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sort the pending queue according to cost/priority
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If multiple pieces of work are waiting in the pending queue, we can sort it according to
their priorities: higher priorities should be scheduled sooner.

They are more often than not wider than pure chains, and this should create more parallelism
opportunities earlier in the pipeline: a high priority piece of work represents more future
pieces of work down the line.

This is a scheduling tradeoff that behaves differently for each project, machine configuration,
amount of available parallelism at a given point in time, etc, but seems to help more often than
hinders, at low-core counts and with enough units of work to be done, so that there is jobserver
token contention where choosing a "better" piece of work to work on next is possible.
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lqd committed Sep 1, 2022
1 parent 04a4081 commit c685acd
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Showing 2 changed files with 16 additions and 1 deletion.
10 changes: 9 additions & 1 deletion src/cargo/core/compiler/job_queue.rs
Original file line number Diff line number Diff line change
Expand Up @@ -583,11 +583,19 @@ impl<'cfg> DrainState<'cfg> {
}
}

// If multiple pieces of work are waiting in the pending queue, we can
// sort it according to their priorities: higher priorities should be
// scheduled sooner.
self.pending_queue
.sort_by_cached_key(|(unit, _)| self.queue.priority(unit));

// Now that we've learned of all possible work that we can execute
// try to spawn it so long as we've got a jobserver token which says
// we're able to perform some parallel work.
// The `pending_queue` is sorted in ascending priority order, and we're
// removing the highest priority items from its end.
while self.has_extra_tokens() && !self.pending_queue.is_empty() {
let (unit, job) = self.pending_queue.remove(0);
let (unit, job) = self.pending_queue.pop().unwrap();
*self.counts.get_mut(&unit.pkg.package_id()).unwrap() -= 1;
if !cx.bcx.build_config.build_plan {
// Print out some nice progress information.
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7 changes: 7 additions & 0 deletions src/cargo/util/dependency_queue.rs
Original file line number Diff line number Diff line change
Expand Up @@ -170,6 +170,13 @@ impl<N: Hash + Eq + Clone, E: Eq + Hash + Clone, V> DependencyQueue<N, E, V> {
self.dep_map.len()
}

/// Returns the relative priority of a node. Higher priorities should be scheduled sooner.
/// Currently computed as the transitive cost of the given node: its own, plus the cost of its
/// reverse dependencies.
pub(crate) fn priority(&self, node: &N) -> usize {
self.priority[node]
}

/// Indicate that something has finished.
///
/// Calling this function indicates that the `node` has produced `edge`. All
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