Which one is faster atomic_compare_and_swap or spin trylock?

Question

Below is my use case

I have one global variable and multiple thread across all CPU are accessing this.

With atomic compare and exchange

auto old = global_var;
auto new_var = old
for (;;) {
            new++;
            bool got_it = atomic_compare_and_swap(global_var,
                                  old,
                                  new_var);
            if (got_it) {
                return new_var;
            }
            old = global_var;
            new_var = old;
    }

With spin trylock

for(;;)
    {
        auto temp = go_for_work();
        if (temp -> spin.trylock() == 0 )
        {
            continue;
        }

    }

... go_for_work
{
    auto old = global_var;
    auto new_var = old
    new_var++;
    global_var  =  new_var;
    return new_var
}

This is rough code I hope it is clear. Let me know if it is not clear.

global_var is not a int it is structure.

So, my main aim is to protect global_var, which one is faster atomic_compare_and_swap or spin.trylock(), also if some other technique ?

Side note: don't name your variable `new` that is a C++ keyword — Cory Kramer, Jan 21 '15 at 19:27
Both are a busy loop - there is no difference (both are bad) — , Jan 21 '15 at 19:32
If the `global_var` type is integral, use an atomic increment. — jxh, Jan 21 '15 at 19:46
This is rough as I can't post my code. global_var is not a int it is structure — eswaat, Jan 21 '15 at 20:25
I can't sleep. can you give use case that fit my above condition. — eswaat, Jan 21 '15 at 20:53
If `global_var` is not integral or a pointer type, the `atomic_compare_and_swap` is misleading. — jxh, Jan 22 '15 at 00:18

score 0 · Answer 1 · answered Jan 22 '15 at 00:36

0

You need to use a blocking test. non-blocking tests like a spin lock which at best is test and sleep in a loop take a lot of processor cycles at the detriment to all other processes. Try using a semaphore

answered Jan 22 '15 at 00:36

phil

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10

Which one is faster atomic_compare_and_swap or spin trylock?

1 Answers1