Thanks for the reply and suggestion, Ralph. I tried to get this working
with semaphores/mutexes/locks/etc. But I've not been having any luck.
Here's a simplified, incomplete version of what I'm trying to do. I'm
hoping that someone can offer a suggestion if they see some sample code.
function localfunction()
files = listfiles()
locks = [Threads.SpinLock() for _ in files]
ranges = getindexranges(length(files))
pmap(pairs(ranges)) do rows_and_cols
rows, cols = rows_and_cols
workerfunction(files, locks, rows, cols)
end
end
function workerfunction(files, locks, rows, cols)
data = kindofexpensive(...)
pairs = pairs(rows, cols)
@sync for idx in unique([rows; cols])
@async begin
lock(locks[idx])
updatefile(files[idx], data[idx])
unlock(locks[idx])
end
end
end
This (obviously) does not work. I think that the problem is that the locks
are being copied when the function is spawned on each process. I've tried
wrapping the locks/semaphores in Futures/RemoteChannels, but that also
hasn't worked for me.
I found that I could do the sort of coordination that I need by starting
Tasks on the local process. More specifically, each file would have an
associated Task to handle the coordination between processes. But this
only worked for me in a simplified situation with the Tasks being declared
globally. When I tried to implement this coordination within localfunction
above, I got an error (really a bunch of errors) that said that a running
Task cannot be serialized.
Sorry for the long post, but I'm really hoping that someone can help me
out. I have a feeling that I'm missing something pretty simple.
---Zachary
On Tuesday, October 11, 2016 at 10:15:06 AM UTC-4, Ralph Smith wrote:
>
> You can do it with 2 (e.g. integer) channels per worker (requests and
> replies) and a task for each pair in the main process. That's so ugly I'd
> be tempted to write an
> interface to named system semaphores. Or just use a separate file for
> each worker.
>
> On Monday, October 10, 2016 at 11:09:39 AM UTC-4, Zachary Roth wrote:
>>
>> Hi, everyone,
>>
>> I'm trying to save to a single file from multiple worker processes, but
>> don't know of a nice way to coordinate this. When I don't coordinate,
>> saving works fine much of the time. But I sometimes get errors with
>> reading/writing of files, which I'm assuming is happening because multiple
>> processes are trying to use the same file simultaneously.
>>
>> I tried to coordinate this with a queue/channel of `Condition`s managed
>> by a task running in process 1, but this isn't working for me. I've tried
>> to simiplify this to track down the problem. At least part of the issue
>> seems to be writing to the channel from process 2. Specifically, when I
>> `put!` something onto a channel (or `push!` onto an array) from process 2,
>> the channel/array is still empty back on process 1. I feel like I'm
>> missing something simple. Is there an easier way to go about coordinating
>> multiple processes that are trying to access the same file? If not, does
>> anyone have any tips?
>>
>> Thanks for any help you can offer.
>>
>> Cheers,
>> ---Zachary
>>
>
