Make number of parallel substituter queries configurable

NixOS · Oct 6, 2021 · b3104c2 · b3104c2
1 parent 53e4794
commit b3104c2
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Showing 2 changed files with 12 additions and 1 deletion.
diff --git a/src/libstore/globals.hh b/src/libstore/globals.hh
@@ -250,6 +250,17 @@ public:
         )",
         {"build-use-substitutes"}};
 
+    Setting<unsigned int> maxQuerySubstitutersJobs{
+        this, 25, "max-query-substituters-jobs",
+        R"(
+          This option defines the maximum number of parallel queries Nix will
+          use to ask substituters for the presence of substitutes (which is a
+          separate step from actually downloading substitutes). For builds of
+          many derivations backed by substituters supporting HTTP/2,
+          significant speedups of this step have been observed by increasing
+          this option thanks to request multiplexing.
+        )"};
+
     Setting<std::string> buildUsersGroup{
         this, "", "build-users-group",
         R"(

diff --git a/src/libstore/misc.cc b/src/libstore/misc.cc
@@ -100,7 +100,7 @@ void Store::queryMissing(const std::vector<DerivedPath> & targets,
 
     downloadSize_ = narSize_ = 0;
 
-    ThreadPool pool;
+    ThreadPool pool(settings.maxQuerySubstitutersJobs);
 
     struct State
     {