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DATE 2016-12-01

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MESSAGE
DATE 2016-12-21
FROM Ruben Safir
SUBJECT Subject: [Learn] (fwd) Re: thread concurancy
From learn-bounces-at-nylxs.com Wed Dec 21 17:19:43 2016
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Subject: [Learn] (fwd) Re: thread concurancy
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On 21.12.2016 22:48, Popping mad wrote:
> :(
>
> I don't know. I'm very confused about the behavior of this test program I've been right.
> I'm trying to move date by worker threads from one blob of memory to another and the debugger
> is saying that the pointers beg and canvas_index is jumping 10 bytes between this two lines
>
> for(int i = 0; i < 10;i++)
> {
> t[i] = std::thread([this]{ readin(beg, canvas_index); });
>
> and I'm not sure why. I lost the concurrency somewhere, but can't seem to figure out what I did wrong

Who knows. Debuggers sometimes also get confused and display wrong data.

Your example is incomplete (truncated?), cannot be compiled and even the
types of beg and canvas_index are not known. So not much can be said
about the code.

From the visible code I can only infer that you have not understood why
and how to protect data with mutexes (mutex lock is in one thread only,
and the data apparently protected by the lock (beg, canvas_index) is
not even accessed in the other threads.

Also, access to the shared data buffer (canvas) is creating a lot of
false sharing as the slicing size 10 is most probably not divisible by
the cache line size, thus causing potentially significant performance
penalties (probably not important for your toy example, but worth to
mention).

>
> #include
> #include
> #include
> std::mutex medco;
> std::mutex master;
>
> namespace testing{
> std::thread t[10];
> class PIC
> {
> public:
> PIC():beg{&source[0]}
> {
> canvas_index = canvas;
> std::cout << "Testing Source" << std::endl;
> for(int i = 0; i<100; i++)
> {
> std::cout << i << " " << source[i] << std::endl ;
> }
> for(int i = 0; i < 10;i++)
> {
> t[i] = std::thread([this]{ readin(beg, canvas_index); });
> std::cerr << i << ": Making a thread" << std::endl;
> sync_canvas_and_input();
> }
> };
>
> void sync_canvas_and_input()
> {
> std::cout << "**LOCKING**" << std::endl;
> std::lock_guard turn(medco);
> beg += 10;
> canvas_index += 10;
> }
>
> ~PIC()
> {
> std::cerr << "In the destructor" << std::endl;
> for(int i=0; i<10; i++)
> {
> t[i].join();
> std::cerr << i << ": Joining a thread" << std::endl;
> }
>
> };
> void readin(char * start, char * loc_canvas_index)
> {
> for( int i = 9; i>=0; i-- )
> {
> *loc_canvas_index = start[i];
> std::cerr << i << ": Copy " << start[i] << std::endl;
> std::cerr << i << ": Copied to loc_canvas_index " << reinterpret_cast(*loc_canvas_index) << std::endl;
> loc_canvas_index++;
> }
>

-- end of forwarded message --
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