How to replace timeSetEvent function without losing functionality? - delphi

Question

Has anybody tried SIP Delphi component? I bought it some time ago at a reasonable price with sources in order to replace the old code written for Dialogic HMP. It looks like mail support was not implied, docs and help were absent either though with available codes I was not going to have troubles. And they didn't appear until now when I got stuck with the problem I cannot find a solution to.
The library during the call sends small RTP data packets via UDP every 20 ms and to keep these intervals equal it uses a winsdk function timeSetEvent. Here is the extracts from the code (I simplified it to make the things clearer):
Interface
type
// RTP packet header
TRTPHeader = packed record
Byte80: Byte;
PayloadType: Byte;
SeqNo: WORD;
TimeStamp: DWORD;
SSRC: DWORD;
end;
//RTP packet structure
TRTP = packed record
H: TRTPHeader;
Payload: packed array [0 .. 1023] of Byte;
end;
//class realisation of ISipCall interface
TCall = class(TInterfacedObject, ISipCall)
FRtpPacketToSend:TRTP;//RTP packet
//callback function, it is invoked by TMicrophoneThread regularly
procedure OnMicrophone(const Buffer: Pointer);
end;
//Thread class for timing purposes
TMicrophoneThread = class(TThread)
public
FCall: TCall;//call associated with this thread
FEvent: THandle;// Event handle
FTimerHandle: THandle;// Timer handle
procedure Execute; override;
constructor Create(const ACall: TCall);
destructor Destroy; override;
end;
implementation
procedure TCall.OnMicrophone(const Buffer: Pointer); //callback function, it is invoked by TMicrophoneThread regularly
var socket: TSocket;
begin
//preparing FRtpPacketToSend data, initializing socket, Remote server address
//win32 function, sends data to the “Remote” server
sendto(socket, FRtpPacketToSend, sizeof(FRtpPacketToSend), 0, #Remote, SizeOf(Remote));
end;
//callback function invoked by windows timer every 20 ms
procedure Timer20ms(uTimerID, uMessage: UINT; dwUser, dw1, dw2: DWORD_PTR); stdcall;
begin
SetEvent(TMicrophoneThread(dwUser).FEvent);//Sets the TMicrophoneThread event
end;
constructor TMicrophoneThread.Create(ACall: TCall);
begin
inherited;
FCall:=ACall;
FEvent := CreateEvent(nil, False, False, nil);
//Setting timer
FTimerHandle := timeSetEvent(20, 0, #Timer20ms, Cardinal(Self), TIME_CALLBACK_FUNCTION + TIME_PERIODIC);
end;
destructor TMicrophoneThread.Destroy;
begin
timeKillEvent(FTimerHandle);//removing timer
CloseHandle(FEvent);
inherited;
end;
procedure TMicrophoneThread.Execute;
var
buf: array [0 .. 159] of SmallInt;//buffer data, looks like for storing data between function calls
begin
FillChar(buf, SizeOf(buf), 0);
Repeat
//waiting for the timer to set FEvent from Timer20ms function
if (WaitForSingleObject(FEvent, INFINITE) <> WAIT_TIMEOUT) and not Terminated then
begin
if not Terminated then
try
FCall.OnMicrophone(#buf);
except
end;
end;
until Terminated;
end;
//Using these classes:
// Sip call object
Call:=TCall.Create;
// TMicrophoneThread object creates timer and every 20 ms invokes OnMicrophone function to send UDP data in realtime
Mth= TMicrophoneThread.Create(Call);
This code works fine, voice data flows smoothly. But to my surprise it works perfectly up until the number of simultaneous calls exceeds 16, the 17th and other calls do not receive timer signals. I found that this function is already marked as obsolete and some people encountered the same undocumented restrictions of this function - no more than 16 threads.
Instead of timeSetEvent I tried using CreateTimerQueue/CreateTimerQueueTimer with different parameters:
implementation
var
TimerQueue: THandle;
....
procedure WaitOrTimerCallback(lpParameter: Pointer; TimerOrWaitFired: BOOL); stdcall;
begin
SetEvent(TMicrophoneThread(lpParameter).FEvent);
end;
constructor TMicrophoneThread.Create(ACall: TCall);
begin
inherited;
FCall:=ACall;
FEvent := CreateEvent(nil, False, False, nil);
//Setting timer
CreateTimerQueueTimer(FTimerHandle, TimerQueue, #WaitOrTimerCallback, Self, 0, 20, 0);
end;
...
initialization
TimerQueue := CreateTimerQueue;
Also I tried Sleep and its more advanced realization based on QueryPerformanceFrequency/QueryPerformanceCounter:
procedure TMicrophoneThread.Execute;
var
buf: array [0 .. 159] of SmallInt;
waittime: integer;
begin
FillChar(buf, SizeOf(buf), 0);
repeat
if not Terminated then
try
FCall.OnMicrophone(#buf);
waittime:=round((Now - FCall.GetStartTime)*MSecsPerDay)
if waittime<20 then
Sleep(20-waittime)
except
end;
until Terminated;
end;
All these possible solutions have the same problem - voice flow stops being continuous and you distinctively hear clicks during the playback especially if you have two or more calls. The only reason I can imagine is that timeSetEvent is more accurate than others. What can be done here?

Answer 1

Given that you have pinpointed a limit in the number of timers, a small design change to keep within that limit seems in order. Each timer currently does a negligible amount of work when procedure Timer20ms is called. So it seems feasible to allow a single timer to set multiple events.
As a first pass I'd try using only a single timer to set all the events.
I doubt this will be a solution because it's unlikely that signalling (resuming) a large number of TMicrophoneThread instances simultaneously won't cause other problems. But it will be useful to see how many can be handled smoothly (let's call it simultaneous-signal-limit); as this will likely be a factor in determining a hard limit before you need to look at scaling to better/more hardware.
constructor TMicrophoneThread.Create(ACall: TCall);
begin
inherited;
FCall:=ACall;
FEvent := CreateEvent(nil, False, False, nil);
{ Instead of setting a new timer, add the event to a list. }
TimerEvents.Add(FEvent);
end;
destructor TMicrophoneThread.Destroy;
begin
{ Instead of removing the timer, remove the event }
TimerEvents.Remove(FEvent);
CloseHandle(FEvent);
inherited;
end;
procedure Timer20ms(uTimerID, uMessage: UINT; dwUser, dw1, dw2: DWORD_PTR); stdcall;
{ The timer callback sets all events in the list. }
var
LTimers: TList;
begin
{ I'm illustrating this code where TimerEvents is implemented as a TThreadList.
If you can ensure all access to the list happens from the same thread,
you'll be able to do away with the locks - which would be better. }
LTimers := TThreadList(dwUser).LockList;
try
for LoopI := 0 to LTimers.Count - 1 do
SetEvent(THandle(LTimers[LoopI]));
finally
TThreadList(dwUser).UnlockList;
end;
end;
Once this experiment is out the way, you could look at running multiple timers. Each with its own list. If you stagger the timers, and manage to get a reasonably fair distribution of TMicrophoneThread instances across each timer; you may be able to get close to handling 16 x simultaneous-signal-limit instance of TMicrophoneThread.