Performance and scalability

The data logger is capable of handling roughly 5000 to 10000 number tag updates per second per GHz of processing power, including logging of all data to disk. The actual performance can vary somewhat depending on many details, in particular the percentage of the tag updates that are logged to disk and what fraction of the tag updates is performed from either Lua scripts or LabVIEW code (which offers the best performance). By virtue of a multi-stage buffering mechanism and O(1) algorithms, the performance is almost independent of the number of different tags being updated. The system is therefore able to scale to several thousand tags with little degradation, and will probably handle 10000 to 20000 tags with minor performance regressions.

Updates of string tags do not scale as well. Essentially, string tag updates perform identically to appending strings to per-tag files. Thus, string logging performance is almost wholly dependent on the disk subsystem. Provided that the strings are not too large, a hundred logged string updates per second is about the limit of what one should expect on a non-RAID system. Because of this relatively high overhead, string tags should be used only to record state that changes infrequently.

Note that not all data more complex than a number needs be logged using a string tag. For example, a vector can be logged by updating, in unison, and with identical timestamps a set of number tags that represent the vector components. When doing so, it is preferable to choose a representation that can be interpolated on a per-component basis. For example, a polar vector is degenerate along the z-axis and as such is ill-suited to component-wise interpolation. Using a set of number tags instead of a string tag will scale better.

When using the system on which the data logger runs to perform time-sensitive control tasks, the relevant performance parameter may not be maximum throughput. Instead, the maximum latency of the control task may be a larger concern. Latency can be reduced by tweaking the execution system and priority of the control task. However the most deterministic way of reducing latencies, short of switching to a real-time operating system, is to lower CPU usage and avoid intensive operations. User interface windows and their display updates are a particularly notorious source of scheduling latency. The client-server architecture helps out by allowing the client (with all the user interface windows) to run on a different machine than the server doing the control and logging. In such a situation it also helps to use the database mirroring mechanism so that queries can be run on a machine not taxed with time-sensitive tasks.

Being able to run all logging and acquisition-related tasks inside a single runtime allows the system to scale with commodity computer hardware. Deploying on a faster computer with ample memory and multiple processors or multi-core processors will do wonders for performance and latency. The incremental cost will likely be less than the hardware cost of real-time controllers, and that is not even counting the additional development hassle these entail. Note that the system has been designed to benefit from multi-processing: when LabVIEW is configured to use multi-threading, busy Lua for LabVIEW tasks gets assigned separate OS threads. Where possible, time-consuming operations were made reentrant so as to prevent one thread from blocking the other.

The performance of queries hardly depends on the amount of data logged to the tag being queried. Instead, the performance scales with the amount of data involved in the query. When retrieving a time or index range, this equals the amount of data being retrieved. However when interpolating, this equals the time range over which the interpolation takes place, even when only few points in time are interpolated at.

The total amount of data logged to a string tag may not exceed two gigabytes. When this limit is still some way off, a warning is generated. An error occurs only when the limit is actually reached. The total amount of data logged to a number tag, which requires 16 bytes per point, has no hard limit. However for very large amounts of data per tag (on the order of a terabyte) the query performance will start to degrade because the index file (which serves to speed up queries) will itself become a bottleneck because of its large size. When a number tag grows to be this large, a warning is generated but no errors are thrown. Obviously the total amount of data in a logging database is limited by the capacity of the logical volume on which it is stored. For this reason it is advisable, when logging, to periodically write the disk usage to a tag with a warning limit. The "disk usage.lua" script included in the example server configuration does so.

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