+![WPA with profile applied](../img/wpa-initial-memory.png)
+
+Next, we will need to tell WPA to load and process debug symbols so that it can properly demangle
+the Rust stack traces. To do this, click "Trace" and then choose "Load Symbols". This step can take
+a while.
+
+Once WPA has loaded symbols for rustc, we can expand the rustc.exe node and begin drilling down
+into the stack with the largest allocations.
+
+To do that, we'll expand the `[Root]` node in the "Commit Stack" column and continue expanding
+until we find interesting stack frames.
+
+> Tip: After selecting the node you want to expand, press the right arrow key. This will expand the
+node and put the selection on the next largest node in the expanded set. You can continue pressing
+the right arrow key until you reach an interesting frame.
+
+![WPA with expanded stack](../img/wpa-stack.png)
+
+In this sample, you can see calls through codegen are allocating ~30gb of memory in total
+throughout this profile.
+
+## Other Analysis Tabs
+
+The profile also includes a few other tabs which can be helpful:
+
+- System Configuration
+    - General information about the system the capture was recorded on.
+- rustc Build Processes
+    - A flat list of relevant processes such as rustc.exe, cargo.exe, link.exe etc.
+    - Each process lists its command line arguments.
+    - Useful for figuring out what a specific rustc process was working on.
+- rustc Build Process Tree
+    - Timeline showing when processes started and exited.
+- rustc CPU Analysis
+    - Contains charts preconfigured to show hotspots in rustc.
+    - These charts are designed to support analyzing where rustc is spending its time.
+- rustc Memory Analysis
+    - Contains charts preconfigured to show where rustc is allocating memory.