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style: Make style parallel traversal more tunable at runtime

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This adds two prefs to configure the parallel traversal work item size
and kick-off threshold, but otherwise shouldn't change behavior.

I switched from iterator generics to just a slice while at it, mostly
for simplicity, but there is a trade-off:

  * When switching from sequential to parallel traversal, we potentially
    pay the price of memmoving the VecDeque around once to make them a
    contiguous slice.

  * However we win in the common case of the smaller-than-work-unit size
    in which case we no longer need to copy stuff to a WorkItem. So I
    think overall this should be an improvement.

Differential Revision: https://phabricator.services.mozilla.com/D178656
This commit is contained in:
Emilio Cobos Álvarez 2023-05-24 06:36:08 +00:00 • committed by Martin Robinson
parent ea3fcce25f
commit d49b014c78
2 changed files with 88 additions and 87 deletions
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103
components/style/driver.rs
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@ -11,7 +11,7 @@ use crate::context::{PerThreadTraversalStatistics, StyleContext};
use crate::context::{ThreadLocalStyleContext, TraversalStatistics};
use crate::dom::{SendNode, TElement, TNode};
use crate::parallel;
use crate::parallel::{DispatchMode, WORK_UNIT_MAX};
use crate::parallel::{work_unit_max, DispatchMode};
use crate::scoped_tls::ScopedTLS;
use crate::traversal::{DomTraversal, PerLevelTraversalData, PreTraverseToken};
use rayon;
@ -48,17 +48,19 @@ fn report_statistics(stats: &PerThreadTraversalStatistics) {
gecko_stats.mStylesReused += stats.styles_reused;
}
/// Do a DOM traversal for top-down and (optionally) bottom-up processing,
/// generic over `D`.
fn parallelism_threshold() -> usize {
static_prefs::pref!("layout.css.stylo-parallelism-threshold") as usize
}
/// Do a DOM traversal for top-down and (optionally) bottom-up processing, generic over `D`.
///
/// We use an adaptive traversal strategy. We start out with simple sequential
/// processing, until we arrive at a wide enough level in the DOM that the
/// parallel traversal would parallelize it. If a thread pool is provided, we
/// then transfer control over to the parallel traversal.
/// We use an adaptive traversal strategy. We start out with simple sequential processing, until we
/// arrive at a wide enough level in the DOM that the parallel traversal would parallelize it.
/// If a thread pool is provided, we then transfer control over to the parallel traversal.
///
/// Returns true if the traversal was parallel, and also returns the statistics
/// object containing information on nodes traversed (on nightly only). Not
/// all of its fields will be initialized since we don't call finish().
/// Returns true if the traversal was parallel, and also returns the statistics object containing
/// information on nodes traversed (on nightly only). Not all of its fields will be initialized
/// since we don't call finish().
pub fn traverse_dom<E, D>(
traversal: &D,
token: PreTraverseToken<E>,
@ -100,7 +102,9 @@ where
// Process the nodes breadth-first, just like the parallel traversal does.
// This helps keep similar traversal characteristics for the style sharing
// cache.
let mut discovered = VecDeque::<SendNode<E::ConcreteNode>>::with_capacity(WORK_UNIT_MAX * 2);
let work_unit_max = work_unit_max();
let parallelism_threshold = parallelism_threshold();
let mut discovered = VecDeque::<SendNode<E::ConcreteNode>>::with_capacity(work_unit_max * 2);
let mut depth = root.depth();
let mut nodes_remaining_at_current_depth = 1;
discovered.push_back(unsafe { SendNode::new(root.as_node()) });
@ -122,45 +126,48 @@ where
);
nodes_remaining_at_current_depth -= 1;
if nodes_remaining_at_current_depth == 0 {
depth += 1;
// If there is enough work to parallelize over, and the caller allows
// parallelism, switch to the parallel driver. We do this only when
// moving to the next level in the dom so that we can pass the same
// depth for all the children.
if pool.is_some() && discovered.len() > WORK_UNIT_MAX {
let pool = pool.unwrap();
let tls = ScopedTLS::<ThreadLocalStyleContext<E>>::new(pool);
let root_opaque = root.as_node().opaque();
let drain = discovered.drain(..);
pool.scope_fifo(|scope| {
// Enable a breadth-first rayon traversal. This causes the work
// queue to be always FIFO, rather than FIFO for stealers and
// FILO for the owner (which is what rayon does by default). This
// ensures that we process all the elements at a given depth before
// proceeding to the next depth, which is important for style sharing.
#[cfg(feature = "gecko")]
gecko_profiler_label!(Layout, StyleComputation);
parallel::traverse_nodes(
drain,
DispatchMode::TailCall,
/* recursion_ok = */ true,
root_opaque,
PerLevelTraversalData {
current_dom_depth: depth,
},
scope,
pool,
traversal,
&tls,
);
});
tls_slots = Some(tls.into_slots());
break;
}
nodes_remaining_at_current_depth = discovered.len();
// If there is enough work to parallelize over, and the caller allows parallelism, switch
// to the parallel driver. We do this only when moving to the next level in the dom so that
// we can pass the same depth for all the children.
if nodes_remaining_at_current_depth != 0 {
continue;
}
depth += 1;
if pool.is_some() &&
discovered.len() > parallelism_threshold &&
parallelism_threshold > 0
{
let pool = pool.unwrap();
let tls = ScopedTLS::<ThreadLocalStyleContext<E>>::new(pool);
let root_opaque = root.as_node().opaque();
pool.scope_fifo(|scope| {
// Enable a breadth-first rayon traversal. This causes the work
// queue to be always FIFO, rather than FIFO for stealers and
// FILO for the owner (which is what rayon does by default). This
// ensures that we process all the elements at a given depth before
// proceeding to the next depth, which is important for style sharing.
#[cfg(feature = "gecko")]
gecko_profiler_label!(Layout, StyleComputation);
parallel::traverse_nodes(
discovered.make_contiguous(),
DispatchMode::TailCall,
/* recursion_ok = */ true,
root_opaque,
PerLevelTraversalData {
current_dom_depth: depth,
},
scope,
pool,
traversal,
&tls,
);
});
tls_slots = Some(tls.into_slots());
break;
}
nodes_remaining_at_current_depth = discovered.len();
}
// Collect statistics from thread-locals if requested.