Re: how to move a TGraph to a new file

Hi Charles,

See your other post and an answer at: <http://root.cern.ch/phpBB3/viewtopic.php?t=13257>

Cheers,
Philippe.

On 8/10/11 1:08 PM, Charles Leggett wrote:
> In order to do event based parallelism in ATLAS, we have a mother
> process that forks worker child processes to individual cores, each of
> which then processes a number of events individually. In order that
> these child processes not interfere with each others' I/O, they each
> write their outputs into individual directories. At the end of the
> job, the outputs are merged. In order to maximize memory sharing, this
> forking happens after the first event is processed, by which time many
> output files have already been opened. So what we do is clone these
> files into the individual worker directories, and reset file
> descriptors to point to the appropriate places.
>
> We need to do this in a way that's transparent to the users, so that
> if they hold a pointer to something in the original file, this pointer
> still needs to be valid after the forking has migrated the files to
> the new locations.
>
> We have a service (the THistSvc in Gaudi) that manages all Histograms,
> Trees and Graphs that are registered with it. It holds all the
> pointers to the individual TTrees, THNs, and TGraphs. When the mother
> process clones the original TFiles in the individual worker
> directories, we update all these objects that are registered with the
> service, so that the pointers point to the objects in the new files,
> but the value of the pointers haven't changed. This way, users can
> still keep using a pointer that they got before the forking.
>
> When dealing with TFiles that contain TTrees and THNs, this isn't a
> problem as after we have copied the file structure to the new
> location, we can just do a SetDirectory() that reflects the new TFile.
>
> However, there's no such method in TGraphs.
>
> What I've tried doing is cloning the TGraph in the new file (either
> via the constructor or a Clone() method), then doing a memcpy to
> overwrite the original pointer with the clone, so that the original
> pointer will reference a TGraph in the new file, but this doesn't
> work.
>
> Can anyone suggest a solution?
>
> cheers, Charles.
>
>
>
>
Received on Wed Aug 10 2011 - 21:08:59 CEST