Start from 2 threads instead of #CORES_COUNT

Author: qwwdfsad

core/kotlinx-coroutines-core/src/main/kotlin/kotlinx/coroutines/experimental/scheduling/CoroutineScheduler.kt

@@ -161,14 +161,13 @@ internal class CoroutineScheduler(
         }
 
         workers = arrayOfNulls(maxPoolSize)
-        // todo: can we lazily create corePool, too?
-        // todo: The goal: when running "small" workload on "large" machine we should not consume extra resource in advance
-        // todo: Can't we just invoke createNewWorker here to get the first one up and running?
-        for (i in 0 until corePoolSize) {
+        // By default create at most 2 workers and allocate next ones lazily
+        val initialSize = corePoolSize.coerceAtMost(2)
+        for (i in 0 until initialSize) {
             workers[i] = PoolWorker(i).apply { start() }
         }
 
-        controlState.value = corePoolSize.toLong()
+        controlState.value = initialSize.toLong()
     }
 
     /*

core/kotlinx-coroutines-core/src/test/kotlin/kotlinx/coroutines/experimental/scheduling/CoroutineDispatcherTest.kt

@@ -92,7 +92,7 @@ class CoroutineDispatcherTest : SchedulerTestBase() {
             assertEquals(thread, Thread.currentThread())
         }
 
-        checkPoolThreadsCreated()
+        checkPoolThreadsCreated(initialPoolSize()..initialPoolSize() + 1)
     }
 
     @Test
@@ -131,7 +131,7 @@ class CoroutineDispatcherTest : SchedulerTestBase() {
             finish(4)
         }
 
-        checkPoolThreadsCreated()
+        checkPoolThreadsCreated(initialPoolSize()..CORES_COUNT)
     }
 
     @Test

core/kotlinx-coroutines-core/src/test/kotlin/kotlinx/coroutines/experimental/scheduling/CoroutineSchedulerShrinkTest.kt

@@ -23,7 +23,7 @@ class CoroutineSchedulerShrinkTest : SchedulerTestBase() {
         // Init dispatcher
         async(dispatcher) { }.await()
         // Pool is initialized with core size in the beginning
-        checkPoolThreadsExist(corePoolSize)
+        checkPoolThreadsExist(initialPoolSize()..initialPoolSize() + 1)
 
         // Run blocking tasks and check increased threads count
         val blockingTasks = launchBlocking()

core/kotlinx-coroutines-core/src/test/kotlin/kotlinx/coroutines/experimental/scheduling/SchedulerTestBase.kt

@@ -44,6 +44,8 @@ abstract class SchedulerTestBase : TestBase() {
             require(threads == expectedThreadsCount) { "Expected $expectedThreadsCount threads, but has $threads" }
         }
 
+        fun initialPoolSize() = Runtime.getRuntime().availableProcessors().coerceAtMost(2)
+
         private fun maxSequenceNumber(): Int? {
             return Thread.getAllStackTraces().keys.filter { it is CoroutineScheduler.PoolWorker }
                 .map { sequenceNumber(it.name) }.max()