Re: [Algorithms] General purpose task parallel threading approach

[Sebastian S. <seb...@gm...>]

On Sat, Apr 4, 2009 at 7:29 AM, <asy...@gm...> wrote:

> Hi Jon,
>
> >you instead just suspend the current thread and
> >create a new thread in the thread pool, what would the difference be?
> The difference here is speed.
>
> I have an implementation which does the same but with threads (for
> debugging purposes - you can find it in the package as well), and it is
> significantly slower. While mine implementation still touches a semaphore on
> each task switch, it can push about a million task switches per second now.
> I actually didn't measure performance with threads, but will try doing that
> later.
> My target goal for a task switch cost is about 1-2 hundred of cycles, which
> is about 10-15 millions of task switches per second on a 3ghz cpu.


Hi, I didn't look at the source code, but am I to understand that this
"switch" you're talking about happens every single time you start a new
task? I.e. when a worker runs tasks A, B, and C, neither of which does any
blocking, you will pay that cost 3 times?
If not, then what's the additional cost of starting a task given that each
task is now a fibre?

Assuming I did understand you correctly, I'll just say that I think adding
(even slight) overhead to the "run" method of every single task in order to
make blocking cheaper is a big mistake. The whole motivating idea behind
task parallelism is that you saturate the system with way more runnable
tasks than there are hardware capabilities, so that performance can scale
with hardware, and that no CPU capability is left idle. If the number of
runnable tasks is much larger than the actual number of worker threads, that
means that most tasks will just run serially on the same worker that spawned
them, which means that most synchronisation points won't block, which means
that the number of thread switches will be few compared to the number of
task executions, which means that optimizing for fast blocking is not as
important as making sure that running a task is dirt cheap.

-- 
Sebastian Sylvan
+44(0)7857-300802
UIN: 44640862