Merge pull request #8328 from urbas/max-substitution-jobs

doc/manual/src/release-notes/rl-next.md

@@ -1,2 +1,6 @@
 # Release X.Y (202?-??-??)
 
+- Speed-up of downloads from binary caches.
+  The number of parallel downloads (also known as substitutions) has been separated from the [`--max-jobs` setting](../command-ref/conf-file.md#conf-max-jobs).
+  The new setting is called [`max-substitution-jobs`](../command-ref/conf-file.md#conf-max-substitution-jobs).
+  The number of parallel downloads is now set to 16 by default (previously, the default was 1 due to the coupling to build jobs).

src/libstore/build/substitution-goal.cc

@@ -200,10 +200,9 @@ void PathSubstitutionGoal::tryToRun()
 {
     trace("trying to run");
 
-    /* Make sure that we are allowed to start a build.  Note that even
-       if maxBuildJobs == 0 (no local builds allowed), we still allow
-       a substituter to run.  This is because substitutions cannot be
-       distributed to another machine via the build hook. */
+    /* Make sure that we are allowed to start a substitution.  Note that even
+       if maxSubstitutionJobs == 0, we still allow a substituter to run. This
+       prevents infinite waiting. */
     if (worker.getNrSubstitutions() >= std::max(1U, (unsigned int) settings.maxSubstitutionJobs)) {
         worker.waitForBuildSlot(shared_from_this());
         return;