From 97b98ec4b9ffcbc1a6f0917c851259d47514bfce Mon Sep 17 00:00:00 2001
From: Ian Jackson <ijackson@chiark.greenend.org.uk>
Date: Sun, 22 Aug 2021 18:58:45 +0100
Subject: [PATCH] discussion of hardcoded backpressure constants

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
---
 server/server.rs | 28 +++++++++++++++++++++++-----
 server/sweb.rs   |  2 +-
 2 files changed, 24 insertions(+), 6 deletions(-)

diff --git a/server/server.rs b/server/server.rs
index 0b5dde3..c28e017 100644
--- a/server/server.rs
+++ b/server/server.rs
@@ -22,7 +22,26 @@ pub struct Opts {
 
 pub const METADATA_MAX_LEN: usize = MAX_OVERHEAD;
 
-pub const MAXQUEUE_ROUTE2WEB: usize = 15; // xxx: config
+
+// ----- Backpressure discussion -----
+
+// These two kinds of channels are sent blockingly, so this means the
+// task which calls route_packet can get this far ahead, before a
+// context switch to the receiving task is forced.
+pub const MAXQUEUE_ROUTE2USER: usize = 15;
+pub const MAXQUEUE_ROUTE2LOCAL: usize = 50;
+
+// This channel is sent with try_send, ie non-blocking.  If the user
+// task becomes overloaded, requests will start to be rejected.
+pub const MAXQUEUE_WEBREQ2USER: usize = 5;
+
+// The user task prioritises 1. returning requests or discarding data,
+// 2. handling data routed to it.  Ie it prefers to drain queues.
+//
+// The slocal task prioritises handling routed data and writing it
+// (synchronously) to the local kernel.  So if the local kernel starts
+// blocking, all tasks may end up blocked waiting for things to drain.
+
 
 #[derive(Debug)]
 pub struct Global {
@@ -116,11 +135,10 @@ async fn main() {
     let (client_handles_send, client_handles_recv) = ics.iter()
       .map(|_ic| {
         let (web_send, web_recv) = mpsc::channel(
-          5 // xxx should me max_requests_outstanding but that's
-          // marked client-only so needs rework
+          MAXQUEUE_WEBREQ2USER
         );
         let (route_send, route_recv) = mpsc::channel(
-          MAXQUEUE_ROUTE2WEB
+          MAXQUEUE_ROUTE2USER
         );
         ((web_send, route_send), (web_recv, route_recv))
       }).unzip::<_,_,Vec<_>,Vec<_>>();
@@ -138,7 +156,7 @@ async fn main() {
     }).collect();
 
     let (local_rx_send, local_tx_recv) = mpsc::channel(
-      50 // xxx configurable?
+      MAXQUEUE_ROUTE2LOCAL
     );
 
     let global = Arc::new(Global {
diff --git a/server/sweb.rs b/server/sweb.rs
index 49416c6..0e52fb9 100644
--- a/server/sweb.rs
+++ b/server/sweb.rs
@@ -192,7 +192,7 @@ pub async fn handle(
     };
 
     client.web.try_send(wreq)
-      .map_err(|_| anyhow!("client task shut down!"))?;
+      .map_err(|_| anyhow!("client user task overloaded"))?;
 
     let reply: WebResponse = reply_recv.await?;
     warnings = reply.warnings;
-- 
2.30.2