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| <h1 id="profiling-with-perf"><a class="header" href="#profiling-with-perf">Profiling with perf</a></h1> |
| <p>This is a guide for how to profile rustc with <a href="https://perf.wiki.kernel.org/index.php/Main_Page">perf</a>.</p> |
| <h2 id="initial-steps"><a class="header" href="#initial-steps">Initial steps</a></h2> |
| <ul> |
| <li>Get a clean checkout of rust-lang/rust</li> |
| <li>Set the following settings in your <code>bootstrap.toml</code>: |
| <ul> |
| <li><code>rust.debuginfo-level = 1</code> - enables line debuginfo</li> |
| <li><code>rust.jemalloc = false</code> - lets you do memory use profiling with valgrind</li> |
| <li>leave everything else the defaults</li> |
| </ul> |
| </li> |
| <li>Run <code>./x build</code> to get a full build</li> |
| <li>Make a rustup toolchain pointing to that result |
| <ul> |
| <li>see <a href="../building/how-to-build-and-run.html#toolchain">the “build and run” section for instructions</a></li> |
| </ul> |
| </li> |
| </ul> |
| <h2 id="gathering-a-perf-profile"><a class="header" href="#gathering-a-perf-profile">Gathering a perf profile</a></h2> |
| <p>perf is an excellent tool on linux that can be used to gather and |
| analyze all kinds of information. Mostly it is used to figure out |
| where a program spends its time. It can also be used for other sorts |
| of events, though, like cache misses and so forth.</p> |
| <h3 id="the-basics"><a class="header" href="#the-basics">The basics</a></h3> |
| <p>The basic <code>perf</code> command is this:</p> |
| <pre><code class="language-bash">perf record -F99 --call-graph dwarf XXX |
| </code></pre> |
| <p>The <code>-F99</code> tells perf to sample at 99 Hz, which avoids generating too |
| much data for longer runs (why 99 Hz you ask? It is often chosen |
| because it is unlikely to be in lockstep with other periodic |
| activity). The <code>--call-graph dwarf</code> tells perf to get call-graph |
| information from debuginfo, which is accurate. The <code>XXX</code> is the |
| command you want to profile. So, for example, you might do:</p> |
| <pre><code class="language-bash">perf record -F99 --call-graph dwarf cargo +<toolchain> rustc |
| </code></pre> |
| <p>to run <code>cargo</code> – here <code><toolchain></code> should be the name of the toolchain |
| you made in the beginning. But there are some things to be aware of:</p> |
| <ul> |
| <li>You probably don’t want to profile the time spend building |
| dependencies. So something like <code>cargo build; cargo clean -p $C</code> may |
| be helpful (where <code>$C</code> is the crate name) |
| <ul> |
| <li>Though usually I just do <code>touch src/lib.rs</code> and rebuild instead. =)</li> |
| </ul> |
| </li> |
| <li>You probably don’t want incremental messing about with your |
| profile. So something like <code>CARGO_INCREMENTAL=0</code> can be helpful.</li> |
| </ul> |
| <p>In case to avoid the issue of <code>addr2line xxx/elf: could not read first record</code> when reading |
| collected data from <code>cargo</code>, you may need use the latest version of <code>addr2line</code>:</p> |
| <pre><code class="language-bash">cargo install addr2line --features="bin" |
| </code></pre> |
| <h3 id="gathering-a-perf-profile-from-a-perfrust-langorg-test"><a class="header" href="#gathering-a-perf-profile-from-a-perfrust-langorg-test">Gathering a perf profile from a <code>perf.rust-lang.org</code> test</a></h3> |
| <p>Often we want to analyze a specific test from <code>perf.rust-lang.org</code>. |
| The easiest way to do that is to use the <a href="https://github.com/rust-lang/rustc-perf">rustc-perf</a> |
| benchmarking suite, this approach is described <a href="with_rustc_perf.html">here</a>.</p> |
| <p>Instead of using the benchmark suite CLI, you can also profile the benchmarks manually. First, |
| you need to clone the <a href="https://github.com/rust-lang/rustc-perf">rustc-perf</a> repository:</p> |
| <pre><code class="language-bash">$ git clone https://github.com/rust-lang/rustc-perf |
| </code></pre> |
| <p>and then find the source code of the test that you want to profile. Sources for the tests |
| are found in <a href="https://github.com/rust-lang/rustc-perf/tree/master/collector/compile-benchmarks">the <code>collector/compile-benchmarks</code> directory</a> |
| and <a href="https://github.com/rust-lang/rustc-perf/tree/master/collector/runtime-benchmarks">the <code>collector/runtime-benchmarks</code> directory</a>. So let’s |
| go into the directory of a specific test; we’ll use <code>clap-rs</code> as an example:</p> |
| <pre><code class="language-bash">cd collector/compile-benchmarks/clap-3.1.6 |
| </code></pre> |
| <p>In this case, let’s say we want to profile the <code>cargo check</code> |
| performance. In that case, I would first run some basic commands to |
| build the dependencies:</p> |
| <pre><code class="language-bash"># Setup: first clean out any old results and build the dependencies: |
| cargo +<toolchain> clean |
| CARGO_INCREMENTAL=0 cargo +<toolchain> check |
| </code></pre> |
| <p>(Again, <code><toolchain></code> should be replaced with the name of the |
| toolchain we made in the first step.)</p> |
| <p>Next: we want record the execution time for <em>just</em> the clap-rs crate, |
| running cargo check. I tend to use <code>cargo rustc</code> for this, since it |
| also allows me to add explicit flags, which we’ll do later on.</p> |
| <pre><code class="language-bash">touch src/lib.rs |
| CARGO_INCREMENTAL=0 perf record -F99 --call-graph dwarf cargo rustc --profile check --lib |
| </code></pre> |
| <p>Note that final command: it’s a doozy! It uses the <code>cargo rustc</code> |
| command, which executes rustc with (potentially) additional options; |
| the <code>--profile check</code> and <code>--lib</code> options specify that we are doing a |
| <code>cargo check</code> execution, and that this is a library (not a binary).</p> |
| <p>At this point, we can use <code>perf</code> tooling to analyze the results. For example:</p> |
| <pre><code class="language-bash">perf report |
| </code></pre> |
| <p>will open up an interactive TUI program. In simple cases, that can be |
| helpful. For more detailed examination, the <a href="https://github.com/nikomatsakis/perf-focus"><code>perf-focus</code> tool</a> |
| can be helpful; it is covered below.</p> |
| <p><strong>A note of caution.</strong> Each of the rustc-perf tests is its own special |
| snowflake. In particular, some of them are not libraries, in which |
| case you would want to do <code>touch src/main.rs</code> and avoid passing |
| <code>--lib</code>. I’m not sure how best to tell which test is which to be |
| honest.</p> |
| <h3 id="gathering-nll-data"><a class="header" href="#gathering-nll-data">Gathering NLL data</a></h3> |
| <p>If you want to profile an NLL run, you can just pass extra options to |
| the <code>cargo rustc</code> command, like so:</p> |
| <pre><code class="language-bash">touch src/lib.rs |
| CARGO_INCREMENTAL=0 perf record -F99 --call-graph dwarf cargo rustc --profile check --lib -- -Z borrowck=mir |
| </code></pre> |
| <h2 id="analyzing-a-perf-profile-with-perf-focus"><a class="header" href="#analyzing-a-perf-profile-with-perf-focus">Analyzing a perf profile with <code>perf focus</code></a></h2> |
| <p>Once you’ve gathered a perf profile, we want to get some information |
| about it. For this, I personally use <a href="https://github.com/nikomatsakis/perf-focus">perf focus</a>. It’s a kind of |
| simple but useful tool that lets you answer queries like:</p> |
| <ul> |
| <li>“how much time was spent in function F” (no matter where it was called from)</li> |
| <li>“how much time was spent in function F when it was called from G”</li> |
| <li>“how much time was spent in function F <em>excluding</em> time spent in G”</li> |
| <li>“what functions does F call and how much time does it spend in them”</li> |
| </ul> |
| <p>To understand how it works, you have to know just a bit about |
| perf. Basically, perf works by <em>sampling</em> your process on a regular |
| basis (or whenever some event occurs). For each sample, perf gathers a |
| backtrace. <code>perf focus</code> lets you write a regular expression that tests |
| which functions appear in that backtrace, and then tells you which |
| percentage of samples had a backtrace that met the regular |
| expression. It’s probably easiest to explain by walking through how I |
| would analyze NLL performance.</p> |
| <h3 id="installing-perf-focus"><a class="header" href="#installing-perf-focus">Installing <code>perf-focus</code></a></h3> |
| <p>You can install perf-focus using <code>cargo install</code>:</p> |
| <pre><code class="language-bash">cargo install perf-focus |
| </code></pre> |
| <h3 id="example-how-much-time-is-spent-in-mir-borrowck"><a class="header" href="#example-how-much-time-is-spent-in-mir-borrowck">Example: How much time is spent in MIR borrowck?</a></h3> |
| <p>Let’s say we’ve gathered the NLL data for a test. We’d like to know |
| how much time it is spending in the MIR borrow-checker. The “main” |
| function of the MIR borrowck is called <code>do_mir_borrowck</code>, so we can do |
| this command:</p> |
| <pre><code class="language-bash">$ perf focus '{do_mir_borrowck}' |
| Matcher : {do_mir_borrowck} |
| Matches : 228 |
| Not Matches: 542 |
| Percentage : 29% |
| </code></pre> |
| <p>The <code>'{do_mir_borrowck}'</code> argument is called the <strong>matcher</strong>. It |
| specifies the test to be applied on the backtrace. In this case, the |
| <code>{X}</code> indicates that there must be <em>some</em> function on the backtrace |
| that meets the regular expression <code>X</code>. In this case, that regex is |
| just the name of the function we want (in fact, it’s a subset of the name; |
| the full name includes a bunch of other stuff, like the module |
| path). In this mode, perf-focus just prints out the percentage of |
| samples where <code>do_mir_borrowck</code> was on the stack: in this case, 29%.</p> |
| <p><strong>A note about c++filt.</strong> To get the data from <code>perf</code>, <code>perf focus</code> |
| currently executes <code>perf script</code> (perhaps there is a better |
| way…). I’ve sometimes found that <code>perf script</code> outputs C++ mangled |
| names. This is annoying. You can tell by running <code>perf script | head</code> yourself — if you see names like <code>5rustc6middle</code> instead of |
| <code>rustc::middle</code>, then you have the same problem. You can solve this |
| by doing:</p> |
| <pre><code class="language-bash">perf script | c++filt | perf focus --from-stdin ... |
| </code></pre> |
| <p>This will pipe the output from <code>perf script</code> through <code>c++filt</code> and |
| should mostly convert those names into a more friendly format. The |
| <code>--from-stdin</code> flag to <code>perf focus</code> tells it to get its data from |
| stdin, rather than executing <code>perf focus</code>. We should make this more |
| convenient (at worst, maybe add a <code>c++filt</code> option to <code>perf focus</code>, or |
| just always use it — it’s pretty harmless).</p> |
| <h3 id="example-how-much-time-does-mir-borrowck-spend-solving-traits"><a class="header" href="#example-how-much-time-does-mir-borrowck-spend-solving-traits">Example: How much time does MIR borrowck spend solving traits?</a></h3> |
| <p>Perhaps we’d like to know how much time MIR borrowck spends in the |
| trait checker. We can ask this using a more complex regex:</p> |
| <pre><code class="language-bash">$ perf focus '{do_mir_borrowck}..{^rustc::traits}' |
| Matcher : {do_mir_borrowck},..{^rustc::traits} |
| Matches : 12 |
| Not Matches: 1311 |
| Percentage : 0% |
| </code></pre> |
| <p>Here we used the <code>..</code> operator to ask “how often do we have |
| <code>do_mir_borrowck</code> on the stack and then, later, some function whose |
| name begins with <code>rustc::traits</code>?” (basically, code in that module). It |
| turns out the answer is “almost never” — only 12 samples fit that |
| description (if you ever see <em>no</em> samples, that often indicates your |
| query is messed up).</p> |
| <p>If you’re curious, you can find out exactly which samples by using the |
| <code>--print-match</code> option. This will print out the full backtrace for |
| each sample. The <code>|</code> at the front of the line indicates the part that |
| the regular expression matched.</p> |
| <h3 id="example-where-does-mir-borrowck-spend-its-time"><a class="header" href="#example-where-does-mir-borrowck-spend-its-time">Example: Where does MIR borrowck spend its time?</a></h3> |
| <p>Often we want to do more “explorational” queries. Like, we know that |
| MIR borrowck is 29% of the time, but where does that time get spent? |
| For that, the <code>--tree-callees</code> option is often the best tool. You |
| usually also want to give <code>--tree-min-percent</code> or |
| <code>--tree-max-depth</code>. The result looks like this:</p> |
| <pre><code class="language-bash">$ perf focus '{do_mir_borrowck}' --tree-callees --tree-min-percent 3 |
| Matcher : {do_mir_borrowck} |
| Matches : 577 |
| Not Matches: 746 |
| Percentage : 43% |
| |
| Tree |
| | matched `{do_mir_borrowck}` (43% total, 0% self) |
| : | rustc_borrowck::nll::compute_regions (20% total, 0% self) |
| : : | rustc_borrowck::nll::type_check::type_check_internal (13% total, 0% self) |
| : : : | core::ops::function::FnOnce::call_once (5% total, 0% self) |
| : : : : | rustc_borrowck::nll::type_check::liveness::generate (5% total, 3% self) |
| : : : | <rustc_borrowck::nll::type_check::TypeVerifier<'a, 'b, 'tcx> as rustc::mir::visit::Visitor<'tcx>>::visit_mir (3% total, 0% self) |
| : | rustc::mir::visit::Visitor::visit_mir (8% total, 6% self) |
| : | <rustc_borrowck::MirBorrowckCtxt<'cx, 'tcx> as rustc_mir_dataflow::DataflowResultsConsumer<'cx, 'tcx>>::visit_statement_entry (5% total, 0% self) |
| : | rustc_mir_dataflow::do_dataflow (3% total, 0% self) |
| </code></pre> |
| <p>What happens with <code>--tree-callees</code> is that</p> |
| <ul> |
| <li>we find each sample matching the regular expression</li> |
| <li>we look at the code that occurs <em>after</em> the regex match and try |
| to build up a call tree</li> |
| </ul> |
| <p>The <code>--tree-min-percent 3</code> option says “only show me things that take |
| more than 3% of the time”. Without this, the tree often gets really |
| noisy and includes random stuff like the innards of |
| malloc. <code>--tree-max-depth</code> can be useful too, it just limits how many |
| levels we print.</p> |
| <p>For each line, we display the percent of time in that function |
| altogether (“total”) and the percent of time spent in <strong>just that |
| function and not some callee of that function</strong> (self). Usually |
| “total” is the more interesting number, but not always.</p> |
| <h3 id="relative-percentages"><a class="header" href="#relative-percentages">Relative percentages</a></h3> |
| <p>By default, all in perf-focus are relative to the <strong>total program |
| execution</strong>. This is useful to help you keep perspective — often as |
| we drill down to find hot spots, we can lose sight of the fact that, |
| in terms of overall program execution, this “hot spot” is actually not |
| important. It also ensures that percentages between different queries |
| are easily compared against one another.</p> |
| <p>That said, sometimes it’s useful to get relative percentages, so <code>perf focus</code> offers a <code>--relative</code> option. In this case, the percentages are |
| listed only for samples that match (vs all samples). So for example we |
| could get our percentages relative to the borrowck itself |
| like so:</p> |
| <pre><code class="language-bash">$ perf focus '{do_mir_borrowck}' --tree-callees --relative --tree-max-depth 1 --tree-min-percent 5 |
| Matcher : {do_mir_borrowck} |
| Matches : 577 |
| Not Matches: 746 |
| Percentage : 100% |
| |
| Tree |
| | matched `{do_mir_borrowck}` (100% total, 0% self) |
| : | rustc_borrowck::nll::compute_regions (47% total, 0% self) [...] |
| : | rustc::mir::visit::Visitor::visit_mir (19% total, 15% self) [...] |
| : | <rustc_borrowck::MirBorrowckCtxt<'cx, 'tcx> as rustc_mir_dataflow::DataflowResultsConsumer<'cx, 'tcx>>::visit_statement_entry (13% total, 0% self) [...] |
| : | rustc_mir_dataflow::do_dataflow (8% total, 1% self) [...] |
| </code></pre> |
| <p>Here you see that <code>compute_regions</code> came up as “47% total” — that |
| means that 47% of <code>do_mir_borrowck</code> is spent in that function. Before, |
| we saw 20% — that’s because <code>do_mir_borrowck</code> itself is only 43% of |
| the total time (and <code>.47 * .43 = .20</code>).</p> |
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