[fpc-pascal] Threading vs Parallelism ?

Gary Doades gpd at gpdnet.co.uk
Sat Apr 1 19:13:59 CEST 2017

>> On Mar 31, 2017, at 5:32 PM, Michael Schnell <mschnell at lumino.de> wrote:
>> Regarding the view of the application (disregarding execution speed) or of the application programmer, there is no difference between real ("Hardware")  and virtual (e.g. threads) parallelism. These dirty basics need to be handled by the software and hardware infrastructure.
>> The use of real (e.g. multi CPU) parallelism that the application allows for being divided into multiple parallel "Threads". his fact given Hardware parallelism can speed up the execution, while even virtual parallelism allows for improving the latency of definable parts the application.

> I’m not understanding how parallelism could apply to anything besides breaking down a task so that it can run on multiple hardware compute units.

> Why would you ever break a task into 100 threads when you could just run it one thread?


One gets into the grey area of threads and "processors". As an example you could divide a program into two threads, one reading and one writing. Immediately after issuing a write request, you could start reading the next item in a separate thread before the write is complete. This works because the I/O subsystem is mostly independent so that the OS can schedule another thread (or process) which is not waiting for the I/O subsystem to reply. Using only a single thread, the whole program has to wait for the I/O write to finish before starting the next read.

In this way a single process on a single "processor" (at least CPU) can interleave tasks to speed up the overall performance of the application. This could be extended to a myriad of cases of course.

Hence the recent upsurge in "async" routines, which only works if used properly of course.


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