Multithreading and partitioned shared memory

Hi all

I'm having no success with this problem multithreading (simple?) on my processor core-i7, using CVI 9.0 (32-bit compiler).

In the snippets below, I have a structure of node 5 integers, and I use 32 calls to calloc() to allocate space of 32 blocks of 128 * 128 nodes (16K) and store pointers returned in a table as a global var.

Size in bytes of the nodes = 20, size in bytes of the block = (roughly) 328KB, total allocated size in bytes = (roughly) 10.5 MB.

I have then spawn 32 threads, each of them is passed a unique index in the pointer_array of 'node_space' (see code below), kind, each thread handles (read/write) a block separated from K 16 knots.

It must be thread-safe and multiply by the number of threads because each thread is addressing a different memory block (with no overlap), but not multithreading go more quickly (maybe a little) to a single thread.

I tried different sizes of thread pool, filling of the nodes to the limits of 16 to 64 bytes, all to nothing does not.

Is this a problem of band bandwidth memory due to the size of the tables? Each thread is somehow loaded the whole 32 blocks?  Any help appreciated.

struct node

{
unsigned int a;
unsigned int b;
unsigned int c;
unsigned int d;
unsigned int e;

} ;
typedef struct lymph nodes;
typedef nodes * Node_Ptr;

Node_Ptr node_space [32];          / * pointer to table in 32 separate blocks (loaded via individual calloc calls for each block) * /.

.... Spawning wire...

for (index = 0; index)< 32;="">

CmtScheduleThreadPoolFunction (my_thread_pool_handle, My_Thread_Function, & index, NULL);

Hello CVI_Rules,

It is difficult to answer your question because it depends on what you do in your thread function. Given that you do not see any speed upward in your program when you change the number of threads in your thread pool, you are done as well (or all the work) in each thread, serialize your threads with locks, or somehow slow down execution of each thread.

Your basic configuration seems fine. You can simplify it slightly by moving the nodes directly to your thread function:

    for (index = 0; index < 32; ++index)
    {
        CmtScheduleThreadPoolFunction(pool, My_Thread_Function, node_space[index], NULL);
    }

...

static int My_Thread_Function(void *functionData){    Node_Ptr nodes = functionData;...

But this will not affect performance.

Things to look at:

  1. Check that you're really work only on a subset of node space in each thread, you are passing and receiving the correct node space in each thread and you work only on that one.
  2. Verify that you do not have locks or other synchronization in your program. It seems that you do not have because you have designed your program so that it would not need any. But check anyway.
  3. Check that you do not have something useless in your thread function. Sometimes people call ProcessSystemEvents or ProcessDrawEvents because they feel that it makes the user interface more responsive. These two functions are expensive (all about 20 ms per call, I think). So, if you call these functions in a loop, with a fixed number of iteraction on all threads, and if the actual calculations are relatively fast, then these functions can easily dominate the runtime of your program. (There aren't necessarily those functions, it may be others. "These are just examples).
  4. Show and explain your code to a colleague. Sometimes, you don't see clearly until you show someone. Or they may have noticed something.

Apart from this, can you explain what you are doing in your thread function so that we can have a better understanding of your program and what might inhibit parallelism?

Tags: NI Software

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