core: Move the subgraph task to the blocking thread pool

It has a lot of database interactions, this should reduce
the amount of thread pool contention in indexing nodes.

Author: leoyvens

Cargo.lock

@@ -14,6 +14,7 @@ serde = "1.0"
 serde_json = "1.0"
 serde_yaml = "0.7"
 uuid = { version = "0.7.2", features = ["v4"] }
+tokio-threadpool = "0.1.14"
 
 [dev-dependencies]
 # We're using the latest ipfs-api for the HTTPS support that was merged in

core/Cargo.toml

@@ -255,7 +255,17 @@ impl SubgraphInstanceManager {
         // creates dynamic data sources. This allows us to recreate the
         // block stream and include events for the new data sources going
         // forward; this is easier than updating the existing block stream.
-        tokio::spawn(loop_fn(ctx, move |ctx| run_subgraph(ctx)));
+        let mut subgraph_task = loop_fn(ctx, |ctx| run_subgraph(ctx));
+
+        // This task has many calls to the store, so mark it as `blocking`.
+        tokio::spawn(future::poll_fn(move || {
+            match tokio_threadpool::blocking(|| subgraph_task.poll()) {
+                Ok(Async::NotReady) | Ok(Async::Ready(Ok(Async::NotReady))) => Ok(Async::NotReady),
+                Ok(Async::Ready(Ok(Async::Ready(())))) => Ok(Async::Ready(())),
+                Ok(Async::Ready(Err(()))) => Err(()),
+                Err(_) => panic!("not inside a tokio thread pool"),
+            }
+        }));
 
         Ok(())
     }