Make find_route's dist map elements fit in 128 bytes

We'd previously aggressively cached elements in the
`PathBuildingHop` struct (and its sub-structs), which resulted in a
rather bloated size. This implied cache misses as we read from and
write to multiple cache lines during processing of a single
channel.

Here, we reduce caching in `DirectedChannelInfo`, fitting the
`(NodeId, PathBuildingHop)` tuple in exactly 128 bytes. While this
should fit in a single cache line, it sadly does not generally lie
in only two lines, as glibc returns large buffers from `malloc`
which are very well aligned, plus 16 bytes (for its own allocation
tracking). Thus, we try to avoid reading from the last 16 bytes of
a `PathBuildingHop`, but luckily that isn't super hard.

Note that here we make accessing
`DirectedChannelInfo::effective_capacity` somewhat slower, but
that's okay as its only ever done once per `DirectedChannelInfo`
anyway.

While our routing benchmarks are quite noisy, this appears to
result in between a 5% and 15% performance improvement in the
probabilistic scoring benchmarks.

lightning/src/routing/gossip.rs

@@ -990,8 +990,6 @@ impl Readable for ChannelInfo {
 pub struct DirectedChannelInfo<'a> {
 	channel: &'a ChannelInfo,
 	direction: &'a ChannelUpdateInfo,
-	htlc_maximum_msat: u64,
-	effective_capacity: EffectiveCapacity,
 	/// The direction this channel is in - if set, it indicates that we're traversing the channel
 	/// from [`ChannelInfo::node_one`] to [`ChannelInfo::node_two`].
 	from_node_one: bool,
@@ -1000,39 +998,30 @@ pub struct DirectedChannelInfo<'a> {
 impl<'a> DirectedChannelInfo<'a> {
 	#[inline]
 	fn new(channel: &'a ChannelInfo, direction: &'a ChannelUpdateInfo, from_node_one: bool) -> Self {
-		let mut htlc_maximum_msat = direction.htlc_maximum_msat;
-		let capacity_msat = channel.capacity_sats.map(|capacity_sats| capacity_sats * 1000);
-
-		let effective_capacity = match capacity_msat {
-			Some(capacity_msat) => {
-				htlc_maximum_msat = cmp::min(htlc_maximum_msat, capacity_msat);
-				EffectiveCapacity::Total { capacity_msat, htlc_maximum_msat }
-			},
-			None => EffectiveCapacity::AdvertisedMaxHTLC { amount_msat: htlc_maximum_msat },
-		};
-
-		Self {
-			channel, direction, htlc_maximum_msat, effective_capacity, from_node_one,
-		}
+		Self { channel, direction, from_node_one }
 	}
 
 	/// Returns information for the channel.
 	#[inline]
 	pub fn channel(&self) -> &'a ChannelInfo { self.channel }
 
-	/// Returns the maximum HTLC amount allowed over the channel in the direction.
-	#[inline]
-	pub fn htlc_maximum_msat(&self) -> u64 {
-		self.htlc_maximum_msat
-	}
-
 	/// Returns the [`EffectiveCapacity`] of the channel in the direction.
 	///
 	/// This is either the total capacity from the funding transaction, if known, or the
 	/// `htlc_maximum_msat` for the direction as advertised by the gossip network, if known,
 	/// otherwise.
+	#[inline]
 	pub fn effective_capacity(&self) -> EffectiveCapacity {
-		self.effective_capacity
+		let mut htlc_maximum_msat = self.direction().htlc_maximum_msat;
+		let capacity_msat = self.channel.capacity_sats.map(|capacity_sats| capacity_sats * 1000);
+
+		match capacity_msat {
+			Some(capacity_msat) => {
+				htlc_maximum_msat = cmp::min(htlc_maximum_msat, capacity_msat);
+				EffectiveCapacity::Total { capacity_msat, htlc_maximum_msat }
+			},
+			None => EffectiveCapacity::AdvertisedMaxHTLC { amount_msat: htlc_maximum_msat },
+		}
 	}
 
 	/// Returns information for the direction.
@@ -1042,11 +1031,13 @@ impl<'a> DirectedChannelInfo<'a> {
 	/// Returns the `node_id` of the source hop.
 	///
 	/// Refers to the `node_id` forwarding the payment to the next hop.
+	#[inline]
 	pub(super) fn source(&self) -> &'a NodeId { if self.from_node_one { &self.channel.node_one } else { &self.channel.node_two } }
 
 	/// Returns the `node_id` of the target hop.
 	///
 	/// Refers to the `node_id` receiving the payment from the previous hop.
+	#[inline]
 	pub(super) fn target(&self) -> &'a NodeId { if self.from_node_one { &self.channel.node_two } else { &self.channel.node_one } }
 }
 

lightning/src/routing/router.rs

@@ -1183,6 +1183,10 @@ impl<'a> CandidateRouteHop<'a> {
 		}
 	}
 
+	/// Fetch the effective capacity of this hop.
+	///
+	/// Note that this may be somewhat expensive, so calls to this should be limited and results
+	/// cached!
 	fn effective_capacity(&self) -> EffectiveCapacity {
 		match self {
 			CandidateRouteHop::FirstHop { details, .. } => EffectiveCapacity::ExactLiquidity {
@@ -1338,6 +1342,18 @@ struct PathBuildingHop<'a> {
 	value_contribution_msat: u64,
 }
 
+// Checks that the entries in the `find_route` `dist` map fit in (exactly) two standard x86-64
+// cache lines. Sadly, they're not guaranteed to actually lie on a cache line (and in fact,
+// generally won't, because at least glibc's malloc will align to a nice, big, round
+// boundary...plus 16), but at least it will reduce the amount of data we'll need to load.
+//
+// Note that these assertions only pass on somewhat recent rustc, and thus are gated on the
+// ldk_bench flag.
+#[cfg(ldk_bench)]
+const _NODE_MAP_SIZE_TWO_CACHE_LINES: usize = 128 - core::mem::size_of::<(NodeId, PathBuildingHop)>();
+#[cfg(ldk_bench)]
+const _NODE_MAP_SIZE_EXACTLY_CACHE_LINES: usize = core::mem::size_of::<(NodeId, PathBuildingHop)>() - 128;
+
 impl<'a> core::fmt::Debug for PathBuildingHop<'a> {
 	fn fmt(&self, f: &mut core::fmt::Formatter) -> Result<(), core::fmt::Error> {
 		let mut debug_struct = f.debug_struct("PathBuildingHop");