Reduce Execution Context Save+Restore #15629

benaadams · 2017-12-24T04:52:42Z

x 1.94 speed up for ExecutionContext.Run(ec, (o) => { }, null); where ec == Default
x 1.57 speed up for ExecutionContext.Run(ec, (o) => { }, null); with AsyncLocals (no notifications)

Results #15629 (comment)

Alternative approach to #11100 as suggested by @kouvel #11100 (comment)

When Default context is used in Run use it as null; which also counts as Default - rather than having two forms of Default context internally (Thread starts will null) and saving and restoring between them.

                Method |   Current |       New |    Change |
---------------------- |----------:|----------:|----------:|
        Run Default EC | 14.134 ns |  7.303 ns |    x 1.94 |
 Run w/ AsyncLocals EC | 11.986 ns |  7.631 ns |    x 1.57 | 
                 await |  3.152 ns |  3.158 ns |    x 1.00 |

ASP.NET Kestrel Plaintext call counts

Pre

Post

Resolves #11126

benaadams · 2017-12-24T04:52:56Z

Total bytes of diff: -1467 (-0.04% of base)
    diff is an improvement.

Total byte diff includes -184 bytes from reconciling methods
        Base had    8 unique methods,      660 unique bytes
        Diff had    3 unique methods,      476 unique bytes

Top file improvements by size (bytes):
       -1467 : System.Private.CoreLib.dasm (-0.04% of base)

1 total files with size differences (1 improved, 0 regressed), 0 unchanged.

Top method regessions by size (bytes):
         452 : System.Private.CoreLib.dasm - ExecutionContext:Restore(byref,ref) (0/1 methods)
          78 : System.Private.CoreLib.dasm - ExecutionContext:SetLocalValue(ref,ref,bool)
          63 : System.Private.CoreLib.dasm - ExecutionContext:Run(ref,ref,ref)
          12 : System.Private.CoreLib.dasm - Thread:get_ExecutionContext():byref:this (0/1 methods)
          12 : System.Private.CoreLib.dasm - Thread:get_SynchronizationContext():byref:this (0/1 methods)
           2 : System.Private.CoreLib.dasm - SynchronizationContext:SetSynchronizationContext(ref)
           2 : System.Private.CoreLib.dasm - SynchronizationContext:get_Current():ref
           2 : System.Private.CoreLib.dasm - ExecutionContext:Capture():ref

Top method improvements by size (bytes):
       -1438 : System.Private.CoreLib.dasm - AsyncTaskMethodBuilder`1:Start(byref):this (12 methods)
        -384 : System.Private.CoreLib.dasm - ExecutionContext:OnContextChanged(ref,ref) (1/0 methods)
        -123 : System.Private.CoreLib.dasm - ExecutionContext:Restore(ref,ref) (1/0 methods)
         -61 : System.Private.CoreLib.dasm - ExecutionContextSwitcher:Undo(ref):this (1/0 methods)
         -40 : System.Private.CoreLib.dasm - ExecutionContext:EstablishCopyOnWriteScope(ref,byref) (1/0 methods)
         -16 : System.Private.CoreLib.dasm - Thread:set_ExecutionContext(ref):this (1/0 methods)
         -16 : System.Private.CoreLib.dasm - Thread:set_SynchronizationContext(ref):this (1/0 methods)
         -10 : System.Private.CoreLib.dasm - Thread:get_ExecutionContext():ref:this (1/0 methods)

21 total methods with size differences (9 improved, 12 regressed), 16738 unchanged.

jkotas · 2017-12-24T17:53:10Z

@dotnet-bot test Windows_NT x64 Checked corefx_baseline
@dotnet-bot test Ubuntu x64 Checked corefx_baseline

benaadams · 2017-12-24T21:36:47Z

@dotnet-bot test Windows_NT x64 Checked corefx_baseline
@dotnet-bot test Ubuntu x64 Checked corefx_baseline

benaadams · 2017-12-24T23:56:21Z

x 1.58 speed up for ExecutionContext.Run(ec, (o) => { }, null); where ec == Default

Pre

 Method |      Mean |     Error |    StdDev |        Op/s |
------- |----------:|----------:|----------:|------------:|
    Run | 14.690 ns | 0.2907 ns | 0.4261 ns |  68,073,519 |

Post

 Method |      Mean |     Error |    StdDev |        Op/s |
------- |----------:|----------:|----------:|------------:|
    Run |  9.263 ns | 0.0123 ns | 0.0115 ns | 107,956,385 |

https://gist.github.com/benaadams/73fedf79313a90bdef4f4f617cbb7aa8

benaadams · 2017-12-25T01:07:22Z

Corresponding corert change dotnet/corert#5153

benaadams · 2017-12-26T01:01:40Z

Previous call tracing

Post call tracing (Restore not invoked)

benaadams · 2017-12-27T21:17:04Z

PTAL @stephentoub

benaadams · 2017-12-27T21:52:46Z

src/mscorlib/src/System/Runtime/CompilerServices/AsyncMethodBuilder.cs

-            Thread currentThread = Thread.CurrentThread;
-            ExecutionContextSwitcher ecs = default(ExecutionContextSwitcher);
-            try
+            // Async state machines are required not to throw, so no need for try/finally here.


corert specifies this corert/.../Start<TStateMachine>(ref TStateMachine stateMachine); valid approach?

Would resolve https://github.com/dotnet/coreclr/issues/11156

As described in #11156, this is debatable. Let's keep PRs focused on one thing rather than trying to cram such changes together.

stephentoub · 2018-01-03T13:53:15Z

x 1.58 speed up for ExecutionContext.Run(ec, (o) => { }, null); where ec == Default

Any impact when not using the default?

stephentoub · 2018-01-03T13:53:58Z

src/mscorlib/shared/System/Threading/ExecutionContext.cs

@@ -130,126 +108,164 @@ public static bool IsFlowSuppressed()

        public static void Run(ExecutionContext executionContext, ContextCallback callback, Object state)
        {
+            // Note: ExecutionContext.Run is an extremly hot function and used by every await, threadpool execution etc


nit: "threadpool execution etc" => "ThreadPool execution, etc."

also typo: extremly

stephentoub · 2018-01-03T13:54:13Z

src/mscorlib/shared/System/Threading/ExecutionContext.cs


            Thread currentThread = Thread.CurrentThread;
-            ExecutionContextSwitcher ecsw = default(ExecutionContextSwitcher);
+            // Capture references to Thread Contexts


Nit: move the comment up a line

stephentoub · 2018-01-03T13:54:40Z

src/mscorlib/shared/System/Threading/ExecutionContext.cs


            Thread currentThread = Thread.CurrentThread;
-            ExecutionContextSwitcher ecsw = default(ExecutionContextSwitcher);
+            // Capture references to Thread Contexts
+            ref ExecutionContext current = ref currentThread.ExecutionContext;


Nit: current => currentExecutionCtx

stephentoub · 2018-01-03T13:55:05Z

src/mscorlib/shared/System/Threading/ExecutionContext.cs

+            // Store current ExecutionContext and SynchronizationContext as "previous"
+            // This allows us to restore them and undo any Context changes made in callback.Invoke
+            // so that they won't "leak" back into caller.
+            ExecutionContext previous = current;


Nit: previous => previousExecutionCtx

stephentoub · 2018-01-03T13:55:20Z

src/mscorlib/shared/System/Threading/ExecutionContext.cs

+            ref ExecutionContext current = ref currentThread.ExecutionContext;
+            ref SynchronizationContext currentSyncCtx = ref currentThread.SynchronizationContext;
+
+            // Store current ExecutionContext and SynchronizationContext as "previous"


Nit: missing period at end of sentence.

stephentoub · 2018-01-03T13:56:48Z

src/mscorlib/shared/System/Threading/ExecutionContext.cs

        }

-        private static void OnContextChanged(ExecutionContext previous, ExecutionContext current)
+        internal static void Restore(ref ExecutionContext current, ExecutionContext next)


Restore seems like the wrong naming for this. SetCurrentContext?

Also, maybe currentRef instead of current would make this easier to understand when reading the code?

Changed to OnValuesChanged as it only gets triggered if at least once side has notifications

stephentoub · 2018-01-03T13:59:57Z

src/mscorlib/shared/System/Threading/ExecutionContext.cs

-                ecsw.Undo(currentThread);
-                throw;
+                // context before any of our callers' EH filters run.
+                edi = ExceptionDispatchInfo.Capture(ex);


Isn't this going to change the call stack from what it would have been, adding in the whole "rethrown from" gook? That's going to affect pretty much every async invocation that incurs an exception, no?

Its the same in both cases

Really? This:

using System; using System.Runtime.ExceptionServices; class Program { static void Main() { try { Foo1(); } catch (Exception e) { Console.WriteLine(e.StackTrace); } Console.WriteLine(); try { Foo2(); } catch (Exception e) { Console.WriteLine(e.StackTrace); } } static void Foo1() { try { throw new Exception("test"); } catch { throw; } } static void Foo2() { ExceptionDispatchInfo edi = null; try { throw new Exception("test"); } catch (Exception e) { edi = ExceptionDispatchInfo.Capture(e); } edi.Throw(); } }

produces this for me:

>corerun test.exe at Program.Foo1() in c:\Users\stoub\Desktop\CoreClrTest\test.cs:line 16 at Program.Main() in c:\Users\stoub\Desktop\CoreClrTest\test.cs:line 8 at Program.Foo2() in c:\Users\stoub\Desktop\CoreClrTest\test.cs:line 22 --- End of stack trace from previous location where exception was thrown --- at Program.Main() in c:\Users\stoub\Desktop\CoreClrTest\test.cs:line 10

You're saying though that with your change that --- End of stack trace... doesn't show up when an exception emerges from EC.Run?

(It's not a deal-breaker, but given how common this method is, it'd be nice to avoid the extra gook. If it's not possible, ok.)

Been trying to get this to show up in async without much luck; but have since realized that async never lets an exception get caught here (i.e. its already caught it to fail the Task)

Added issue for EDI https://github.com/dotnet/coreclr/issues/15779

Just need to change StackTrace from

if (sf.GetIsLastFrameFromForeignExceptionStackTrace() && !isAsync) // Skip EDI boundary for async

to

if (sf.GetIsLastFrameFromForeignExceptionStackTrace() && !isAsync && // Skip EDI boundary for async declaringType != typeof(ExecutionContext)) // Skip EDI boundary for ExecutionContext.Run

However would need to rebase; and also I said I'd finished making changes to this PR 😉

stephentoub · 2018-01-03T14:03:30Z

src/mscorlib/shared/System/Threading/ExecutionContext.cs

-        internal static void EstablishCopyOnWriteScope(Thread currentThread, ref ExecutionContextSwitcher ecsw)
-        {
-            Debug.Assert(currentThread == Thread.CurrentThread);
+            if (current != previous)


Any benefit to storing a bool earlier when we do the previous != executionContext check and using that bool here rather than redoing the comparison with the ref?

I guess current could have changed back to previous already, could be if(executionContextChanged && current != previous)

Yes it would be better, however the callee may have changed the EC by setting an AsyncLocal or unpaired SuppressFlow/RestoreFlow call; so it needs to check the Thread's contexts again?

so it needs to check the Thread's contexts again?

Yup, you're right.

stephentoub · 2018-01-03T14:09:12Z

src/mscorlib/shared/System/Threading/ExecutionContext.cs

+                    foreach (IAsyncLocal local in previous.m_localChangeNotifications)
+                    {
+                        previous.m_localValues.TryGetValue(local, out object previousValue);
+                        object currentValue = null;


The = null should not be necessary, in which case it can be out object currentValue as on the previous line.

Nevermind, I missed the ? in the next? on the next line.

stephentoub · 2018-01-03T14:13:09Z

src/mscorlib/shared/System/Threading/ExecutionContext.cs

+            {
+                newValues = AsyncLocalValueMap.Empty.Set(local, newValue);;
+                newChangeNotifications = Array.Empty<IAsyncLocal>();
+            }


Have you measured the cost of setting async locals with this change?

Wasn't entirely happy with this; will see if can improve

Is better now

stephentoub · 2018-01-03T14:14:35Z

src/mscorlib/src/System/Runtime/CompilerServices/AsyncMethodBuilder.cs

-            // Run the MoveNext method within a copy-on-write ExecutionContext scope.
-            // This allows us to undo any ExecutionContext changes made in MoveNext,
+            Thread currentThread = Thread.CurrentThread;
+            // Capture references to Thread Contexts


Nit: same nits as earlier

kouvel

LGTM aside from other comments, thanks!

kouvel · 2018-01-03T20:48:53Z

src/mscorlib/shared/System/Threading/ExecutionContext.cs

@@ -130,126 +108,164 @@ public static bool IsFlowSuppressed()

        public static void Run(ExecutionContext executionContext, ContextCallback callback, Object state)
        {
+            // Note: ExecutionContext.Run is an extremly hot function and used by every await, threadpool execution etc


also typo: extremly

kouvel · 2018-01-03T21:00:39Z

src/mscorlib/shared/System/Threading/ExecutionContext.cs

-        internal static void EstablishCopyOnWriteScope(Thread currentThread, ref ExecutionContextSwitcher ecsw)
-        {
-            Debug.Assert(currentThread == Thread.CurrentThread);
+            if (current != previous)


I guess current could have changed back to previous already, could be if(executionContextChanged && current != previous)

benaadams · 2018-01-04T11:29:39Z

Any impact when not using the default?

Think I have a version that may improve both default and non-default (without notifications); setting up some scenarios to test properly.

kouvel · 2018-01-30T21:16:13Z

@stephentoub, should we go ahead and merge this?

stephentoub · 2018-01-31T01:43:33Z

I'll take one more look tomorrow. Thanks.

stephentoub · 2018-01-31T11:24:48Z

@dotnet-bot test CentOS7.1 x64 Release Innerloop Build and Test please
@dotnet-bot test Ubuntu arm Cross Debug Innerloop Build please

benaadams · 2018-01-31T11:26:54Z

@dotnet-bot test CentOS7.1 x64 Release Innerloop Build and Test please
@dotnet-bot test Ubuntu arm Cross Debug Innerloop Build please

No longer exist on CI

stephentoub · 2018-01-31T11:27:26Z

No longer exist on CI

Oh, you're right. Ok.

stephentoub · 2018-01-31T11:28:03Z

src/mscorlib/shared/System/Runtime/CompilerServices/AsyncMethodBuilder.cs

+        /// <summary>Initiates the builder's execution with the associated state machine.</summary>
+        /// <typeparam name="TStateMachine">Specifies the type of the state machine.</typeparam>
+        /// <param name="stateMachine">The state machine instance, passed by reference.</param>
+        [DebuggerStepThrough]


Why DebuggerStepThrough?

Hmm, nevermind, I see that it's there on the original for some reason. Not sure why it's there either, but we can leave this for consistency.

stephentoub · 2018-01-31T11:36:04Z

src/mscorlib/shared/System/Threading/ExecutionContext.cs

-            Thread.CurrentThread.ExecutionContext =
-                new ExecutionContext(newValues, newChangeNotifications, current.m_isFlowSuppressed);
+            Thread.CurrentThread.ExecutionContext = 
+                (!isFlowSuppressed && newValues.GetType() == typeof(AsyncLocalValueMap.EmptyAsyncLocalValueMap)) ?


In what situation could newValues be empty here?

Ah, if null was set as the value for an already empty map I guess?

If the value in OneElementAsyncLocalValueMap is set to null

stephentoub · 2018-01-31T11:39:24Z

src/mscorlib/src/System/Threading/Overlapped.cs

@@ -94,7 +94,8 @@ static internal void IOCompletionCallback_Context(Object state)
                overlapped = OverlappedData.GetOverlappedFromNative(pOVERLAP).m_overlapped;
                helper = overlapped.iocbHelper;

-                if (helper == null || helper._executionContext == null || helper._executionContext == ExecutionContext.Default)
+                ExecutionContext context = helper?._executionContext;
+                if (context == null || context.IsDefault)


If helper is null, don't we end up with an extra null check (checking context for null when we otherwise wouldn't)? Or is the JIT able to remove it?

No doesn't, will change

stephentoub · 2018-01-31T11:41:49Z

src/mscorlib/src/System/Threading/ThreadPool.cs

-                (IThreadPoolWorkItem)new QueueUserWorkItemCallback(callBack, state, context);
+            IThreadPoolWorkItem tpcallBack = (context == null || !context.IsDefault) ?
+                new QueueUserWorkItemCallback(callBack, state, context) :
+                (IThreadPoolWorkItem)new QueueUserWorkItemCallbackDefaultContext(callBack, state);


It's fine, but I'm curious why you swapped the polarity here. Did that have an impact on something?

So could combine the context == null || !context.IsDefault tests; otherwise it was a bit ugly

context != null && context.IsDefault, no?

I may have over thought it 😄

benaadams · 2018-01-31T12:06:48Z

@dotnet-bot test Windows_NT x64 Checked corefx_baseline
@dotnet-bot test Ubuntu x64 Checked corefx_baseline

benaadams · 2018-01-31T12:51:44Z

coreclr and corefx out of sync

MembersMustExist :
 Member 'System.Threading.Tasks.ValueTask.CreateAsyncMethodBuilder()'
 does not exist in the implementation but it does exist in the contract.

stephentoub · 2018-01-31T12:54:03Z

I'll fix it. My change hasn't mirrored to corefx yet, and I didn't actually know that these legs did ref validation.

stephentoub · 2018-01-31T14:05:12Z

@dotnet-bot test Windows_NT x64 Checked corefx_baseline
@dotnet-bot test Ubuntu x64 Checked corefx_baseline

stephentoub · 2018-01-31T15:14:19Z

@benaadams, have you run corefx tests with your change locally? Things are still out-of-sync, e.g.

error : MembersMustExist : Member 'System.Runtime.Intrinsics.X86.Avx.BroadcastElementToVector128(System.Single)' does not exist in the implementation but it does exist in the contract.

and if you've verified this locally we can just get it merged.

benaadams · 2018-01-31T15:23:38Z

have you run corefx tests with your change locally?

Yes and on here on coreclr CI prior to the two feedback tweaks

stephentoub · 2018-01-31T15:24:24Z

Ok. Thanks!

* Reduce Save+Restore for ExecutionContext * Use flag rather than comparison to static * Skip null check for pre-checked EC.Run * Feedback * Add static helper lookup for default context for TP * Add note for enregistering * Return to Default context when no values * Default + FlowSuppressed Context * Move AsyncMethodBuilder.Start to static non-generic * Feedback Signed-off-by: dotnet-bot <[email protected]>

* Reduce Save+Restore for ExecutionContext * Use flag rather than comparison to static * Skip null check for pre-checked EC.Run * Feedback * Add static helper lookup for default context for TP * Add note for enregistering * Return to Default context when no values * Default + FlowSuppressed Context * Move AsyncMethodBuilder.Start to static non-generic * Feedback Signed-off-by: dotnet-bot-corefx-mirror <[email protected]>

* Reduce Save+Restore for ExecutionContext * Use flag rather than comparison to static * Skip null check for pre-checked EC.Run * Feedback * Add static helper lookup for default context for TP * Add note for enregistering * Return to Default context when no values * Default + FlowSuppressed Context * Move AsyncMethodBuilder.Start to static non-generic * Feedback Signed-off-by: dotnet-bot <[email protected]>

benaadams force-pushed the executioncontext branch from 843c467 to 698ca00 Compare December 24, 2017 21:29

benaadams changed the title ~~[WIP] Reduce EC Save+Restore for Default context~~ Reduce EC Save+Restore for Default context Dec 24, 2017

benaadams force-pushed the executioncontext branch from 698ca00 to 6188b76 Compare December 24, 2017 21:35

benaadams mentioned this pull request Dec 25, 2017

Reduce EC Save+Restore for Default context dotnet/corert#5153

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benaadams commented Dec 27, 2017

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stephentoub reviewed Jan 3, 2018

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kouvel approved these changes Jan 3, 2018

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benaadams force-pushed the executioncontext branch from 6188b76 to d18b7c9 Compare January 4, 2018 02:25

benaadams changed the title ~~Reduce EC Save+Restore for Default context~~ [Wip] Reduce EC Save+Restore for Default context Jan 4, 2018

benaadams force-pushed the executioncontext branch from d18b7c9 to 016ed58 Compare January 6, 2018 00:25

benaadams changed the title ~~[Wip] Reduce EC Save+Restore for Default context~~ Reduce EC Save+Restore for Default context Jan 6, 2018

stephentoub reviewed Jan 31, 2018

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stephentoub approved these changes Jan 31, 2018

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Feedback

ab17e75

stephentoub approved these changes Jan 31, 2018

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stephentoub merged commit 1816802 into dotnet:master Jan 31, 2018

benaadams deleted the executioncontext branch January 31, 2018 15:25

benaadams mentioned this pull request Aug 9, 2019

[WIP] Support Write-Thru of EH variables in LSRA #26087

Closed

AndyAyersMS mentioned this pull request Dec 17, 2019

Support Write-Thru of EH variables in LSRA dotnet/runtime#543

Merged

benaadams mentioned this pull request Jan 31, 2020

[Jit] Overzealous w/ do-not-enreg in presence of EH dotnet/runtime#9514

Open

kunalspathak mentioned this pull request Jan 27, 2021

JIT: Enable EH Write Thru by default dotnet/runtime#35923

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4 tasks

Reduce Execution Context Save+Restore #15629

Reduce Execution Context Save+Restore #15629

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