Why my solo chain block time is 12 seconds when set to 6?

Question

I have an my own Substrate node (not a parachain, based on the template-node, so Aura consensus is used) and I changed the block time to 6 seconds in the runtime/src/lib.rs by setting

pub const MILLISECS_PER_BLOCK: u64 = 6000;

Though, when running the node locally with --dev configuration, it takes 12 seconds to produce blocks:

2022-09-01 12:50:36.010  INFO tokio-runtime-worker sc_basic_authorship::basic_authorship: 🙌 Starting consensus session on top of parent 0x7b2d27b8c9d1bacda74bf281bcf1f3fa6fdc9e017490860e55356b695aec9d4d    
2022-09-01 12:50:36.029 DEBUG tokio-runtime-worker runtime::system: [1] length: 11 (normal 0%, op: 0%, mandatory 0%) / normal weight: 0 (0%) / op weight 0 (0%) / mandatory weight 5921651000 (0%)    
2022-09-01 12:50:36.030  INFO tokio-runtime-worker sc_basic_authorship::basic_authorship: 🎁 Prepared block for proposing at 1 (3 ms) [hash: 0x017f1e5a02dcccb639a13c5fb9e5c5844b4efdfc596613db9da255000b89e414; parent_hash: 0x7b2d…9d4d; extrinsics (1): [0x7932…5556]]    
2022-09-01 12:50:36.031  INFO tokio-runtime-worker aura: 🔖 Pre-sealed block for proposal at 1. Hash now 0xdd564786a31bceefd7b4468173234870866615cd9433b438e84214e9debb9a18, previously 0x017f1e5a02dcccb639a13c5fb9e5c5844b4efdfc596613db9da255000b89e414.    
2022-09-01 12:50:36.032  INFO tokio-runtime-worker substrate: ✨ Imported #1 (0xdd56…9a18)    
2022-09-01 12:50:39.100  INFO tokio-runtime-worker substrate: 💤 Idle (0 peers), best: #1 (0xdd56…9a18), finalized #0 (0x7b2d…9d4d), ⬇ 0 ⬆ 0    
2022-09-01 12:50:44.101  INFO tokio-runtime-worker substrate: 💤 Idle (0 peers), best: #1 (0xdd56…9a18), finalized #0 (0x7b2d…9d4d), ⬇ 0 ⬆ 0    
2022-09-01 12:50:48.001  INFO tokio-runtime-worker sc_basic_authorship::basic_authorship: 🙌 Starting consensus session on top of parent 0xdd564786a31bceefd7b4468173234870866615cd9433b438e84214e9debb9a18    

Why is it taking 12 seconds to produce blocks?

Answer 1

Does your Aura chain_spec.rs have 2 validators in it? Aura is a round-robin based protocol, if you have more than 1 validator, it will expect another block to be made in order of the list of authorities of your chain. Therefore, it might be the case that Bob fails to make a block after 6 seconds and loses his turn (since an extra 6 seconds are given to buffer the possibility that Bob is slow).