added by_blocks method

rayon-demo/src/find/mod.rs

@@ -1,4 +1,4 @@
-/// Simple benchmarks of `find_any()` performance
+/// Simple benchmarks of `find_any()` and `find_first` performance
 
 macro_rules! make_tests {
     ($n:expr, $m:ident) => {
@@ -21,56 +21,237 @@ macro_rules! make_tests {
                     .collect()
             });
 
+            // this is a very dumb find_first algorithm.
+            // no early aborts so we have a linear best case cost.
+            fn find_dumb<I: ParallelIterator, P: Fn(&I::Item) -> bool + Send + Sync>(
+                iter: I,
+                cond: P,
+            ) -> Option<I::Item> {
+                iter.map(|e| if cond(&e) { Some(e) } else { None })
+                    .reduce(|| None, |left, right| left.or(right))
+            }
+
+            #[bench]
+            fn parallel_find_any_start(b: &mut Bencher) {
+                let needle = HAYSTACK[0][0];
+                b.iter(|| assert!(HAYSTACK.par_iter().find_any(|&&x| x[0] == needle).is_some()));
+            }
+
             #[bench]
-            fn parallel_find_first(b: &mut Bencher) {
+            fn parallel_find_first_start(b: &mut Bencher) {
                 let needle = HAYSTACK[0][0];
                 b.iter(|| assert!(HAYSTACK.par_iter().find_any(|&&x| x[0] == needle).is_some()));
             }
 
             #[bench]
-            fn serial_find_first(b: &mut Bencher) {
+            fn parallel_find_first_blocks_start(b: &mut Bencher) {
+                let needle = HAYSTACK[0][0];
+                b.iter(|| {
+                    assert!(HAYSTACK
+                        .par_iter()
+                        .by_exponential_blocks()
+                        .find_first(|&&x| x[0] == needle)
+                        .is_some())
+                });
+            }
+
+            #[bench]
+            fn serial_find_start(b: &mut Bencher) {
                 let needle = HAYSTACK[0][0];
                 b.iter(|| assert!(HAYSTACK.iter().find(|&&x| x[0] == needle).is_some()));
             }
 
             #[bench]
-            fn parallel_find_last(b: &mut Bencher) {
+            fn parallel_find_any_end(b: &mut Bencher) {
                 let needle = HAYSTACK[HAYSTACK.len() - 1][0];
                 b.iter(|| assert!(HAYSTACK.par_iter().find_any(|&&x| x[0] == needle).is_some()));
             }
+            #[bench]
+            fn parallel_find_first_end(b: &mut Bencher) {
+                let needle = HAYSTACK[HAYSTACK.len() - 1][0];
+                b.iter(|| {
+                    assert!(HAYSTACK
+                        .par_iter()
+                        .find_first(|&&x| x[0] == needle)
+                        .is_some())
+                });
+            }
+            #[bench]
+            fn parallel_find_first_blocks_end(b: &mut Bencher) {
+                let needle = HAYSTACK[HAYSTACK.len() - 1][0];
+                b.iter(|| {
+                    assert!(HAYSTACK
+                        .par_iter()
+                        .by_exponential_blocks()
+                        .find_first(|&&x| x[0] == needle)
+                        .is_some())
+                });
+            }
 
             #[bench]
-            fn serial_find_last(b: &mut Bencher) {
+            fn serial_find_end(b: &mut Bencher) {
                 let needle = HAYSTACK[HAYSTACK.len() - 1][0];
                 b.iter(|| assert!(HAYSTACK.iter().find(|&&x| x[0] == needle).is_some()));
             }
 
             #[bench]
-            fn parallel_find_middle(b: &mut Bencher) {
-                let needle = HAYSTACK[HAYSTACK.len() / 3 * 2][0];
+            fn parallel_find_any_third(b: &mut Bencher) {
+                let needle = HAYSTACK[HAYSTACK.len() / 3][0];
+                b.iter(|| assert!(HAYSTACK.par_iter().find_any(|&&x| x[0] == needle).is_some()));
+            }
+
+            #[bench]
+            fn parallel_find_first_third(b: &mut Bencher) {
+                let needle = HAYSTACK[HAYSTACK.len() / 3][0];
+                b.iter(|| {
+                    assert!(HAYSTACK
+                        .par_iter()
+                        .find_first(|&&x| x[0] == needle)
+                        .is_some())
+                });
+            }
+
+            #[bench]
+            fn parallel_find_dumb_third(b: &mut Bencher) {
+                let needle = HAYSTACK[HAYSTACK.len() / 3][0];
+                b.iter(
+                    || assert!(find_dumb(HAYSTACK.par_iter(), (|&&x| x[0] == needle)).is_some()),
+                );
+            }
+
+            #[bench]
+            fn parallel_find_first_blocks_third(b: &mut Bencher) {
+                let needle = HAYSTACK[HAYSTACK.len() / 3][0];
+                b.iter(|| {
+                    assert!(HAYSTACK
+                        .par_iter()
+                        .by_exponential_blocks()
+                        .find_first(|&&x| x[0] == needle)
+                        .is_some())
+                });
+            }
+
+            #[bench]
+            fn serial_find_third(b: &mut Bencher) {
+                let needle = HAYSTACK[HAYSTACK.len() / 3][0];
+                b.iter(|| assert!(HAYSTACK.iter().find(|&&x| x[0] == needle).is_some()));
+            }
+
+            #[bench]
+            fn parallel_find_any_middle(b: &mut Bencher) {
+                let needle = HAYSTACK[(HAYSTACK.len() / 2).saturating_sub(1)][0];
                 b.iter(|| assert!(HAYSTACK.par_iter().find_any(|&&x| x[0] == needle).is_some()));
             }
 
+            #[bench]
+            fn parallel_find_first_middle(b: &mut Bencher) {
+                let needle = HAYSTACK[(HAYSTACK.len() / 2).saturating_sub(1)][0];
+                b.iter(|| {
+                    assert!(HAYSTACK
+                        .par_iter()
+                        .find_first(|&&x| x[0] == needle)
+                        .is_some())
+                });
+            }
+
+            #[bench]
+            fn parallel_find_dumb_middle(b: &mut Bencher) {
+                let needle = HAYSTACK[(HAYSTACK.len() / 2).saturating_sub(1)][0];
+                b.iter(
+                    || assert!(find_dumb(HAYSTACK.par_iter(), (|&&x| x[0] == needle)).is_some()),
+                );
+            }
+
+            #[bench]
+            fn parallel_find_first_blocks_middle(b: &mut Bencher) {
+                let needle = HAYSTACK[(HAYSTACK.len() / 2).saturating_sub(1)][0];
+                b.iter(|| {
+                    assert!(HAYSTACK
+                        .par_iter()
+                        .by_exponential_blocks()
+                        .find_first(|&&x| x[0] == needle)
+                        .is_some())
+                });
+            }
+
             #[bench]
             fn serial_find_middle(b: &mut Bencher) {
+                let needle = HAYSTACK[(HAYSTACK.len() / 2).saturating_sub(1)][0];
+                b.iter(|| assert!(HAYSTACK.iter().find(|&&x| x[0] == needle).is_some()));
+            }
+
+            #[bench]
+            fn parallel_find_any_two_thirds(b: &mut Bencher) {
+                let needle = HAYSTACK[HAYSTACK.len() / 3 * 2][0];
+                b.iter(|| assert!(HAYSTACK.par_iter().find_any(|&&x| x[0] == needle).is_some()));
+            }
+
+            #[bench]
+            fn parallel_find_first_two_thirds(b: &mut Bencher) {
+                let needle = HAYSTACK[HAYSTACK.len() / 3 * 2][0];
+                b.iter(|| {
+                    assert!(HAYSTACK
+                        .par_iter()
+                        .find_first(|&&x| x[0] == needle)
+                        .is_some())
+                });
+            }
+
+            #[bench]
+            fn parallel_find_first_blocks_two_thirds(b: &mut Bencher) {
+                let needle = HAYSTACK[HAYSTACK.len() / 3 * 2][0];
+                b.iter(|| {
+                    assert!(HAYSTACK
+                        .par_iter()
+                        .by_exponential_blocks()
+                        .find_first(|&&x| x[0] == needle)
+                        .is_some())
+                });
+            }
+
+            #[bench]
+            fn serial_find_two_thirds(b: &mut Bencher) {
                 let needle = HAYSTACK[HAYSTACK.len() / 3 * 2][0];
                 b.iter(|| assert!(HAYSTACK.iter().find(|&&x| x[0] == needle).is_some()));
             }
 
             #[bench]
-            fn parallel_find_missing(b: &mut Bencher) {
+            fn parallel_find_any_missing(b: &mut Bencher) {
                 let needle = HAYSTACK.iter().map(|v| v[0]).max().unwrap() + 1;
                 b.iter(|| assert!(HAYSTACK.par_iter().find_any(|&&x| x[0] == needle).is_none()));
             }
 
+            #[bench]
+            fn parallel_find_first_missing(b: &mut Bencher) {
+                let needle = HAYSTACK.iter().map(|v| v[0]).max().unwrap() + 1;
+                b.iter(|| {
+                    assert!(HAYSTACK
+                        .par_iter()
+                        .find_first(|&&x| x[0] == needle)
+                        .is_none())
+                });
+            }
+
+            #[bench]
+            fn parallel_find_first_blocks_missing(b: &mut Bencher) {
+                let needle = HAYSTACK.iter().map(|v| v[0]).max().unwrap() + 1;
+                b.iter(|| {
+                    assert!(HAYSTACK
+                        .par_iter()
+                        .by_exponential_blocks()
+                        .find_first(|&&x| x[0] == needle)
+                        .is_none())
+                });
+            }
+
             #[bench]
             fn serial_find_missing(b: &mut Bencher) {
                 let needle = HAYSTACK.iter().map(|v| v[0]).max().unwrap() + 1;
                 b.iter(|| assert!(HAYSTACK.iter().find(|&&x| x[0] == needle).is_none()));
             }
 
             #[bench]
-            fn parallel_find_common(b: &mut Bencher) {
+            fn parallel_find_any_common(b: &mut Bencher) {
                 b.iter(|| {
                     assert!(HAYSTACK
                         .par_iter()

src/compile_fail/must_use.rs

@@ -30,6 +30,8 @@ macro_rules! must_use {
 }
 
 must_use! {
+    by_exponential_blocks  /** v.par_iter().by_exponential_blocks(); */
+    by_uniform_blocks   /** v.par_iter().by_uniform_blocks(2); */
     step_by             /** v.par_iter().step_by(2); */
     chain               /** v.par_iter().chain(&v); */
     chunks              /** v.par_iter().chunks(2); */

src/iter/blocks.rs

@@ -0,0 +1,131 @@
+use super::plumbing::*;
+use super::*;
+
+struct BlocksCallback<S, C> {
+    sizes: S,
+    consumer: C,
+    len: usize,
+}
+
+impl<T, S, C> ProducerCallback<T> for BlocksCallback<S, C>
+where
+    C: UnindexedConsumer<T>,
+    S: Iterator<Item = usize>,
+{
+    type Output = C::Result;
+
+    fn callback<P: Producer<Item = T>>(mut self, mut producer: P) -> Self::Output {
+        let mut remaining_len = self.len;
+        let mut consumer = self.consumer;
+
+        // we need a local variable for the accumulated results
+        // we call the reducer's identity by splitting at 0
+        let (left_consumer, right_consumer, _) = consumer.split_at(0);
+        let mut leftmost_res = left_consumer.into_folder().complete();
+        consumer = right_consumer;
+
+        // now we loop on each block size
+        while remaining_len > 0 && !consumer.full() {
+            // we compute the next block's size
+            let size = self.sizes.next().unwrap_or(std::usize::MAX);
+            let capped_size = remaining_len.min(size);
+            remaining_len -= capped_size;
+
+            // split the producer
+            let (left_producer, right_producer) = producer.split_at(capped_size);
+            producer = right_producer;
+
+            // split the consumer
+            let (left_consumer, right_consumer, _) = consumer.split_at(capped_size);
+            consumer = right_consumer;
+
+            leftmost_res = consumer.to_reducer().reduce(
+                leftmost_res,
+                bridge_producer_consumer(capped_size, left_producer, left_consumer),
+            );
+        }
+        leftmost_res
+    }
+}
+
+/// `ExponentialBlocks` is a parallel iterator that consumes itself as a sequence
+/// of parallel blocks of increasing sizes (exponentially).
+///
+/// This struct is created by the [`by_exponential_blocks()`] method on [`IndexedParallelIterator`]
+///
+/// [`by_exponential_blocks()`]: trait.IndexedParallelIterator.html#method.by_exponential_blocks
+/// [`IndexedParallelIterator`]: trait.IndexedParallelIterator.html
+#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
+#[derive(Debug, Clone)]
+pub struct ExponentialBlocks<I> {
+    base: I,
+}
+
+impl<I> ExponentialBlocks<I> {
+    pub(super) fn new(base: I) -> Self {
+        Self { base }
+    }
+}
+
+impl<I> ParallelIterator for ExponentialBlocks<I>
+where
+    I: IndexedParallelIterator,
+{
+    type Item = I::Item;
+
+    fn drive_unindexed<C>(self, consumer: C) -> C::Result
+    where
+        C: UnindexedConsumer<Self::Item>,
+    {
+        let first = crate::current_num_threads();
+        let callback = BlocksCallback {
+            consumer,
+            sizes: std::iter::successors(Some(first), exponential_size),
+            len: self.base.len(),
+        };
+        self.base.with_producer(callback)
+    }
+}
+
+fn exponential_size(size: &usize) -> Option<usize> {
+    Some(size.saturating_mul(2))
+}
+
+/// `UniformBlocks` is a parallel iterator that consumes itself as a sequence
+/// of parallel blocks of constant sizes.
+///
+/// This struct is created by the [`by_uniform_blocks()`] method on [`IndexedParallelIterator`]
+///
+/// [`by_uniform_blocks()`]: trait.IndexedParallelIterator.html#method.by_uniform_blocks
+/// [`IndexedParallelIterator`]: trait.IndexedParallelIterator.html
+#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
+#[derive(Debug, Clone)]
+pub struct UniformBlocks<I> {
+    base: I,
+    block_size: usize,
+}
+
+impl<I> UniformBlocks<I> {
+    pub(super) fn new(base: I, block_size: usize) -> Self {
+        Self { base, block_size }
+    }
+}
+
+impl<I> ParallelIterator for UniformBlocks<I>
+where
+    I: IndexedParallelIterator,
+{
+    type Item = I::Item;
+
+    fn drive_unindexed<C>(self, consumer: C) -> C::Result
+    where
+        C: UnindexedConsumer<Self::Item>,
+    {
+        let callback = BlocksCallback {
+            consumer,
+            sizes: std::iter::repeat(self.block_size),
+            len: self.base.len(),
+        };
+        self.base.with_producer(callback)
+    }
+}