From: Matt Corallo Date: Wed, 6 Dec 2023 05:02:07 +0000 (+0000) Subject: Reorder `PathBuildingHop` fields somewhat X-Git-Tag: v0.0.119~18^2~1 X-Git-Url: http://git.bitcoin.ninja/?a=commitdiff_plain;h=8ba3e83bb0a38b9fc8a3e492f70e6f78bd575d73;p=rust-lightning Reorder `PathBuildingHop` fields somewhat Given `PathBuildingHop` is now an even multiple of cache lines, we can pick which fields "fall off" the cache line we have visible when dealing with hops, which we do here. --- diff --git a/lightning/src/routing/router.rs b/lightning/src/routing/router.rs index 652c8cd32..e2ccd1f73 100644 --- a/lightning/src/routing/router.rs +++ b/lightning/src/routing/router.rs @@ -1311,15 +1311,15 @@ fn iter_equal(mut iter_a: I1, mut iter_b: I2) /// Fee values should be updated only in the context of the whole path, see update_value_and_recompute_fees. /// These fee values are useful to choose hops as we traverse the graph "payee-to-payer". #[derive(Clone)] +#[repr(C)] // Force fields to appear in the order we define them. struct PathBuildingHop<'a> { candidate: CandidateRouteHop<'a>, - fee_msat: u64, - - /// All the fees paid *after* this channel on the way to the destination - next_hops_fee_msat: u64, - /// Fee paid for the use of the current channel (see candidate.fees()). - /// The value will be actually deducted from the counterparty balance on the previous link. - hop_use_fee_msat: u64, + /// If we've already processed a node as the best node, we shouldn't process it again. Normally + /// we'd just ignore it if we did as all channels would have a higher new fee, but because we + /// may decrease the amounts in use as we walk the graph, the actual calculated fee may + /// decrease as well. Thus, we have to explicitly track which nodes have been processed and + /// avoid processing them again. + was_processed: bool, /// Used to compare channels when choosing the for routing. /// Includes paying for the use of a hop and the following hops, as well as /// an estimated cost of reaching this hop. @@ -1331,12 +1331,20 @@ struct PathBuildingHop<'a> { /// All penalties incurred from this channel on the way to the destination, as calculated using /// channel scoring. path_penalty_msat: u64, - /// If we've already processed a node as the best node, we shouldn't process it again. Normally - /// we'd just ignore it if we did as all channels would have a higher new fee, but because we - /// may decrease the amounts in use as we walk the graph, the actual calculated fee may - /// decrease as well. Thus, we have to explicitly track which nodes have been processed and - /// avoid processing them again. - was_processed: bool, + + // The last 16 bytes are on the next cache line by default in glibc's malloc. Thus, we should + // only place fields which are not hot there. Luckily, the next three fields are only read if + // we end up on the selected path, and only in the final path layout phase, so we don't care + // too much if reading them is slow. + + fee_msat: u64, + + /// All the fees paid *after* this channel on the way to the destination + next_hops_fee_msat: u64, + /// Fee paid for the use of the current channel (see candidate.fees()). + /// The value will be actually deducted from the counterparty balance on the previous link. + hop_use_fee_msat: u64, + #[cfg(all(not(ldk_bench), any(test, fuzzing)))] // In tests, we apply further sanity checks on cases where we skip nodes we already processed // to ensure it is specifically in cases where the fee has gone down because of a decrease in