Re: [racket-users] Places code not using all the CPU

['Paulo Matos' via Racket Users]

On 05/10/2018 14:15, Matthew Flatt wrote:
> It's difficult to be sure from your description, but it sounds like the
> problem may just be the usual one of scaling parallelism when
> communication is involved.
> 

Matthew, thanks for the reply.

The interesting thing here is that there is no communication between
places _most of the time_. It works as a ring topology where every
worker only communicates with the master and the master with all workers.

This communication is relatively rare, as in a message sent every few
minutes.

> Red is probably synchronization. It might be synchronization due to the
> communication you have between places, it might be synchronization on
> Racket's internal data structures, or it might be that the OS has to
> synchronize actions from multiple places within the same process (e.g.,
> multiple places are allocating and calling OS functions like mmap and
> mprotect, which the OS has to synchronize within a process). We've
> tried to minimize sharing among places, and it's important that they
> can GC independently, but there are still various forms of sharing to
> manage internally. In contrast, running separate processes for Z3
> should scale well, especially if the Z3 task is compute-intensive with
> minimal I/0 --- a best-case scenario for the OS.
> 

So, here you have pointed out to something that's surprising to me:
"OS has to synchronize actions from multiple places within the same
process (e.g., multiple places are allocating and calling OS functions
like mmap and mprotect, which the OS has to synchronize within a process)."

I thought each place was its own process similar to issuing a call of
racket itself on the body of the place. Now it seems somehow places are
all in the same process... in which case they'll probably even share
mutexes, although these low level details are a bit foggy in my mind.

> A parallel `raco setup` runs into similar issues. In recent development
> builds, you might experiment with passing `--processes` to `raco setup`
> to have it use separate processes instead of places within a single OS
> process, but I think you'll still find that it tops out well below your
> machine's compute capacity. Partly, dependencies constrain parallelism.
> Partly, the processes have to communicate more and there's a lot of
> I/O.

Again, I am really surprised that you mention that places are not
separate processes. Documentation does say they are separate racket
virtual machines, how is this accomplished if not by using separate
processes?

My workers are really doing Z3 style work - number crushing and lots of
searching. No IO (writing to disk) or communication so I would expect
them to really max out all CPUs.

-- 
Paulo Matos

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