6.6 Poll Mode

In some cases you may want a server to deposit images only when it is ready to do so. For this case the server will not register a callback function that triggers image generation, but rather the server will deposit an image at its convenience. For example a server may want to create a movie or series of images corresponding to some timesteps in a simulation. The server will have a timestep loop in which an array computes some data for a timestep. At the end of each iteration the server will deposit the image. The use of LiveViz's Poll Mode supports this type of server generation of images.

Poll Mode contains a few significant differences to the standard mode. First we describe the use of Poll Mode, and then we will describe the differences. liveVizPoll must get control during the creation of your array, so you call liveVizPollInit with no parameters.

 liveVizPollInit();
 CkArrayOptions opts(nChares);
 arr = CProxy_lvServer::ckNew(opts);

To deposit an image, the server just calls liveVizPollDeposit. The server must take care not to generate too many images, before a client requests them. Each server generated image is buffered until the client can get the image. The buffered images will be stored in memory on processor 0.

  liveVizPollDeposit( this,
                      startX,startY,            // Location of local piece
                      localSizeX,localSizeY,    // Dimensions of the piece I'm depositing
                      globalSizeX,globalSizeY,  // Dimensions of the entire image
                      img,                      // Image byte array
                      sum_image_data,           // Desired image combiner
                      3                         // Bytes/pixel
                    );

The last two parameters are optional. By default they are set to sum_image_data and 3 bytes per pixel.

A sample liveVizPoll server and client are available at:

           .../charm/examples/charm++/lvServer
           .../charm/java/bin/lvClient

This example server uses a PythonCCS command to cause an image to be generated by the server. The client also then gets the image.

LiveViz provides multiple image combiner types. Any supported type can be used as a parameter to liveVizPollDeposit. Valid combiners include: sum_float_image_data, max_float_image_data, sum_image_data, and max_image_data.

The differences in Poll Mode may be apparent. There is no callback function which causes the server to generate and deposit an image. Furthermore, a server may generate an image before or after a client has sent a request. The deposit function, therefore is more complicated, as the server will specify information about the image that it is generating. The client will no longer specify the desired size or other configuration options, since the server may generate the image before the client request is available to the server. The liveVizPollInit call takes no parameters.

The server should call Deposit with the same global size and combiner type on all of the array elements which correspond to the ``this'' parameter.

The latest version of liveVizPoll is not backwards compatable with older versions. The old version had some fundamental problems which would occur if a server generated an image before a client requested it. Thus the new version buffers server generated images until requested by a client. Furthermore the client requests are also buffered if they arrive before the server generates the images. Problems could also occur during migration with the old version.