Change erasure ratio to 0.25 and increase data shreds to 16 (#5931)

* Change erasure ratio to 0.25 and increase data shreds to 16

* Fix case where no coding shreds are requested

core/src/broadcast_stage/broadcast_utils.rs

@@ -1,7 +1,7 @@
 use crate::entry::Entry;
 use crate::poh_recorder::WorkingBankEntries;
 use crate::result::Result;
-use crate::shred::{Shred, ShredInfo, Shredder};
+use crate::shred::{Shred, ShredInfo, Shredder, RECOMMENDED_FEC_RATE};
 use solana_runtime::bank::Bank;
 use solana_sdk::signature::Keypair;
 use std::sync::mpsc::Receiver;
@@ -88,9 +88,14 @@ pub(super) fn entries_to_shreds(
         .for_each(|(i, entries_tuple)| {
             let (entries, _): (Vec<_>, Vec<_>) = entries_tuple.into_iter().unzip();
             //entries
-            let mut shredder =
-                Shredder::new(slot, parent_slot, 1.0, keypair, latest_shred_index as u32)
-                    .expect("Expected to create a new shredder");
+            let mut shredder = Shredder::new(
+                slot,
+                parent_slot,
+                RECOMMENDED_FEC_RATE,
+                keypair,
+                latest_shred_index as u32,
+            )
+            .expect("Expected to create a new shredder");
 
             bincode::serialize_into(&mut shredder, &entries)
                 .expect("Expect to write all entries to shreds");

core/src/shred.rs

@@ -168,7 +168,9 @@ pub enum Shred {
 
 /// This limit comes from reed solomon library, but unfortunately they don't have
 /// a public constant defined for it.
-const MAX_DATA_SHREDS_PER_FEC_BLOCK: u32 = 4;
+const MAX_DATA_SHREDS_PER_FEC_BLOCK: u32 = 16;
+/// Based on rse benchmarks, the optimal erasure config uses 16 data shreds and 4 coding shreds
+pub const RECOMMENDED_FEC_RATE: f32 = 0.25;
 
 const LAST_SHRED_IN_SLOT: u8 = 0b0000_0001;
 const DATA_COMPLETE_SHRED: u8 = 0b0000_0010;
@@ -624,7 +626,8 @@ impl Shredder {
     fn generate_coding_shreds(&mut self) {
         if self.fec_rate != 0.0 {
             let num_data = (self.index - self.fec_set_index) as usize;
-            let num_coding = (self.fec_rate * num_data as f32) as usize;
+            // always generate at least 1 coding shred even if the fec_rate doesn't allow it
+            let num_coding = 1.max((self.fec_rate * num_data as f32) as usize);
             let session =
                 Session::new(num_data, num_coding).expect("Failed to create erasure session");
             let start_index = self.index - num_data as u32;