src/hydra-queue-runner/build-remote.cc

@@ -386,6 +386,16 @@ void RemoteResult::updateWithBuildResult(const nix::BuildResult & buildResult)
 
 }
 
+/* Utility guard object to auto-release a semaphore on destruction. */
+template <typename T>
+class SemaphoreReleaser {
+public:
+    SemaphoreReleaser(T* s) : sem(s) {}
+    ~SemaphoreReleaser() { sem->release(); }
+
+private:
+    T* sem;
+};
 
 void State::buildRemote(ref<Store> destStore,
     ::Machine::ptr machine, Step::ptr step,
@@ -527,6 +537,14 @@ void State::buildRemote(ref<Store> destStore,
             result.logFile = "";
         }
 
+        /* Throttle CPU-bound work. Opportunistically skip updating the current
+         * step, since this requires a DB roundtrip. */
+        if (!localWorkThrottler.try_acquire()) {
+            updateStep(ssWaitingForLocalSlot);
+            localWorkThrottler.acquire();
+        }
+        SemaphoreReleaser releaser(&localWorkThrottler);
+
         StorePathSet outputs;
         for (auto & [_, realisation] : buildResult.builtOutputs)
             outputs.insert(realisation.outPath);

src/hydra-queue-runner/hydra-queue-runner.cc

@@ -85,6 +85,7 @@ State::State(std::optional<std::string> metricsAddrOpt)
     : config(std::make_unique<HydraConfig>())
     , maxUnsupportedTime(config->getIntOption("max_unsupported_time", 0))
     , dbPool(config->getIntOption("max_db_connections", 128))
+    , localWorkThrottler(config->getIntOption("max_local_worker_threads", std::min(maxSupportedLocalWorkers, std::max(4u, std::thread::hardware_concurrency()) - 2)))
     , maxOutputSize(config->getIntOption("max_output_size", 2ULL << 30))
     , maxLogSize(config->getIntOption("max_log_size", 64ULL << 20))
     , uploadLogsToBinaryCache(config->getBoolOption("upload_logs_to_binary_cache", false))

src/hydra-queue-runner/state.hh

@@ -6,6 +6,8 @@
 #include <map>
 #include <memory>
 #include <queue>
+#include <regex>
+#include <semaphore>
 
 #include <prometheus/counter.h>
 #include <prometheus/gauge.h>
@@ -58,6 +60,7 @@ typedef enum {
     ssConnecting = 10,
     ssSendingInputs = 20,
     ssBuilding = 30,
+    ssWaitingForLocalSlot = 35,
     ssReceivingOutputs = 40,
     ssPostProcessing = 50,
 } StepState;
@@ -353,6 +356,10 @@ private:
     typedef std::map<nix::StoreReference::Variant, Machine::ptr> Machines;
     nix::Sync<Machines> machines; // FIXME: use atomic_shared_ptr
 
+    /* Throttler for CPU-bound local work. */
+    static constexpr unsigned int maxSupportedLocalWorkers = 1024;
+    std::counting_semaphore<maxSupportedLocalWorkers> localWorkThrottler;
+
     /* Various stats. */
     time_t startedAt;
     counter nrBuildsRead{0};

src/root/common.tt

@@ -270,6 +270,8 @@ BLOCK renderBusyStatus;
     <strong>Sending inputs</strong>
   [% ELSIF step.busy == 30 %]
     <strong>Building</strong>
+  [% ELSIF step.busy == 35 %]
+    <strong>Waiting to receive outputs</strong>
   [% ELSIF step.busy == 40 %]
     <strong>Receiving outputs</strong>
   [% ELSIF step.busy == 50 %]