/*
* //----------------------------------------------------------------------------
* // NAME: SharedCircularBufferChannel::ReaderThreadLoop
* //
* // PURPOSE:
* // Reader loop, receives frames and processes them.
* //
* // RETURNS:
* //
* // NOTES:
* //
* void SharedCircularBufferChannel::ReaderThreadLoop(
* CircularBuffer& buffer,
* DispatchEntry* dispatchTable,
* size_t dispatchEntryCount)
* {
* // Receiver thread.
* //
* while (true)
* {
* if (WaitAndDispatchFrame(buffer, dispatchTable, dispatchEntryCount))
* {
* continue;
* }
*
* // The wait has been interrupted, we are done.
* //
* break;
* }
* }
*
* }
* }
*/
/// <inheritdoc/>
public void SendMessage <TMessage>(ref TMessage msg)
where TMessage : ICodegenType
{
// Calculate frame size.
//
int frameLength = FrameHeader.TypeSize + (int)CodegenTypeExtensions.GetSerializedSize(msg);
frameLength = Utils.Align(frameLength, sizeof(int));
// Acquire a write region to write the frame.
//
uint writeOffset = AcquireWriteRegionForFrame(ref frameLength);
if (writeOffset == uint.MaxValue)
{
// The write has been interrupted.
//
return;
}
MlosProxy.FrameHeader frame = Frame(writeOffset);
// Optimization. Store the frame length with incomplete bit.
//
frame.Length.Store(frameLength | 1);
// Store type index and hash.
//
frame.CodegenTypeIndex = msg.CodegenTypeIndex();
frame.CodegenTypeHash = msg.CodegenTypeHash();
// Copy the structure to the buffer.
//
IntPtr payload = Payload(writeOffset);
CodegenTypeExtensions.Serialize(msg, payload);
// Frame is ready for the reader.
//
SignalFrameIsReady(frame, frameLength);
// If there are readers in the waiting state, we need to notify them.
//
if (HasReadersInWaitingState())
{
ChannelPolicy.NotifyExternalReader();
}
}
/// <summary>
/// Acquire a region to write the frame.
/// </summary>
/// <param name="frameLength"></param>
/// <returns>Returns an offset to acquired memory region that can hold a full frame.</returns>
/// <remarks>The acquired region is contiguous.</remarks>
internal uint AcquireWriteRegionForFrame(ref int frameLength)
{
uint expectedFrameLength = (uint)frameLength;
// Acquire writing region in the buffer.
//
while (true)
{
// Align frame length to the integer.
// Otherwise the next frame might have unaligned offset for Length field.
//
frameLength = (int)expectedFrameLength;
// Acquire region for writes. Function might adjust frame length.
//
uint writeOffset = AcquireRegionForWrite(ref frameLength);
if (writeOffset == uint.MaxValue)
{
// The reader has terminated, abandon the write.
//
return(writeOffset);
}
// We might acquired a circular region,
// check if we can store a full frame without overlapping the buffer.
//
if (writeOffset + frameLength > Size)
{
// There is not enough space in the buffer to write the full frame.
// Create an empty frame to adjust the write offset to the beginning of the buffer.
// Retry the whole operation until we write a full frame.
//
MlosProxy.FrameHeader frame = Frame(writeOffset);
frame.CodegenTypeIndex = 0;
SignalFrameIsReady(frame, frameLength);
continue;
}
// Acquired a region that we can write a full frame.
//
return(writeOffset);
}
}
internal static void SignalFrameForCleanup(MlosProxy.FrameHeader frame, int frameLength)
{
frame.Length.Store(-frameLength);
}
internal static void SignalFrameIsReady(
MlosProxy.FrameHeader frame,
int frameLength)
{
frame.Length.Store(frameLength);
}
/// <inheritdoc/>
public bool WaitAndDispatchFrame(DispatchEntry[] dispatchTable)
{
TChannelPolicy channelPolicy = default;
uint readOffset = WaitForFrame();
if (readOffset == uint.MaxValue)
{
// Invalid offset, the wait was interrupted.
//
return(false);
}
uint dispatchEntryCount = (uint)dispatchTable.Length;
// Verify frame and call dispatcher.
//
MlosProxy.FrameHeader frame = Frame(readOffset);
uint codegenTypeIndex = frame.CodegenTypeIndex;
ulong codegenTypeHash = frame.CodegenTypeHash;
int frameLength = frame.Length.LoadRelaxed();
// Check if this is valid frame or just the link to the beginning of the buffer.
//
if (codegenTypeIndex != 0 && codegenTypeIndex <= dispatchEntryCount)
{
// Use hash to check the message.
//
ulong expectedCodegenTypeHash = dispatchTable[codegenTypeIndex - 1].CodegenTypeHash;
bool isMessageValid = ((uint)frameLength < Size) && (expectedCodegenTypeHash == codegenTypeHash);
if (isMessageValid)
{
unsafe
{
// Call dispatcher only if type hash is correct.
//
isMessageValid = dispatchTable[codegenTypeIndex - 1].Callback(Payload(readOffset), frameLength);
}
}
if (!isMessageValid)
{
// Received invalid frame, channel policy decides what how to handle it.
//
channelPolicy.ReceivedInvalidFrame();
}
// Cleanup. Writer requires clean memory buffer (frameLength != 0).
// Clear the whole frame except the length.
//
ClearPayload(readOffset, frameLength);
}
else
{
// Just a link frame, clean circular region.
//
ClearOverlappedPayload(readOffset, frameLength, Size);
}
// Mark frame that processing is completed (negative length).
//
SignalFrameForCleanup(frame, frameLength);
return(true);
}
/// <summary>
/// Wait for the frame become available.
/// </summary>
/// <returns>Returns an offset to the frame buffer.</returns>
/// <remarks>
/// Reader function.
/// If the wait has been aborted, it returns uint32_t::max.
/// </remarks>
internal uint WaitForFrame()
{
uint readPosition;
TChannelSpinPolicy channelSpinPolicy = default;
// Create AtomicUInt32 proxy once.
//
StdTypesProxy.AtomicUInt32 atomicReadPosition = Sync.ReadPosition;
StdTypesProxy.AtomicUInt32 atomicReaderInWaitingStateCount = Sync.ReaderInWaitingStateCount;
StdTypesProxy.AtomicBool atomicTerminateChannel = Sync.TerminateChannel;
MlosProxy.FrameHeader frame = default;
uint shouldWait = 0;
// int spinIndex = 0;
while (true)
{
// Wait for the frame become available.
// Spin on current frame (ReadOffset).
//
readPosition = atomicReadPosition.Load();
uint readOffset = readPosition % Size;
frame = Frame(readOffset);
int frameLength = frame.Length.Load();
if (frameLength > 0)
{
// Writer had updated the length.
// Frame is ready and available for the reader.
// Advance ReadIndex to end of the frame, and allow other reads to process next frame.
//
uint expectedReadPosition = readPosition;
uint nextReadPosition = readPosition + (uint)(frameLength & (~1));
if (atomicReadPosition.LoadRelaxed() != expectedReadPosition ||
atomicReadPosition.CompareExchange(nextReadPosition, expectedReadPosition) != expectedReadPosition)
{
// Other reader advanced ReadPosition therefore it will process the frame.
//
channelSpinPolicy.FailedToAcquireReadRegion();
continue;
}
// Current reader owns the frame. Wait untill the reader completes the write.
//
while ((frameLength & 1) == 1)
{
channelSpinPolicy.WaitForFrameCompletion();
frameLength = frame.Length.Load();
}
break;
}
channelSpinPolicy.WaitForNewFrame();
// if ((++spinIndex & 0xff) == 0)
{
// No frame yet, spin if the channel is stil active.
//
if (atomicTerminateChannel.LoadRelaxed())
{
return(uint.MaxValue);
}
}
// Wait for the synchronization primitive.
//
if (shouldWait != 0)
{
ChannelPolicy.WaitForFrame();
atomicReaderInWaitingStateCount.FetchSub(shouldWait);
shouldWait = 0;
}
else
{
// Before reader enters wait state it will increase ReaderInWaitingState count and then check if are there any messages in the channel.
//
shouldWait = 1;
atomicReaderInWaitingStateCount.FetchAdd(shouldWait);
}
// If (frameLength < 0) there is active cleaning up on this frame by the writer.
// The read offset had already advanced, so retry.
//
}
// Reset InWaitingState counter.
//
atomicReaderInWaitingStateCount.FetchSub(shouldWait);
// Reader acquired read region, frame is ready.
//
return(readPosition % Size);
}
/// <summary>
/// Follows free links until we reach read position.
/// </summary>
/// <remarks>
/// While we follow the links, the method is not cleaning the memory.
/// The memory is cleared by the reader after processing the frame.
/// The whole memory region is clean except locations where negative frame length values are stored
/// to signal that the message has been read and the frame is free-able.
/// Those locations are always aligned to the size of uint32_t. The current reader continues to spin if it reads negative frame length.
/// </remarks>
internal void AdvanceFreePosition()
{
// Create AtomicUInt32 proxies once.
//
StdTypesProxy.AtomicUInt32 atomicFreePosition = Sync.FreePosition;
StdTypesProxy.AtomicUInt32 atomicReadPosition = Sync.ReadPosition;
// Move free position and allow the writer to advance.
//
uint freePosition = atomicFreePosition.Load();
uint readPosition = atomicReadPosition.LoadRelaxed();
if (freePosition == readPosition)
{
// Free position points to the current read position.
//
return;
}
// For diagnostic purposes, following the free links we should get the same distance.
//
uint distance = readPosition - freePosition;
// Follow the free links up to a current read position.
// Cleanup is completed when free position is equal to read position.
// However by the time this cleanup is completed,
// the reader threads might process more frames and advance read position.
//
while (freePosition != readPosition)
{
// Load a frame from the beginning of the free region.
// However, other writer threads might already advance the free position.
// In this case, local free position points to the write region and
// we will fail to advance free offset.
//
uint freeOffset = freePosition % Size;
MlosProxy.FrameHeader frame = Frame(freeOffset);
int frameLength = frame.Length.Load();
if (frameLength >= 0)
{
// Frame is currently processed or has been already cleared.
// Other writer thread advanced free position, there is no point to check using compare_exchange_weak.
// Local free offset is now the write region.
//
return;
}
// Advance free position. The frame length is negative.
//
uint expectedFreePosition = freePosition;
uint nextFreePosition = freePosition - (uint)frameLength;
if (atomicFreePosition.LoadRelaxed() != expectedFreePosition ||
atomicFreePosition.CompareExchange(nextFreePosition, expectedFreePosition) != expectedFreePosition)
{
// Advanced by another writer, local free offset is now the write region.
//
return;
}
freePosition = nextFreePosition;
// Frame length is negative.
//
distance += (uint)frameLength;
}
}