feat postgres: new connections reservation strategy in postgres connections auto-limiter

Changes a number of reserved connection from 5 to 5% of the total limit capped to mimimum 2 and maximum 10 connections.
commit_hash:c1407d39e0c870b796b90a69c356f9e6aa5bbec9

Author: lexeyo

core/include/userver/utils/impl/userver_experiments.hpp

@@ -60,6 +60,7 @@ extern UserverExperiment kPgCcExperiment;
 extern UserverExperiment kYdbDeadlinePropagationExperiment;
 extern UserverExperiment kWaitAllCheckedUpgradeExperiment;
 extern UserverExperiment kPgConnlimitWatchdogFallbackExperiment;
+extern UserverExperiment kPgConnlimitWatchdogReservationExperiment;
 
 }  // namespace utils::impl
 

core/src/utils/impl/userver_experiments.cpp

@@ -96,6 +96,7 @@ UserverExperiment kPgCcExperiment{"pg-cc"};
 UserverExperiment kYdbDeadlinePropagationExperiment{"ydb-deadline-propagation"};
 UserverExperiment kWaitAllCheckedUpgradeExperiment{"wait-all-checked-upgrade"};
 UserverExperiment kPgConnlimitWatchdogFallbackExperiment{"pg-connlimit-watchdog-new-fallback"};
+UserverExperiment kPgConnlimitWatchdogReservationExperiment{"pg-connlimit-watchdog-new-reservation"};
 
 }  // namespace utils::impl
 

postgresql/src/storages/postgres/connlimit_watchdog.cpp

@@ -13,6 +13,9 @@ namespace {
 constexpr CommandControl kCommandControl{std::chrono::seconds(2), std::chrono::seconds(2)};
 constexpr std::size_t kTestsuiteConnlimit = 100;
 constexpr std::size_t kReservedConn = 5;
+constexpr std::size_t kMinReservedConnections = 2;
+constexpr std::size_t kMaxReservedConnections = 10;
+constexpr double kReservedConnectionsPercentage = 0.05;
 
 constexpr int kMaxStepsWithError = 3;
 constexpr std::size_t kFallbackConnlimit = 20;
@@ -27,8 +30,18 @@ std::size_t GetMaxConnections(Transaction& trx) {
     }
     std::size_t max_connections = std::min(max_server_connections, max_user_connections);
 
-    if (max_connections > kReservedConn) {
-        max_connections -= kReservedConn;
+    const auto reserved_connections =
+        !USERVER_NAMESPACE::utils::impl::kPgConnlimitWatchdogReservationExperiment.IsEnabled()
+            ? kReservedConn
+            : std::max(
+                  kMinReservedConnections,
+                  std::min(
+                      kMaxReservedConnections,
+                      static_cast<std::size_t>(max_connections * kReservedConnectionsPercentage)
+                  )
+              );
+    if (max_connections > reserved_connections) {
+        max_connections -= reserved_connections;
     } else {
         max_connections = 1;
     }

postgresql/src/storages/postgres/tests/connlimit_watchdog_pgtest.cpp

@@ -69,7 +69,8 @@ pg::Transaction GetTransaction(pgd::ClusterImpl& cluster) {
 }
 
 constexpr size_t kReservedConn = 5;
-constexpr size_t kTestsuiteConnlimit = 100 - kReservedConn;
+constexpr size_t kTestsuiteServerConnlimit = 100;
+constexpr size_t kTestsuiteConnlimit = kTestsuiteServerConnlimit - kReservedConn;
 constexpr size_t kFallbackConnlimit = 17;
 constexpr size_t kMaxStepsWithError = 3;
 
@@ -127,6 +128,17 @@ class Watchdog : public PostgreSQLBase {
     utils::impl::UserverExperimentsScope scope_;
 };
 
+class WatchdogWithNewConnectionsReservation : public Watchdog {
+public:
+    void SetUp() override {
+        Watchdog::SetUp();
+        scope_.Set(utils::impl::kPgConnlimitWatchdogReservationExperiment, true);
+    }
+
+private:
+    utils::impl::UserverExperimentsScope scope_;
+};
+
 UTEST_F(Watchdog, Basic) {
     EXPECT_EQ(kTestsuiteConnlimit, DoStepV1());
 
@@ -243,4 +255,17 @@ UTEST_F(Watchdog, FallbackConnlimit) {
     ASSERT_EQ(kFallbackConnlimit, watchdog.GetConnlimit());
 }
 
+UTEST_F(WatchdogWithNewConnectionsReservation, CheckLimit) {
+    constexpr auto
+        kConnectionsLimit = kTestsuiteServerConnlimit - static_cast<std::size_t>(kTestsuiteServerConnlimit * 0.05);
+
+    EXPECT_EQ(kConnectionsLimit, DoStepV1());
+
+    // There are two hosts after 'StepV2'.
+    EXPECT_EQ(kConnectionsLimit / 2, DoStepV2());
+
+    // There are two hosts after 'StepV2'.
+    EXPECT_EQ(kConnectionsLimit / 2, DoStepV1());
+}
+
 USERVER_NAMESPACE_END