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Document everything, and several further improvements

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- Only update all runtimes and the minimum runtime when queuing new
   exectors. We only need to update the current executor's runtime.
 - Fix overflows when comparing times within TimeoutState.
   System.nanotime() may (though probably won't) return negative values.
 - Hopefully explain how the scheduler works a little bit.
This commit is contained in:
SquidDev 2019-03-04 10:22:17 +00:00
parent f7cb526793
commit 0bfb7049b0
2 changed files with 54 additions and 23 deletions
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61
src/main/java/dan200/computercraft/core/computer/ComputerThread.java
View File

@ -10,7 +10,6 @@
import dan200.computercraft.shared.util.ThreadUtils;
import javax.annotation.Nonnull;
import javax.annotation.Nullable;
import java.util.TreeSet;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.TimeUnit;
@ -26,7 +25,26 @@
* Responsible for running all tasks from a {@link Computer}.
*
* This is split into two components: the {@link TaskRunner}s, which pull an executor from the queue and execute it, and
* a single {@link Monitor} which observes all runners and kills them if they are behaving badly.
* a single {@link Monitor} which observes all runners and kills them if they have not been terminated by
* {@link TimeoutState#isSoftAborted()}.
*
* Computers are executed using a priority system, with those who have spent less time executing having a higher
* priority than those hogging the thread. This, combined with {@link TimeoutState#isPaused()} means we can reduce the
* risk of badly behaved computers stalling execution for everyone else.
*
* This is done using an implementation of Linux's Completely Fair Scheduler. When a computer executes, we compute what
* share of execution time it has used (time executed/number of tasks). We then pick the computer who has the least
* "virtual execution time" (aka {@link ComputerExecutor#virtualRuntime}).
*
* When adding a computer to the queue, we make sure its "virtual runtime" is at least as big as the smallest runtime.
* This means that adding computers which have slept a lot do not then have massive priority over everyone else. See
* {@link #queue(ComputerExecutor)} for how this is implemented.
*
* In reality, it's unlikely that more than a few computers are waiting to execute at once, so this will not have much
* effect unless you have a computer hogging execution time. However, it is pretty effective in those situations.
*
* @see TimeoutState For how hard timeouts are handled.
* @see ComputerExecutor For how computers actually do execution.
*/
public class ComputerThread
{
@ -116,7 +134,7 @@ static void start()
if( runners == null )
{
// TODO: Change the runners lenght on config reloads
// TODO: Change the runners length on config reloads
runners = new TaskRunner[ComputerCraft.computer_threads];
// latency and minPeriod are scaled by 1 + floor(log2(threads)). We can afford to execute tasks for
@ -172,7 +190,10 @@ public static void stop()
}
/**
* Mark a computer as having work, enqueuing it on the thread.
* Mark a computer as having work, enqueuing it on the thread
*
* You must be holding {@link ComputerExecutor}'s {@code queueLock} when calling this method - it should only
* be called from {@code enqueue}.
*
* @param executor The computer to execute work on.
*/
@ -184,7 +205,7 @@ static void queue( @Nonnull ComputerExecutor executor )
if( executor.onComputerQueue ) throw new IllegalStateException( "Cannot queue already queued executor" );
executor.onComputerQueue = true;
updateRuntimes( null );
updateRuntimes();
// We're not currently on the queue, so update its current execution time to
// ensure its at least as high as the minimum.
@ -215,23 +236,25 @@ static void queue( @Nonnull ComputerExecutor executor )
/**
* Update the {@link ComputerExecutor#virtualRuntime}s of all running tasks, and then increment the
* {@link #minimumVirtualRuntime} of the executor.
* Update the {@link ComputerExecutor#virtualRuntime}s of all running tasks, and then update the
* {@link #minimumVirtualRuntime} based on the current tasks.
*
* This is called before queueing tasks, to ensure that {@link #minimumVirtualRuntime} is up-to-date.
*/
private static void updateRuntimes( @Nullable ComputerExecutor current )
private static void updateRuntimes()
{
long minRuntime = Long.MAX_VALUE;
// If we've a task on the queue, use that as our base time.
if( !computerQueue.isEmpty() ) minRuntime = computerQueue.first().virtualRuntime;
long now = System.nanoTime();
int tasks = 1 + computerQueue.size();
// Update all the currently executing tasks
TaskRunner[] currentRunners = runners;
if( currentRunners != null )
{
long now = System.nanoTime();
int tasks = 1 + computerQueue.size();
for( TaskRunner runner : currentRunners )
{
if( runner == null ) continue;
@ -245,12 +268,6 @@ private static void updateRuntimes( @Nullable ComputerExecutor current )
}
}
// And update the most recently executed one if set.
if( current != null )
{
minRuntime = Math.min( minRuntime, current.virtualRuntime += (now - current.vRuntimeStart) / tasks );
}
if( minRuntime > minimumVirtualRuntime && minRuntime < Long.MAX_VALUE )
{
minimumVirtualRuntime = minRuntime;
@ -258,7 +275,8 @@ private static void updateRuntimes( @Nullable ComputerExecutor current )
}
/**
* Re-add this task to the queue
* Ensure the "currently working" state of the executor is reset, the timings are updated, and then requeue the
* executor if needed.
*
* @param runner The runner this task was on.
* @param executor The executor to requeue
@ -278,11 +296,14 @@ private static void afterWork( TaskRunner runner, ComputerExecutor executor )
computerLock.lock();
try
{
updateRuntimes( executor );
// Update the virtual runtime of this task.
long now = System.nanoTime();
executor.virtualRuntime += (now - executor.vRuntimeStart) / (1 + computerQueue.size());
// Add to the queue, and signal the workers.
// If we've no more tasks, just return.
if( !executor.afterWork() ) return;
// Otherwise, add to the queue, and signal any waiting workers.
computerQueue.add( executor );
hasWork.signal();
}

16
src/main/java/dan200/computercraft/core/computer/TimeoutState.java
View File

@ -6,6 +6,9 @@
package dan200.computercraft.core.computer;
import dan200.computercraft.core.lua.ILuaMachine;
import dan200.computercraft.core.lua.MachineResult;
import java.util.concurrent.TimeUnit;
/**
@ -22,8 +25,13 @@
* abort ({@link #ABORT_TIMEOUT}), we trigger a hard abort (note, this is done from the computer thread manager). This
* will destroy the entire Lua runtime and shut the computer down.
*
* The Lua runtime is also allowed to pause execution if there are other computers contesting for work. All computers
* are allowed to run for {@link ComputerThread#scaledPeriod()} nanoseconds (see {@link #currentDeadline}). After that
* period, if any computers are waiting to be executed then we'll set the paused flag to true ({@link #isPaused()}.
*
* @see ComputerThread
* @see dan200.computercraft.core.lua.ILuaMachine
* @see ILuaMachine
* @see MachineResult#isPause()
*/
public final class TimeoutState
{
@ -81,9 +89,11 @@ long nanoCurrent()
*/
public void refresh()
{
// Important: The weird arithmetic here is important, as nanoTime may return negative values, and so we
// need to handle overflow.
long now = System.nanoTime();
if( !paused ) paused = now >= currentDeadline && ComputerThread.hasPendingWork();
if( !softAbort ) softAbort = (now - cumulativeStart) >= TIMEOUT;
if( !paused ) paused = currentDeadline - now <= 0 && ComputerThread.hasPendingWork(); // now >= currentDeadline
if( !softAbort ) softAbort = (now - cumulativeStart - TIMEOUT) >= 0; // now - cumulativeStart >= TIMEOUT
}
/**