8356000: C1/C2-only modes use 2 compiler threads on low CPU count machines

Reviewed-by: kvn, dfenacci, galder

src/hotspot/share/compiler/compilationPolicy.cpp

@@ -550,6 +550,7 @@ void CompilationPolicy::initialize() {
     int count = CICompilerCount;
     bool c1_only = CompilerConfig::is_c1_only();
     bool c2_only = CompilerConfig::is_c2_or_jvmci_compiler_only();
+    int min_count = (c1_only || c2_only) ? 1 : 2;
 
 #ifdef _LP64
     // Turn on ergonomic compiler count selection
@@ -560,7 +561,7 @@ void CompilationPolicy::initialize() {
       // Simple log n seems to grow too slowly for tiered, try something faster: log n * log log n
       int log_cpu = log2i(os::active_processor_count());
       int loglog_cpu = log2i(MAX2(log_cpu, 1));
-      count = MAX2(log_cpu * loglog_cpu * 3 / 2, 2);
+      count = MAX2(log_cpu * loglog_cpu * 3 / 2, min_count);
       // Make sure there is enough space in the code cache to hold all the compiler buffers
       size_t c1_size = 0;
 #ifdef COMPILER1
@@ -574,7 +575,7 @@ void CompilationPolicy::initialize() {
       int max_count = (ReservedCodeCacheSize - (CodeCacheMinimumUseSpace DEBUG_ONLY(* 3))) / (int)buffer_size;
       if (count > max_count) {
         // Lower the compiler count such that all buffers fit into the code cache
-        count = MAX2(max_count, c1_only ? 1 : 2);
+        count = MAX2(max_count, min_count);
       }
       FLAG_SET_ERGO(CICompilerCount, count);
     }
@@ -593,9 +594,10 @@ void CompilationPolicy::initialize() {
 #endif
 
     if (c1_only) {
-      // No C2 compiler thread required
+      // No C2 compiler threads are needed
       set_c1_count(count);
     } else if (c2_only) {
+      // No C1 compiler threads are needed
       set_c2_count(count);
     } else {
 #if INCLUDE_JVMCI
@@ -613,6 +615,9 @@ void CompilationPolicy::initialize() {
     }
     assert(count == c1_count() + c2_count(), "inconsistent compiler thread count");
     set_increase_threshold_at_ratio();
+  } else {
+    // Interpreter mode creates no compilers
+    FLAG_SET_ERGO(CICompilerCount, 0);
   }
   set_start_time(nanos_to_millis(os::javaTimeNanos()));
 }

test/hotspot/jtreg/compiler/arguments/TestCompilerCounts.java

@@ -0,0 +1,177 @@
+/*
+ * Copyright Amazon.com Inc. or its affiliates. All Rights Reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @library /test/lib /
+ * @bug 8356000
+ * @requires vm.flagless
+ * @requires vm.bits == "64"
+ * @run driver compiler.arguments.TestCompilerCounts
+ */
+
+package compiler.arguments;
+
+import java.io.IOException;
+import java.util.List;
+import java.util.Arrays;
+import java.util.ArrayList;
+import java.util.function.Function;
+import jdk.test.lib.Asserts;
+import jdk.test.lib.process.ProcessTools;
+import jdk.test.lib.process.OutputAnalyzer;
+import jdk.test.lib.cli.CommandLineOptionTest;
+
+public class TestCompilerCounts {
+
+    // Try not to go over max CPU count on the machine, since we do not
+    // know if the rest of runtime would accept it.
+    // For local testing, feel free to override this to a larger value
+    // if you want to see how heuristics works on even larger machines.
+    static final int MAX_CPUS = Runtime.getRuntime().availableProcessors();
+
+    // Test at most 16 CPUs linearly, to limit test execution time.
+    // After this limit, go for exponential steps.
+    static final int MAX_LINEAR_CPUS = Math.min(16, MAX_CPUS);
+
+    public static void main(String[] args) throws Throwable {
+        // CICompilerCount=0 is incorrect in default modes.
+        fail("-XX:CICompilerCount=0");
+
+        // Interpreter-only mode accepts all values, but sets 0 threads
+        pass(0, "-XX:CICompilerCount=1", "-XX:TieredStopAtLevel=0");
+
+        // C1/C2 only modes accept CICompilerCount=1
+        pass(1, "-XX:CICompilerCount=1", "-XX:TieredStopAtLevel=1");
+        pass(1, "-XX:CICompilerCount=1", "-XX:TieredStopAtLevel=2");
+        pass(1, "-XX:CICompilerCount=1", "-XX:TieredStopAtLevel=3");
+        pass(1, "-XX:CICompilerCount=1", "-XX:-TieredCompilation");
+
+        // C1+C2 modes need at least 2 threads
+        fail("-XX:CICompilerCount=1");
+        fail("-XX:CICompilerCount=1", "-XX:TieredStopAtLevel=4");
+
+        // Overriding the CICompilerCount overrides compiler counts hard.
+        for (int count = 2; count <= MAX_CPUS; count += (count >= MAX_LINEAR_CPUS ? count : 1)) {
+            String opt = "-XX:CICompilerCount=" + count;
+
+            // Interpreter-only mode always sets 0 threads
+            pass(0, opt, "-XX:TieredStopAtLevel=0");
+
+            // All compiled modes accept reasonable CICompilerCount
+            pass(count, opt);
+            pass(count, opt, "-XX:TieredStopAtLevel=1");
+            pass(count, opt, "-XX:TieredStopAtLevel=2");
+            pass(count, opt, "-XX:TieredStopAtLevel=3");
+            pass(count, opt, "-XX:TieredStopAtLevel=4");
+            pass(count, opt, "-XX:-TieredCompilation");
+        }
+
+        // Per CPU heuristics is disabled, we are going to set up defaults.
+
+        for (int cpus = 2; cpus <= MAX_CPUS; cpus += (cpus >= MAX_LINEAR_CPUS ? cpus : 1)) {
+            String opt = "-XX:ActiveProcessorCount=" + cpus;
+            String opt2 = "-XX:-CICompilerCountPerCPU";
+
+            // Interpreter-only mode always set 0 threads
+            pass(0, opt, opt2, "-XX:TieredStopAtLevel=0");
+
+            // All compiled modes default to 2 threads, statically compiled in
+            pass(2, opt, opt2);
+            pass(2, opt, opt2, "-XX:TieredStopAtLevel=1");
+            pass(2, opt, opt2, "-XX:TieredStopAtLevel=2");
+            pass(2, opt, opt2, "-XX:TieredStopAtLevel=3");
+            pass(2, opt, opt2, "-XX:TieredStopAtLevel=4");
+            pass(2, opt, opt2, "-XX:-TieredCompilation");
+        }
+
+        // Otherwise, we set CICompilerCount heuristically.
+
+        // Check hitting the lower values exactly first.
+        for (int cpus = 1; cpus <= 3; cpus++) {
+            String opt = "-XX:ActiveProcessorCount=" + cpus;
+
+            // Interpreter-only mode always set 0 threads
+            pass(0, opt, "-XX:TieredStopAtLevel=0");
+
+            // Non-tiered modes set 1 thread
+            pass(1, opt, "-XX:TieredStopAtLevel=1");
+            pass(1, opt, "-XX:TieredStopAtLevel=2");
+            pass(1, opt, "-XX:TieredStopAtLevel=3");
+            pass(1, opt, "-XX:-TieredCompilation");
+
+            // Tiered modes set 2 threads
+            pass(2, opt);
+            pass(2, opt, "-XX:TieredStopAtLevel=4");
+        }
+
+        // Check what heuristics sets past the trivial number of CPUs.
+        for (int cpus = 4; cpus <= MAX_CPUS; cpus += (cpus >= MAX_LINEAR_CPUS ? cpus : 1)) {
+            String opt = "-XX:ActiveProcessorCount=" + cpus;
+
+            // Interpreter-only mode always set 0 threads
+            pass(0, opt, "-XX:TieredStopAtLevel=0");
+
+            // Non-tiered modes
+            int nonTieredCount = heuristicCount(cpus, false);
+            pass(nonTieredCount, opt, "-XX:TieredStopAtLevel=1");
+            pass(nonTieredCount, opt, "-XX:TieredStopAtLevel=2");
+            pass(nonTieredCount, opt, "-XX:TieredStopAtLevel=3");
+            pass(nonTieredCount, opt, "-XX:-TieredCompilation");
+
+            // Tiered modes
+            int tieredCount = heuristicCount(cpus, true);
+            pass(tieredCount, opt);
+            pass(tieredCount, opt, "-XX:TieredStopAtLevel=4");
+
+            // Also check that heuristics did not set up more threads than CPUs available
+            Asserts.assertTrue(nonTieredCount <= cpus,
+                "Non-tiered count is larger than number of CPUs: " + nonTieredCount + " > " + cpus);
+            Asserts.assertTrue(tieredCount <= cpus,
+                "Tiered count is larger than number of CPUs: " + tieredCount + " > " + cpus);
+        }
+    }
+
+    // Direct translation from CompilationPolicy::initialize:
+    public static int heuristicCount(int cpus, boolean tiered) {
+        int log_cpu = log2(cpus);
+        int loglog_cpu = log2(Math.max(log_cpu, 1));
+        int min_count = tiered ? 2 : 1;
+        return Math.max(log_cpu * loglog_cpu * 3 / 2, min_count);
+    }
+
+    public static int log2(int v) {
+        return (int)(Math.log(v) / Math.log(2));
+    }
+
+    public static void pass(int count, String... args) throws Throwable {
+        CommandLineOptionTest.verifyOptionValueForSameVM("CICompilerCount", "" + count, "", args);
+    }
+
+    public static void fail(String... args) throws Throwable {
+        ProcessBuilder pb = ProcessTools.createLimitedTestJavaProcessBuilder(args);
+        OutputAnalyzer output = new OutputAnalyzer(pb.start());
+        output.shouldNotHaveExitValue(0);
+    }
+
+}