[rust-dev] Reading shared immutable data

Michael Neumann mneumann at ntecs.de
Mon Dec 17 18:27:45 PST 2012

Am 18.12.2012 03:11, schrieb Patrick Walton:
> On 12/17/12 6:03 PM, Michael Neumann wrote:
>> Hi,
>> We have a very huge immutable data structure that we want to share
>> (read-only) between many light-weight threads.
>>  From what I have seen, I should use Arc. Is there any other way to
>> share the data between threads?
> You can also:
> * Use a reader-writer lock (RWLock). This is still safe.

I will look into this.

> * Use unsafe code by casting to an unsafe pointer and sending the 
> unsafe pointer over a channel; do this only as a last resort.

But don't I get into problems with GC when I do that? So I create the 
data structure heap allocated in one thread, then get an unsafe pointer 
to it and sent
this to all other threads. Of course in the originating thread I need to 
keep the reference to the data, otherwise the unsafe pointer will point 
to garbage.

> * Turn your shared state into a task and have other tasks access the 
> data by sending it messages. This is the classical actor model 
> solution, but usually ARC will be more efficient.

But this doesn't allow for parallelism, and that's what we want.

>> And when using Arc, can I access the data in parallel by all threads?
> Yes. The only synchronization cost you'll pay is the cost of an atomic 
> CPU instruction whenever you send the data.

I would just send the data once at thread creation. Hm, I have to play 
with that.

Thanks a lot!


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