Re: [Lazarus] How does one add an event to a blocking system?

[Michael Schnell]

On 10/21/2015 01:44 PM, Bo Berglund wrote:

So if I use a thread to read data from the socket, how do I signal to
the main application that data are arriving using an event?

TThread.Synchronize(), TThread.Queue()

In an "Application" (other than in a "Program" you also can use 
"Application.QueueAsyncCall()".


You supposedly want to transfer Data to the main thread. Here IMHO it's 
a nice idea to use Finalizers.


-Michael

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Re: [Lazarus] How does one add an event to a blocking system?

[leledumbo]

> So if I use a thread to read data from the socket, how do I signal to 
the main application that data are arriving using an event?

Exactly, using an event:
http://www.freepascal.org/docs-html/fcl/syncobjs/tevent.html
Here's an example of how to use:
RoundRobinExample.zip

  



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Re: [Lazarus] How does one add an event to a blocking system?

[Mark Morgan Lloyd]

Michael Schnell wrote:

On 10/21/2015 01:44 PM, Bo Berglund wrote:

So if I use a thread to read data from the socket, how do I signal to
the main application that data are arriving using an event?

TThread.Synchronize(), TThread.Queue()


Please remind us what a non-LCL program has to call to make sure that 
stuff is dequeued.


In an "Application" (other than in a "Program" you also can use 
"Application.QueueAsyncCall()".


You supposedly want to transfer Data to the main thread. Here IMHO it's 
a nice idea to use Finalizers.


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Re: [Lazarus] How does one add an event to a blocking system?

[Michael Schnell]

On 10/21/2015 02:39 PM, Mark Morgan Lloyd wrote:

TThread.Synchronize(), TThread.Queue()

Please remind us what a non-LCL program has to call to make sure that 
stuff is dequeued.

CheckSynchronize()

-Michael



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Re: [Lazarus] How does one add an event to a blocking system?

[Michael Schnell]

On 10/21/2015 02:42 PM, Michael Schnell wrote:

CheckSynchronize()


Unfortunately there is no decent documentation on CheckSynchronize().

So in very short (and just hopefully correct):

CheckSynchronize() needs to be called by the main thread (in fact a 
threat becomes the main thread _because_ it calls CheckSynchronize().


The Events thrown by other Threads via TThread.Synchronize and 
TThread.Queue are executed as sub-procedures of CheckSynchronize() and 
hence they are executed in the main thread and they are executed in the 
order they have been queued by other threads.


It is a very bad idea to do a closed loop such as

while (TRUE) CheckSynchronize();

As this will use 100 % CPU time and hence will reduce the performance of 
all other threads (and every application that is running on the processor).


That is why CheckSynchronize() provides using the parameter "Timeout", 
that defines how long to wait until returning in case no event is 
queued.  (0 or no parameter => return immediately)


AFAIR if an Event is in the queue same is executed and 
CheckSynchronize(I) returns without checking if more Events are queued.


AFAIK the return parameter of CheckSynchronize() does not give any 
decent result and needs to be ignored.


-Michael

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Re: [Lazarus] How does one add an event to a blocking system?

[Mark Morgan Lloyd]

Michael Schnell wrote:

On 10/21/2015 02:39 PM, Mark Morgan Lloyd wrote:

TThread.Synchronize(), TThread.Queue()

Please remind us what a non-LCL program has to call to make sure that 
stuff is dequeued.



CheckSynchronize()


Thanks, it's one of those things I can never remember.

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Re: [Lazarus] How does one add an event to a blocking system?

[Michael Schnell]

On 10/21/2015 03:13 PM, Mark Morgan Lloyd wrote:


Thanks, it's one of those things I can never remember.
Because it is an fpc rtl thingy and not really meant to be used by 
Lazarus users ;-) .


-Michael

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Re: [Lazarus] How does one add an event to a blocking system?

[Mark Morgan Lloyd]

Michael Schnell wrote:

On 10/21/2015 02:42 PM, Michael Schnell wrote:

CheckSynchronize()


Unfortunately there is no decent documentation on CheckSynchronize().

So in very short (and just hopefully correct):

CheckSynchronize() needs to be called by the main thread (in fact a 
threat becomes the main thread _because_ it calls CheckSynchronize().


The Events thrown by other Threads via TThread.Synchronize and 
TThread.Queue are executed as sub-procedures of CheckSynchronize() and 
hence they are executed in the main thread and they are executed in the 
order they have been queued by other threads.


It is a very bad idea to do a closed loop such as

while (TRUE) CheckSynchronize();

As this will use 100 % CPU time and hence will reduce the performance of 
all other threads (and every application that is running on the processor).


That is why CheckSynchronize() provides using the parameter "Timeout", 
that defines how long to wait until returning in case no event is 
queued.  (0 or no parameter => return immediately)


AFAIR if an Event is in the queue same is executed and 
CheckSynchronize(I) returns without checking if more Events are queued.


AFAIK the return parameter of CheckSynchronize() does not give any 
decent result and needs to be ignored.


Seems reasonable. I think I've used it in one place, which is a console 
program which spends most of its time waiting for keyboard input but 
also has


procedure MainThreadYield;

begin
  Assert(InterlockedExchange(mainThreadYieldEntryCounter, 1) = 0, 
'MainThreadYield reentered');

{$ifndef LCL }
  CheckSynchronize;
{$endif LCL  }
  Assert(InterlockedExchange(mainThreadYieldEntryCounter, 0) = 1, 
'MainThreadYield reexited')

end { MainThreadYield } ;

I don't know whether I've overlooked anything significant there, but it 
appears robust and activity triggered by around half a dozen background 
threads appears as expected.


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Re: [Lazarus] How does one add an event to a blocking system?

[Michael Schnell]

On 10/21/2015 04:19 PM, Mark Morgan Lloyd wrote:



I don't know whether I've overlooked anything significant there, but 
it appears robust and activity triggered by around half a dozen 
background threads appears as expected.


Of course it's robust.

But check the CPU usage. It it's 100% all threads get just time slices- 
If it's just one other thread, same will never get more than 50 %.


If it's not 100% the system is waiting somewhere else in a sleep, a 
blocking read or similar.


-Michael


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Re: [Lazarus] How does one add an event to a blocking system?

[Sven Barth]

Am 21.10.2015 14:59 schrieb "Michael Schnell" :
> CheckSynchronize() needs to be called by the main thread (in fact a
threat becomes the main thread _because_ it calls CheckSynchronize().

The part in parenthesis is wrong. CheckSynchronize checks for MainThreadID
which is determined independently of CheckSynchronize.

> The Events thrown by other Threads via TThread.Synchronize and
TThread.Queue are executed as sub-procedures of CheckSynchronize() and
hence they are executed in the main thread and they are executed in the
order they have been queued by other threads.

Better usage of terms: Exceptions are thrown (or raised), events are
signaled.

> AFAIR if an Event is in the queue same is executed and
CheckSynchronize(I) returns without checking if more Events are queued.

CheckSynchronize completely cwalks the lost before it returns.

> AFAIK the return parameter of CheckSynchronize() does not give any decent
result and needs to be ignored.

That is indeed always "false" right now. I should check sometime what it is
supposed to return... Probably whether at least one event got executed :/

Regards,
Sven
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Re: [Lazarus] How does one add an event to a blocking system?

[Mark Morgan Lloyd]

Michael Schnell wrote:

On 10/21/2015 04:19 PM, Mark Morgan Lloyd wrote:


I don't know whether I've overlooked anything significant there, but 
it appears robust and activity triggered by around half a dozen 
background threads appears as expected.


Of course it's robust.


But my use of it might not have been.

But check the CPU usage. It it's 100% all threads get just time slices- 
If it's just one other thread, same will never get more than 50 %.


If it's not 100% the system is waiting somewhere else in a sleep, a 
blocking read or similar.


CPU usage depends on what the hardware I'm emulating is doing :-)

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Re: [Lazarus] How does one add an event to a blocking system?

[Mark Morgan Lloyd]

Michael Schnell wrote:

On 10/21/2015 05:27 PM, Sven Barth wrote:


Better usage of terms: Exceptions are thrown (or raised), events are 
signaled.
You are right. "thrown" is not a good wording for Events. I mostly read 
"fired" on that behalf.


"Signalled" is not a good choice IMO, because of unix-style signals and 
their handlers (e.g. sending a HUP signal to a process, to get it to 
re-read its configuration).


"Triggered" can have problems if a database is being used.

"Fired" seems like a fairly good choice, since it also implies immediacy.

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Re: [Lazarus] How does one add an event to a blocking system?

[Sven Barth]

Am 22.10.2015 10:21 schrieb "Mark Morgan Lloyd" <
markmll.laza...@telemetry.co.uk>:
>
> Michael Schnell wrote:
>
>> On 10/21/2015 05:27 PM, Sven Barth wrote:
>
>
>>> Better usage of terms: Exceptions are thrown (or raised), events are
signaled.
>>
>> You are right. "thrown" is not a good wording for Events. I mostly read
"fired" on that behalf.
>
>
> "Signalled" is not a good choice IMO, because of unix-style signals and
their handlers (e.g. sending a HUP signal to a process, to get it to
re-read its configuration).
>
> "Triggered" can have problems if a database is being used.
>
> "Fired" seems like a fairly good choice, since it also implies immediacy.

Yes, there is a bit of ambiguity with "signaled" and "fired" indeed seems
best :)

Regards,
Sven
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