2011-04-21 Not enough CPU time

The datarate of the Demux board of the Safari project is quite high for a desktop PC to work with, even considering the fact that the raw data is being thoroughly filtered before it reaches the PC running the Generic EGSE (the central part of the software, which allows the user to control electronics and plot output).

We've made some tweaks in the past to allow more breathing space, such as good optical GBit network adapters (we dumped cheap DLinks and bought some expensive Intel ones). A separate PC was installed to run a daemon, which concatenates the large amount of small ethernet packets into a couple of big CCSDS packets. We tweaked the kernel TCP buffers and the driver settings on both PCs.

The running problem with the EGSE was unresponsiveness. When retrieving packets and plotting them, the whole software stopped reacting to user input. The cause was that the plotting (and its computations) took a lot of time because computations run in the same thread as the plotting, and although the GUI gets time to accept user interaction at regular interval but this interval is too long. A solution is to allow the GUI to accept user interactions more often. We are testing this solution.

(What didn't help, was that several bugs popped up in the meantime. For instance, it's possible to create two plots for one packet type. What would happen, is that packets would get divided between plots, or packets would interrupt the plotting right after the first plot was drawn, but before the second plot was started. A colleague also found and fixed these.)

We brainstormed with the users, and one had experience with this type of situation. After talking, we had a proposal for a situation where the daemon would maintain a very large buffer in memory or on disk, and the EGSE would then lazily retrieve the buffer. Since the daemon runs on a separate PC, memory contention wouldn't be a problem.


We thought about this solution, but although it's a solution for other (older) programming models, we use the Qt library. The big idea of Qt is that it generates signals when an event happens, and you just write code (a "slot") that handles the signal. The Signal/Slot system of Qt provides an elegant and robust solution to manage a buffer which is filled in with packet coming from a daemon and is read lazily by the EGSE. We're testing this solution.