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| <h1 id="parallel-compilation"><a class="header" href="#parallel-compilation">Parallel compilation</a></h1> |
| <div class="warning"> |
| As of <!-- date-check --> November 2024, |
| the parallel front-end is undergoing significant changes, |
| so this page contains quite a bit of outdated information. |
| <p>Tracking issue: <a href="https://github.com/rust-lang/rust/issues/113349">https://github.com/rust-lang/rust/issues/113349</a></p> |
| </div> |
| <p>As of <!-- date-check --> November 2024, most of the rust compiler is now |
| parallelized.</p> |
| <ul> |
| <li>The codegen part is executed concurrently by default. You can use the <code>-C codegen-units=n</code> option to control the number of concurrent tasks.</li> |
| <li>The parts after HIR lowering to codegen such as type checking, borrowing |
| checking, and mir optimization are parallelized in the nightly version. |
| Currently, they are executed in serial by default, and parallelization is |
| manually enabled by the user using the <code>-Z threads = n</code> option.</li> |
| <li>Other parts, such as lexical parsing, HIR lowering, and macro expansion, are |
| still executed in serial mode.</li> |
| </ul> |
| <div class="warning"> |
| The following sections are kept for now but are quite outdated. |
| </div> |
| <hr /> |
| <h2 id="code-generation"><a class="header" href="#code-generation">Code generation</a></h2> |
| <p>During monomorphization the compiler splits up all the code to |
| be generated into smaller chunks called <em>codegen units</em>. These are then generated by |
| independent instances of LLVM running in parallel. At the end, the linker |
| is run to combine all the codegen units together into one binary. This process |
| occurs in the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/base/index.html"><code>rustc_codegen_ssa::base</code></a> module.</p> |
| <h2 id="data-structures"><a class="header" href="#data-structures">Data structures</a></h2> |
| <p>The underlying thread-safe data-structures used in the parallel compiler |
| can be found in the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_data_structures/sync/index.html"><code>rustc_data_structures::sync</code></a> module. These data structures |
| are implemented differently depending on whether <code>parallel-compiler</code> is true.</p> |
| <div class="table-wrapper"><table><thead><tr><th>data structure</th><th>parallel</th><th>non-parallel</th></tr></thead><tbody> |
| <tr><td>Lock<T></td><td>(parking_lot::Mutex<T>)</td><td>(std::cell::RefCell)</td></tr> |
| <tr><td>RwLock<T></td><td>(parking_lot::RwLock<T>)</td><td>(std::cell::RefCell)</td></tr> |
| <tr><td>MTLock<T></td><td>(Lock<T>)</td><td>(T)</td></tr> |
| <tr><td>ReadGuard</td><td>parking_lot::RwLockReadGuard</td><td>std::cell::Ref</td></tr> |
| <tr><td>MappedReadGuard</td><td>parking_lot::MappedRwLockReadGuard</td><td>std::cell::Ref</td></tr> |
| <tr><td>WriteGuard</td><td>parking_lot::RwLockWriteGuard</td><td>std::cell::RefMut</td></tr> |
| <tr><td>MappedWriteGuard</td><td>parking_lot::MappedRwLockWriteGuard</td><td>std::cell::RefMut</td></tr> |
| <tr><td>LockGuard</td><td>parking_lot::MutexGuard</td><td>std::cell::RefMut</td></tr> |
| </tbody></table> |
| </div> |
| <ul> |
| <li> |
| <p>These thread-safe data structures are interspersed during compilation which |
| can cause lock contention resulting in degraded performance as the number of |
| threads increases beyond 4. So we audit the use of these data structures |
| which leads to either a refactoring so as to reduce the use of shared state, |
| or the authoring of persistent documentation covering the specific of the |
| invariants, the atomicity, and the lock orderings.</p> |
| </li> |
| <li> |
| <p>On the other hand, we still need to figure out what other invariants |
| during compilation might not hold in parallel compilation.</p> |
| </li> |
| </ul> |
| <h3 id="workerlocal"><a class="header" href="#workerlocal">WorkerLocal</a></h3> |
| <p><a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_data_structures/sync/worker_local/struct.WorkerLocal.html"><code>WorkerLocal</code></a> is a special data structure implemented for parallel compilers. It |
| holds worker-locals values for each thread in a thread pool. You can only |
| access the worker local value through the <code>Deref</code> <code>impl</code> on the thread pool it |
| was constructed on. It panics otherwise.</p> |
| <p><code>WorkerLocal</code> is used to implement the <code>Arena</code> allocator in the parallel |
| environment, which is critical in parallel queries. Its implementation is |
| located in the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_data_structures/sync/worker_local/index.html"><code>rustc_data_structures::sync::worker_local</code></a> module. However, |
| in the non-parallel compiler, it is implemented as <code>(OneThread<T>)</code>, whose <code>T</code> |
| can be accessed directly through <code>Deref::deref</code>.</p> |
| <h2 id="parallel-iterator"><a class="header" href="#parallel-iterator">Parallel iterator</a></h2> |
| <p>The parallel iterators provided by the <a href="https://crates.io/crates/rayon"><code>rayon</code></a> crate are easy ways to |
| implement parallelism. In the current implementation of the parallel compiler |
| we use a custom <a href="https://github.com/rust-lang/rustc-rayon">fork</a> of <code>rayon</code> to run tasks in parallel.</p> |
| <p>Some iterator functions are implemented to run loops in parallel |
| when <code>parallel-compiler</code> is true.</p> |
| <div class="table-wrapper"><table><thead><tr><th>Function(Omit <code>Send</code> and <code>Sync</code>)</th><th>Introduction</th><th>Owning Module</th></tr></thead><tbody> |
| <tr><td><strong>par_iter</strong><T: IntoParallelIterator>(t: T) -> T::Iter</td><td>generate a parallel iterator</td><td>rustc_data_structure::sync</td></tr> |
| <tr><td><strong>par_for_each_in</strong><T: IntoParallelIterator>(t: T, for_each: impl Fn(T::Item))</td><td>generate a parallel iterator and run <code>for_each</code> on each element</td><td>rustc_data_structure::sync</td></tr> |
| <tr><td><strong>Map::par_body_owners</strong>(self, f: impl Fn(LocalDefId))</td><td>run <code>f</code> on all hir owners in the crate</td><td>rustc_middle::hir::map</td></tr> |
| <tr><td><strong>Map::par_for_each_module</strong>(self, f: impl Fn(LocalDefId))</td><td>run <code>f</code> on all modules and sub modules in the crate</td><td>rustc_middle::hir::map</td></tr> |
| <tr><td><strong>ModuleItems::par_items</strong>(&self, f: impl Fn(ItemId))</td><td>run <code>f</code> on all items in the module</td><td>rustc_middle::hir</td></tr> |
| <tr><td><strong>ModuleItems::par_trait_items</strong>(&self, f: impl Fn(TraitItemId))</td><td>run <code>f</code> on all trait items in the module</td><td>rustc_middle::hir</td></tr> |
| <tr><td><strong>ModuleItems::par_impl_items</strong>(&self, f: impl Fn(ImplItemId))</td><td>run <code>f</code> on all impl items in the module</td><td>rustc_middle::hir</td></tr> |
| <tr><td><strong>ModuleItems::par_foreign_items</strong>(&self, f: impl Fn(ForeignItemId))</td><td>run <code>f</code> on all foreign items in the module</td><td>rustc_middle::hir</td></tr> |
| </tbody></table> |
| </div> |
| <p>There are a lot of loops in the compiler which can possibly be parallelized |
| using these functions. As of <!-- date-check--> August 2022, scenarios where |
| the parallel iterator function has been used are as follows:</p> |
| <div class="table-wrapper"><table><thead><tr><th>caller</th><th>scenario</th><th>callee</th></tr></thead><tbody> |
| <tr><td>rustc_metadata::rmeta::encoder::prefetch_mir</td><td>Prefetch queries which will be needed later by metadata encoding</td><td>par_iter</td></tr> |
| <tr><td>rustc_monomorphize::collector::collect_crate_mono_items</td><td>Collect monomorphized items reachable from non-generic items</td><td>par_for_each_in</td></tr> |
| <tr><td>rustc_interface::passes::analysis</td><td>Check the validity of the match statements</td><td>Map::par_body_owners</td></tr> |
| <tr><td>rustc_interface::passes::analysis</td><td>MIR borrow check</td><td>Map::par_body_owners</td></tr> |
| <tr><td>rustc_typeck::check::typeck_item_bodies</td><td>Type check</td><td>Map::par_body_owners</td></tr> |
| <tr><td>rustc_interface::passes::hir_id_validator::check_crate</td><td>Check the validity of hir</td><td>Map::par_for_each_module</td></tr> |
| <tr><td>rustc_interface::passes::analysis</td><td>Check the validity of loops body, attributes, naked functions, unstable abi, const bodys</td><td>Map::par_for_each_module</td></tr> |
| <tr><td>rustc_interface::passes::analysis</td><td>Liveness and intrinsic checking of MIR</td><td>Map::par_for_each_module</td></tr> |
| <tr><td>rustc_interface::passes::analysis</td><td>Deathness checking</td><td>Map::par_for_each_module</td></tr> |
| <tr><td>rustc_interface::passes::analysis</td><td>Privacy checking</td><td>Map::par_for_each_module</td></tr> |
| <tr><td>rustc_lint::late::check_crate</td><td>Run per-module lints</td><td>Map::par_for_each_module</td></tr> |
| <tr><td>rustc_typeck::check_crate</td><td>Well-formedness checking</td><td>Map::par_for_each_module</td></tr> |
| </tbody></table> |
| </div> |
| <p>There are still many loops that have the potential to use parallel iterators.</p> |
| <h2 id="query-system"><a class="header" href="#query-system">Query system</a></h2> |
| <p>The query model has some properties that make it actually feasible to evaluate |
| multiple queries in parallel without too much effort:</p> |
| <ul> |
| <li>All data a query provider can access is via the query context, so |
| the query context can take care of synchronizing access.</li> |
| <li>Query results are required to be immutable so they can safely be used by |
| different threads concurrently.</li> |
| </ul> |
| <p>When a query <code>foo</code> is evaluated, the cache table for <code>foo</code> is locked.</p> |
| <ul> |
| <li>If there already is a result, we can clone it, release the lock and |
| we are done.</li> |
| <li>If there is no cache entry and no other active query invocation computing the |
| same result, we mark the key as being "in progress", release the lock and |
| start evaluating.</li> |
| <li>If there <em>is</em> another query invocation for the same key in progress, we |
| release the lock, and just block the thread until the other invocation has |
| computed the result we are waiting for. <strong>Cycle error detection</strong> in the parallel |
| compiler requires more complex logic than in single-threaded mode. When |
| worker threads in parallel queries stop making progress due to interdependence, |
| the compiler uses an extra thread <em>(named deadlock handler)</em> to detect, remove and |
| report the cycle error.</li> |
| </ul> |
| <p>The parallel query feature still has implementation to do, most of which is |
| related to the previous <code>Data Structures</code> and <code>Parallel Iterators</code>. See <a href="https://github.com/rust-lang/rust/issues/48685">this |
| open feature tracking issue</a>.</p> |
| <h2 id="rustdoc"><a class="header" href="#rustdoc">Rustdoc</a></h2> |
| <p>As of <!-- date-check--> November 2022, there are still a number of steps to |
| complete before <code>rustdoc</code> rendering can be made parallel (see a open discussion |
| of <a href="https://github.com/rust-lang/rust/issues/82741">parallel <code>rustdoc</code></a>).</p> |
| <h2 id="resources"><a class="header" href="#resources">Resources</a></h2> |
| <p>Here are some resources that can be used to learn more:</p> |
| <ul> |
| <li><a href="https://internals.rust-lang.org/t/help-test-parallel-rustc/11503">This IRLO thread by alexchricton about performance</a></li> |
| <li><a href="https://internals.rust-lang.org/t/parallelizing-rustc-using-rayon/6606">This IRLO thread by Zoxc, one of the pioneers of the effort</a></li> |
| <li><a href="https://github.com/nikomatsakis/rustc-parallelization/blob/master/interior-mutability-list.md">This list of interior mutability in the compiler by nikomatsakis</a></li> |
| </ul> |
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