down samples outgoing gossip pull requests

Push message propagation has improved in recent versions of the gossip
code and we don't rely on pull requests as much as before. Handling pull
requests is also inefficient and expensive.
The commit reduces number of outgoing pull requests by down sampling.

gossip/src/crds_gossip_pull.rs

@@ -53,8 +53,6 @@ use {
 pub const CRDS_GOSSIP_PULL_CRDS_TIMEOUT_MS: u64 = 15000;
 // Retention period of hashes of received outdated values.
 const FAILED_INSERTS_RETENTION_MS: u64 = 20_000;
-// Maximum number of pull requests to send out each time around.
-const MAX_NUM_PULL_REQUESTS: usize = 1024;
 pub const FALSE_RATE: f64 = 0.1f64;
 pub const KEYS: f64 = 8f64;
 
@@ -143,19 +141,26 @@ impl CrdsFilter {
 
 /// A vector of crds filters that together hold a complete set of Hashes.
 struct CrdsFilterSet {
-    filters: Vec<AtomicBloom<Hash>>,
+    filters: Vec<Option<AtomicBloom<Hash>>>,
     mask_bits: u32,
 }
 
 impl CrdsFilterSet {
     fn new(num_items: usize, max_bytes: usize) -> Self {
+        const SAMPLE_RATE: usize = 8;
+        const MAX_NUM_FILTERS: usize = 1024;
         let max_bits = (max_bytes * 8) as f64;
         let max_items = CrdsFilter::max_items(max_bits, FALSE_RATE, KEYS);
         let mask_bits = CrdsFilter::mask_bits(num_items as f64, max_items);
-        let filters =
-            repeat_with(|| Bloom::random(max_items as usize, FALSE_RATE, max_bits as usize).into())
-                .take(1 << mask_bits)
-                .collect();
+        let mut filters: Vec<_> = repeat_with(|| None).take(1usize << mask_bits).collect();
+        let mut indices: Vec<usize> = (0..filters.len()).collect();
+        let size = (filters.len() + SAMPLE_RATE - 1) / SAMPLE_RATE;
+        for _ in 0..MAX_NUM_FILTERS.min(size) {
+            let k = rand::thread_rng().gen_range(0..indices.len());
+            let k = indices.swap_remove(k);
+            let filter = Bloom::random(max_items as usize, FALSE_RATE, max_bits as usize);
+            filters[k] = Some(AtomicBloom::<Hash>::from(filter));
+        }
         Self { filters, mask_bits }
     }
 
@@ -167,7 +172,9 @@ impl CrdsFilterSet {
                 .unwrap_or_default(),
         )
         .unwrap();
-        self.filters[index].add(&hash_value);
+        if let Some(filter) = &self.filters[index] {
+            filter.add(&hash_value);
+        }
     }
 }
 
@@ -177,10 +184,12 @@ impl From<CrdsFilterSet> for Vec<CrdsFilter> {
         cfs.filters
             .into_iter()
             .enumerate()
-            .map(|(seed, filter)| CrdsFilter {
-                filter: filter.into(),
-                mask: CrdsFilter::compute_mask(seed as u64, mask_bits),
-                mask_bits,
+            .filter_map(|(seed, filter)| {
+                Some(CrdsFilter {
+                    filter: Bloom::<Hash>::from(filter?),
+                    mask: CrdsFilter::compute_mask(seed as u64, mask_bits),
+                    mask_bits,
+                })
             })
             .collect()
     }
@@ -269,14 +278,7 @@ impl CrdsGossipPull {
         if nodes.is_empty() {
             return Err(CrdsGossipError::NoPeers);
         }
-        let mut filters = self.build_crds_filters(thread_pool, crds, bloom_size);
-        if filters.len() > MAX_NUM_PULL_REQUESTS {
-            for i in 0..MAX_NUM_PULL_REQUESTS {
-                let j = rng.gen_range(i..filters.len());
-                filters.swap(i, j);
-            }
-            filters.truncate(MAX_NUM_PULL_REQUESTS);
-        }
+        let filters = self.build_crds_filters(thread_pool, crds, bloom_size);
         // Associate each pull-request filter with a randomly selected peer.
         let dist = WeightedIndex::new(weights).unwrap();
         let nodes = repeat_with(|| nodes[dist.sample(&mut rng)].clone());
@@ -674,44 +676,55 @@ pub(crate) mod tests {
     #[test]
     fn test_crds_filter_set_add() {
         let crds_filter_set =
-            CrdsFilterSet::new(/*num_items=*/ 9672788, /*max_bytes=*/ 8196);
-        let hash_values: Vec<_> = repeat_with(Hash::new_unique).take(1024).collect();
+            CrdsFilterSet::new(/*num_items=*/ 59672788, /*max_bytes=*/ 8196);
+        let mut rng = rand::thread_rng();
+        let hash_values: Vec<_> = repeat_with(|| {
+            let buf: [u8; 32] = rng.gen();
+            solana_sdk::hash::hashv(&[&buf])
+        })
+        .take(1024)
+        .collect();
+        assert_eq!(crds_filter_set.filters.len(), 8192);
         for hash_value in &hash_values {
             crds_filter_set.add(*hash_value);
         }
         let filters: Vec<CrdsFilter> = crds_filter_set.into();
+        let mut num_hits = 0;
         assert_eq!(filters.len(), 1024);
         for hash_value in hash_values {
-            let mut num_hits = 0;
+            let mut hit = false;
             let mut false_positives = 0;
             for filter in &filters {
                 if filter.test_mask(&hash_value) {
                     num_hits += 1;
+                    assert!(!hit);
+                    hit = true;
                     assert!(filter.contains(&hash_value));
                     assert!(filter.filter.contains(&hash_value));
                 } else if filter.filter.contains(&hash_value) {
                     false_positives += 1;
                 }
             }
-            assert_eq!(num_hits, 1);
             assert!(false_positives < 5);
         }
+        assert!(num_hits > 96, "num_hits: {num_hits}");
     }
 
     #[test]
     fn test_crds_filter_set_new() {
         // Validates invariances required by CrdsFilterSet::get in the
         // vector of filters generated by CrdsFilterSet::new.
-        let filters: Vec<CrdsFilter> =
-            CrdsFilterSet::new(/*num_items=*/ 55345017, /*max_bytes=*/ 4098).into();
-        assert_eq!(filters.len(), 16384);
+        let filters = CrdsFilterSet::new(/*num_items=*/ 55345017, /*max_bytes=*/ 4098);
+        assert_eq!(filters.filters.len(), 16384);
+        let filters = Vec::<CrdsFilter>::from(filters);
+        assert_eq!(filters.len(), 1024);
         let mask_bits = filters[0].mask_bits;
         let right_shift = 64 - mask_bits;
         let ones = !0u64 >> mask_bits;
-        for (i, filter) in filters.iter().enumerate() {
+        for filter in &filters {
             // Check that all mask_bits are equal.
             assert_eq!(mask_bits, filter.mask_bits);
-            assert_eq!(i as u64, filter.mask >> right_shift);
+            assert!((0..16384).contains(&(filter.mask >> right_shift)));
             assert_eq!(ones, ones & filter.mask);
         }
     }
@@ -744,7 +757,7 @@ pub(crate) mod tests {
         let crds = RwLock::new(crds);
         assert!(num_inserts > 30_000, "num inserts: {num_inserts}");
         let filters = crds_gossip_pull.build_crds_filters(&thread_pool, &crds, MAX_BLOOM_SIZE);
-        assert_eq!(filters.len(), MIN_NUM_BLOOM_FILTERS.max(32));
+        assert_eq!(filters.len(), MIN_NUM_BLOOM_FILTERS.max(4));
         let crds = crds.read().unwrap();
         let purged: Vec<_> = thread_pool.install(|| crds.purged().collect());
         let hash_values: Vec<_> = crds.values().map(|v| v.value_hash).chain(purged).collect();
@@ -755,21 +768,24 @@ pub(crate) mod tests {
             "hash_values.len(): {}",
             hash_values.len()
         );
+        let mut num_hits = 0;
         let mut false_positives = 0;
         for hash_value in hash_values {
-            let mut num_hits = 0;
+            let mut hit = false;
             for filter in &filters {
                 if filter.test_mask(&hash_value) {
                     num_hits += 1;
+                    assert!(!hit);
+                    hit = true;
                     assert!(filter.contains(&hash_value));
                     assert!(filter.filter.contains(&hash_value));
                 } else if filter.filter.contains(&hash_value) {
                     false_positives += 1;
                 }
             }
-            assert_eq!(num_hits, 1);
         }
-        assert!(false_positives < 150_000, "fp: {false_positives}");
+        assert!(num_hits > 4000, "num_hits: {num_hits}");
+        assert!(false_positives < 20_000, "fp: {false_positives}");
     }
 
     #[test]