diff --git a/core/src/subgraph/context/instance/mod.rs b/core/src/subgraph/context/instance/mod.rs
index 494ae227e97..77e66457ad8 100644
--- a/core/src/subgraph/context/instance/mod.rs
+++ b/core/src/subgraph/context/instance/mod.rs
@@ -1,7 +1,7 @@
 mod hosts;
+mod module_cache;
 
 use anyhow::ensure;
-use graph::futures01::sync::mpsc::Sender;
 use graph::{
     blockchain::{Blockchain, TriggerData as _},
     data_source::{
@@ -11,7 +11,7 @@ use graph::{
     prelude::*,
 };
 use hosts::{OffchainHosts, OnchainHosts};
-use std::collections::HashMap;
+use module_cache::ModuleCache;
 
 pub(crate) struct SubgraphInstance<C: Blockchain, T: RuntimeHostBuilder<C>> {
     subgraph_id: DeploymentHash,
@@ -36,7 +36,7 @@ pub(crate) struct SubgraphInstance<C: Blockchain, T: RuntimeHostBuilder<C>> {
     offchain_hosts: OffchainHosts<C, T>,
 
     /// Maps the hash of a module to a channel to the thread in which the module is instantiated.
-    module_cache: HashMap<[u8; 32], Sender<T::Req>>,
+    module_cache: ModuleCache<T::Req>,
 
     /// This manages the sequence of causality regions for the subgraph.
     causality_region_seq: CausalityRegionSeq,
@@ -85,7 +85,7 @@ where
             onchain_hosts: OnchainHosts::new(),
             subgraph_hosts: OnchainHosts::new(),
             offchain_hosts: OffchainHosts::new(),
-            module_cache: HashMap::new(),
+            module_cache: ModuleCache::new(),
             templates,
             host_metrics,
             causality_region_seq,
@@ -103,21 +103,14 @@ where
             Some(module_bytes) => module_bytes.cheap_clone(),
         };
 
-        let mapping_request_sender = {
-            let module_hash = alloy::primitives::keccak256(module_bytes.as_ref()).0;
-            if let Some(sender) = self.module_cache.get(&module_hash) {
-                sender.clone()
-            } else {
-                let sender = T::spawn_mapping(
-                    module_bytes.as_ref(),
-                    logger,
-                    self.subgraph_id.clone(),
-                    self.host_metrics.cheap_clone(),
-                )?;
-                self.module_cache.insert(module_hash, sender.clone());
-                sender
-            }
-        };
+        let mapping_request_sender = self.module_cache.sender(&module_bytes, |bytes| {
+            T::spawn_mapping(
+                bytes,
+                logger,
+                self.subgraph_id.clone(),
+                self.host_metrics.cheap_clone(),
+            )
+        })?;
 
         let host = self.host_builder.build(
             self.network.clone(),
diff --git a/core/src/subgraph/context/instance/module_cache.rs b/core/src/subgraph/context/instance/module_cache.rs
new file mode 100644
index 00000000000..21b66432dfd
--- /dev/null
+++ b/core/src/subgraph/context/instance/module_cache.rs
@@ -0,0 +1,190 @@
+use graph::futures01::sync::mpsc::Sender;
+use graph::prelude::Error;
+use std::collections::HashMap;
+use std::sync::Arc;
+
+/// Maps the hash of a module to a channel to the thread in which the module
+/// is instantiated.
+///
+/// Hashing a module reads all of its bytes, and a subgraph can have millions
+/// of data sources that share a handful of modules. The hash is therefore
+/// memoized by the identity of the `Arc` that holds the bytes: every data
+/// source created from the same manifest mapping or template shares that
+/// mapping's `Arc`. The `Arc` is held by the memo, so its address can't be
+/// reused for other bytes while the entry exists. Bytes in another `Arc` are
+/// hashed again: equal contents give the same hash, hence the same module.
+pub(super) struct ModuleCache<R> {
+    senders: HashMap<[u8; 32], Sender<R>>,
+    hashes: HashMap<usize, (Arc<Vec<u8>>, [u8; 32])>,
+    #[cfg(test)]
+    hashes_computed: usize,
+}
+
+impl<R> ModuleCache<R> {
+    pub fn new() -> Self {
+        ModuleCache {
+            senders: HashMap::new(),
+            hashes: HashMap::new(),
+            #[cfg(test)]
+            hashes_computed: 0,
+        }
+    }
+
+    /// The keccak256 hash of `module_bytes`, computed once per `Arc`.
+    fn hash(&mut self, module_bytes: &Arc<Vec<u8>>) -> [u8; 32] {
+        let key = Arc::as_ptr(module_bytes) as usize;
+        if let Some((bytes, hash)) = self.hashes.get(&key)
+            && Arc::ptr_eq(bytes, module_bytes)
+        {
+            return *hash;
+        }
+        let hash = alloy::primitives::keccak256(module_bytes.as_slice()).0;
+        #[cfg(test)]
+        {
+            self.hashes_computed += 1;
+        }
+        self.hashes.insert(key, (module_bytes.clone(), hash));
+        hash
+    }
+
+    /// The sender to the module `module_bytes`, spawned by `spawn` if no
+    /// module with the same hash was spawned before.
+    pub fn sender(
+        &mut self,
+        module_bytes: &Arc<Vec<u8>>,
+        spawn: impl FnOnce(&[u8]) -> Result<Sender<R>, Error>,
+    ) -> Result<Sender<R>, Error> {
+        let module_hash = self.hash(module_bytes);
+        if let Some(sender) = self.senders.get(&module_hash) {
+            return Ok(sender.clone());
+        }
+        let sender = spawn(module_bytes.as_slice())?;
+        self.senders.insert(module_hash, sender.clone());
+        Ok(sender)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use graph::futures01::sync::mpsc::{Receiver, channel};
+
+    /// A cache whose spawns are recorded: the receivers, by spawn order.
+    struct Spawns {
+        cache: ModuleCache<u32>,
+        receivers: Vec<Receiver<u32>>,
+    }
+
+    impl Spawns {
+        fn new() -> Self {
+            Spawns {
+                cache: ModuleCache::new(),
+                receivers: Vec::new(),
+            }
+        }
+
+        fn sender(&mut self, bytes: &Arc<Vec<u8>>) -> Sender<u32> {
+            let receivers = &mut self.receivers;
+            self.cache
+                .sender(bytes, |_| {
+                    let (tx, rx) = channel(1);
+                    receivers.push(rx);
+                    Ok(tx)
+                })
+                .unwrap()
+        }
+    }
+
+    /// Whether `sender` sends to the receiver of spawn `idx`: drop that
+    /// receiver and see if the sender is disconnected.
+    fn sends_to(mut spawns: Spawns, sender: &mut Sender<u32>, idx: usize) -> bool {
+        drop(spawns.receivers.remove(idx));
+        sender.try_send(0).is_err_and(|e| e.is_disconnected())
+    }
+
+    fn module(n: u8) -> Vec<u8> {
+        (0..66_000u32).map(|i| (i as u8) ^ n).collect()
+    }
+
+    #[test]
+    fn two_templates_two_hashes() {
+        let a = Arc::new(module(1));
+        let b = Arc::new(module(2));
+        let mut s = Spawns::new();
+        let ha = s.cache.hash(&a);
+        let hb = s.cache.hash(&b);
+        assert_ne!(ha, hb);
+        assert_eq!(ha, alloy::primitives::keccak256(a.as_slice()).0);
+        assert_eq!(hb, alloy::primitives::keccak256(b.as_slice()).0);
+
+        s.sender(&a);
+        let mut sb = s.sender(&b);
+        assert_eq!(s.receivers.len(), 2, "one module per template");
+        assert!(sends_to(s, &mut sb, 1));
+    }
+
+    #[test]
+    fn sources_of_one_template_share_hash_and_sender() {
+        let template = Arc::new(module(1));
+        let other = Arc::new(module(2));
+        let mut s = Spawns::new();
+        s.sender(&other);
+        s.sender(&template);
+        let mut second = s.sender(&template.clone());
+        assert_eq!(s.cache.hashes_computed, 2);
+        assert_eq!(s.receivers.len(), 2);
+        assert!(sends_to(s, &mut second, 1), "the template's sender");
+    }
+
+    #[test]
+    fn many_sources_hash_once_per_template() {
+        const N: usize = 100_000;
+        let a = Arc::new(module(1));
+        let b = Arc::new(module(2));
+        let mut s = Spawns::new();
+        for i in 0..N {
+            let bytes = if i % 2 == 0 { a.clone() } else { b.clone() };
+            s.sender(&bytes);
+        }
+        assert_eq!(s.cache.hashes_computed, 2);
+        assert_eq!(s.receivers.len(), 2);
+
+        let mut s = Spawns::new();
+        for _ in 0..N {
+            s.sender(&a.clone());
+        }
+        assert_eq!(s.cache.hashes_computed, 1);
+        assert_eq!(s.receivers.len(), 1);
+    }
+
+    #[test]
+    fn equal_unshared_bytes_give_same_hash_and_module() {
+        let shared = Arc::new(module(1));
+        let copy = Arc::new(module(1));
+        assert!(!Arc::ptr_eq(&shared, &copy));
+        let mut s = Spawns::new();
+        s.sender(&Arc::new(module(2)));
+        s.sender(&shared);
+        let hash = s.cache.hash(&shared);
+        assert_eq!(s.cache.hash(&copy), hash);
+        assert_eq!(s.cache.hashes_computed, 3, "the copy is hashed");
+        let mut from_copy = s.sender(&copy);
+        assert_eq!(s.receivers.len(), 2, "no new module for the copy");
+        assert!(sends_to(s, &mut from_copy, 1), "the shared bytes' sender");
+    }
+
+    #[test]
+    fn dropped_bytes_are_not_confused_with_new_ones() {
+        // Allocate and drop many `Arc`s: the memo holds each one, so no
+        // later allocation can take the address of a memoized one.
+        let mut s = Spawns::new();
+        for n in 0..64u8 {
+            let bytes = Arc::new(vec![n; 64]);
+            assert_eq!(
+                s.cache.hash(&bytes),
+                alloy::primitives::keccak256(bytes.as_slice()).0
+            );
+        }
+        assert_eq!(s.cache.hashes_computed, 64);
+    }
+}