Unexpected threading performance result

[Alfred Perlstein]

* Ivan Voras <ivoras at freebsd.org> [071007 07:52] wrote:
> Hi,
> 
> For an unrelated purpose, I'm benchmarking performance of tree 
> algorithms in SMP environments and my preliminary run has an unexpected 
> result. Here's the typical output from the (small) benchmark program, 
> run on a dual-core Athlon64 (i386 mode):
> 
> Running benchmarks on small_nonuniform, 1000000 samples
> Step 1: Running 100 loops
> ** Step 1 benchmark completed 100 loops in 84.44 seconds.
> Step 2: Running 2 threads with 100 loops each
> ** Step 2 benchmark completed 100 loops in 2 threads in 167.46 seconds.
> 
> The interpretation is: running the same loop twice, in two parallel 
> threads doesn't result in a speedup, but it looks like the execution is 
> serialized. The problem is: the loops are completely independent, no 
> locking in their execution, and 'top' confirms that both threads in the 
> program are running at 100% CPU each.

...

1)
Could you please make this example program compile/work, it's a bit
difficult to diagnose the problem if we don't know if things like
n_samples are initialised properly.

2)
Please try to pthread_attr_setscope() to PTHREAD_SCOPE_SYSTEM
on a pthread_attr_t object to pass into pthread_create().  That
may help.

3) 
What's the deal with RB_FIND?  What does that do?  Is that
data structure locked with a an exclusive lock?





> 
> I verified this behaviour on:
> 
> - 7-CURRENT, i386, ULE
> - 7-CURRENT, i386, 4BSD
> - 6-STABLE, amd64, 4BSD
> 
> I can't really explain this behaviour, but it might not be related to 
> FreeBSD - maybe I made a mistake in the program or there's a 
> hardware-related reason for it (maybe CPU cache trashing from the tree 
> traversal?). In any case, can someone shed some light on this?
> 
> The main part of the (small) program is pasted below.
> 
> 
>  47 double time_start, time_b1, time_b2;
>  48 int n_data, n_samples;
>  49 int *data, *samples;
>  50
>  51
>  52 void bench_loop()
>  53 {
>  54         int i;
>  55         struct node *nd, find;
>  56         for (i = 0; i < n_samples; i++) {
>  57                 find.data = samples[i];
>  58                 nd = RB_FIND(node_tree, &head, &find);
>  59                 if (nd == NULL)
>  60                         errx(1, "Sample %d was not found", find.data);
>  61         }
>  62 }
>  63
>  64 void step1()
>  65 {
>  66         int n;
>  67         /* step 1 - simple tree traversal */
>  68         printf("Step 1: Running %d loops\n", STEP1_ITER);
>  69         for (n = 0; n < STEP1_ITER; n++)
>  70                 bench_loop();
>  71         time_b1 = gettime();
>  72         printf("** Step 1 benchmark completed %d loops in %0.2lf 
> seconds.\n", STEP1_ITER, time_b1 - time_start);
>  73 }
>  74
>  75 void *step2_thread(void *arg) {
>  76         int n;
>  77         for (n = 0; n < STEP2_ITER; n++)
>  78                 bench_loop();
>  79         return NULL;
>  80 }
>  81
>  82 void step2()
>  83 {
>  84         /* step 2 - run tree traversal in parallel threads */
>  85         int n;
>  86         pthread_t threads[STEP2_THREADS];
>  87
>  88         printf("Step 2: Running %d threads with %d loops each\n", 
> STEP2_THREADS, STEP2_ITER);
>  89         for (n = 0; n < STEP2_THREADS; n++) {
>  90                 if (pthread_create(&threads[n], NULL, step2_thread, 
> NULL) != 0)
>  91                         err(1, "Cannot spawn thread");
>  92         }
>  93         for (n = 0; n < STEP2_THREADS; n++)
>  94                 pthread_join(threads[n], NULL);
>  95         time_b2 = gettime();
>  96         printf("** Step 2 benchmark completed %d loops in %d 
> threads in %0.2lf seconds.\n",
>  97                         STEP2_ITER, STEP2_THREADS, time_b2 - 
> time_start);
>  98 }
> 
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-- 
- Alfred Perlstein