threading using for..next

  • Thread starter Thread starter baldrick
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baldrick

Hi,
I have some intensive number crunching code that I want to break up
into threads so PCs with several processors can do the job quicker.

I have a loop that will go from say 1 to 10, and for each value,
different numbers will be crunched depending on what the value is.

As it is an intensive process the user may cancel the loop. I need to
know at what value of the loop the process was cancelled, so it can be
restarted at the next value next time.

My initial thought was to allocate 1-5 to thread 1 and 6-10 to thread
2 (for a 2 processor machine) - which shouldn't be too difficult. But
the issue with this if that if the user cancels then its not so neat
to resume as 1 and 6 might be the only ones completed.

Ideally I would like to do the following but am not quite sure how.
Any pointers or code snippets would be appreciated.

for 1 = 1 to 10 step numProcessors (say 2)



next i
 
Hi,
I have some intensive number crunching code that I want to break up
into threads so PCs with several processors can do the job quicker.

I have a loop that will go from say 1 to 10, and for each value,
different numbers will be crunched depending on what the value is.

As it is an intensive process the user may cancel the loop. I need to
know at what value of the loop the process was cancelled, so it can be
restarted at the next value next time.

My initial thought was to allocate 1-5 to thread 1 and 6-10 to thread
2 (for a 2 processor machine) - which shouldn't be too difficult. But
the issue with this if that if the user cancels then its not so neat
to resume as 1 and 6 might be the only ones completed.

Ideally I would like to do the following but am not quite sure how.
Any pointers or code snippets would be appreciated.
Click to expand...


for i = 1 to 10 step numThreads (say 2)

allocate i to the next free thread

next i

This way as I loop through 1 to 10, and the user cancels at x, then I
know I have completed 1 through x and need to resume at x+1.

How do I set up the loop to wait for one of the 2 threads to finish
and then allocate the current unprocessed 'i' to that thread?

Hope this makes sense?

Pb
 
that doesn't require threading. you can use a delegate to stop a loop, and
start it again from sT (startThread)

for i = sT to eT
 
that doesn't require threading.  you can use a delegate to stop a loop, and
start it again from sT (startThread)

for i = sT to eT

--
David Glienna
MVP - Visual Developer (Visual Basic)












- Show quoted text -
Click to expand...

David,
I must have not made myself very clear. How to stop the loop was not
the question. I can do that quite easily.

I want to loop through 1 to 10, allocate 1 to the first thread, 2 to
the second, then 3 to either thread 1 or 2, depending on which one has
finished, or wait if neither have finished, same for 4 etc...

I could easily allocate 1-5 to thread 1 and 6-10 to thread 2 but I
want to do it a bit more in order.

The reason for threading is that each i of the loop will take about 10
minutes so I want to do 2 at once so it will run on 2 processors an be
done in half the time.

Would appreciate any pointers or code snippets.

Phil
 
"baldrick"
I have some intensive number crunching code that I want to break up
into threads so PCs with several processors can do the job quicker.
Click to expand...
It is exactly as you write here, it "can" do the job quicker with threading,
but in the same case it can do the job "slower".

Threading uses managment, so be aware that you are not only because of
marketing information are doing things that has a reverse result than you
think. Threading can be greath as you are doing several jobs which have wait
times like retrieving data from internet.

Have a look simple at the TaskManager and see how many threads are already
running and how they perform on your 2 processors. As it is a function for
your own organisation, and you are allowed to set threadpriority for all
threads to the highest, then maybe you can get some quicker througput time.
Your total processor time will however always be more.

As you persist on using threads in this case, have then a look at the que
class as with that and locking the enque en deque operations you can in a
simple way create a smooth operation.

Cor
 
David,
I must have not made myself very clear. How to stop the loop was not
the question. I can do that quite easily.

I want to loop through 1 to 10, allocate 1 to the first thread, 2 to
the second, then 3 to either thread 1 or 2, depending on which one has
finished, or wait if neither have finished, same for 4 etc...

I could easily allocate 1-5 to thread 1 and 6-10 to thread 2 but I
want to do it a bit more in order.

The reason for threading is that each i of the loop will take about 10
minutes so I want to do 2 at once so it will run on 2 processors an be
done in half the time.

Would appreciate any pointers or code snippets.

=======

Maybe I'm wrong (early in the morning), though..:
1. Enqueue _all_ work items into a Queue object. (symbol search "queue")
2. Start two (or more) threads (one per processor (core))
3. Each thread has access to the queue object. Each thread runs a loop
that pulls one work item from the queue (use Synclock on the queue
object while doing this).
4. That's all
5. Really

However, I don't know if additional work items will have to be processed
later, that means after they some been enqueued in step 1.

I forgot: In order to cancel a thread, just set a boolean flag that is
accessible to the threads and signals them to return.


AZ
 
Armin Zingler said:
However, I don't know if additional work items will have to be
processed later, that means after they some been enqueued in step 1.
Click to expand...

correction:
....after they have been enqueued...
....after some have been...

Choose one.


AZ
 
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