Re: Prevent synchronizing a certain symbol between main and parallel

*To*: mathgroup at smc.vnet.net*Subject*: [mg124084] Re: Prevent synchronizing a certain symbol between main and parallel*From*: "Oleksandr Rasputinov" <oleksandr_rasputinov at hmamail.com>*Date*: Sun, 8 Jan 2012 04:25:54 -0500 (EST)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*References*: <je96n4$j6k$1@smc.vnet.net>

On Sat, 07 Jan 2012 10:27:48 -0000, Szabolcs Horv=E1t <szhorvat at gmail.com> wrote: > > Hello, > > I am trying to use LibraryLink with remote parallel kernels. I end up > with a library function (let's call it libFun[]) that has a different > value on the main and parallel kernels due to different paths on the two > machines. > > Unfortunately it is very easy to inadvertently synchronize the value of > a symbol between the main and subkernels. My question is about how I > can ensure that this syncing will not happen for libFun[]. > > > I'll show in an isolated example how this might accidentally happen: > > In[1]:= LaunchKernels[2] > Out[1]= {KernelObject[1, "local"], KernelObject[2, "local"]} > > Set value of x in main kernel: > > In[2]:= x = 1 > Out[2]= 1 > > Note that it gets the same value in remote kernels too: > > In[3]:= ParallelEvaluate[x] > Out[3]= {1, 1} > > Set a different value for x in the parallel kernels and verify that they > keep it: > > In[4]:= ParallelEvaluate[x = 2] > Out[4]= {2, 2} > > In[5]:= {x, ParallelEvaluate[x]} > Out[5]= {1, {2, 2}} > > Now "innocently" use Parallelize on something containing x: > > In[6]:= Parallelize[Table[x, {10}]] > Out[6]= {1, 1, 1, 1, 1, 1, 1, 1, 1, 1} > > And see how the value of x got re-synced between the main and subkernels. > > In[7]:= {x, ParallelEvaluate[x]} > Out[7]= {1, {1, 1}} > > > So the question is: > > How can I prevent a certain symbol from ever auto-syncing between the > main and the subkernels? > This behaviour is new in 8 and controlled using the DistributedContexts option (which has default value of $DefaultContexts). It applies to all Parallel` functions that in some sense iterate over a list (these are now built around the new internal function parallelIterate which performs the distribution of values). Here Parallelize[Table[...]] gets changed into ParallelTable which meets this criterion. Apart from changing the option value, an easy way to avoid distribution for specific symbols is to define them in a context that is not a member of $DistributedContexts: In[1]:= LaunchKernels[2] Out[1]= {KernelObject[1, "local"], KernelObject[2, "local"]} In[2]:= InputForm[$DistributedContexts] Out[2]//InputForm= "Global`" In[3]:= undistributed`x = 1 Out[3]= 1 In[4]:= ParallelEvaluate[undistributed`x] Out[4]= {1, 1} In[5]:= ParallelEvaluate[undistributed`x = 2] Out[5]= {2, 2} In[6]:= Parallelize[Table[undistributed`x, {10}]] Out[6]= {2, 2, 2, 2, 2, 2, 2, 2, 2, 2}