Performance Tips

LensKit strives to provide pretty good performance (in terms of computation speed), but sometimes it needs a little nudging.


If you are implementing an algorithm, see the implementation tips for information on good performance.

Quick Tips

  • Use Conda-based Python, with tbb installed.

  • Set the MKL_THREADING_LAYER environment variable to tbb, so both MKL and LensKit will use TBB and can coordinate their thread pools.

  • Use LK_NUM_PROCS if you want to control LensKit’s batch prediction and recommendation parallelism, and NUMBA_NUM_THREADS to control its model training parallelism.

We generally find the best performance using MKL with TBB throughout the stack. If both LensKit’s Numba-accelerated code and MKL are using TBB, they will coordinate their thread pools to coordinate threading levels.

Controlling Parallelism

LensKit has two forms of parallelism. Algorithm training processes can be parallelized through a number of mechanisms:

  • Our own parallel code uses Numba, which in turn uses TBB (preferred) or OpenMP. The thread count is controlled by NUMBA_NUM_THREADS.

  • The BLAS library may parallelize underlying operations using its threading library. This is usually OpenMP; MKL also supports TBB, but unlike Numba, it defaults to OpenMP even if TBB is available.

  • Underlying libraries such as TensorFlow and scikit-learn may provide their own parallelism.

The LensKit batch functions use Python multiprocessing, and their concurrency level is controlled by the LK_NUM_PROCS environment variable. The default number of processes is one-half the number of cores as reported by multiprocessing.cpu_count(). The batch functions also set the thread count for some libraries within the worker procesess, to prevent over-subscribing the CPU. Right now, the worker will configure Numba and MKL. In the rest of this section, this will be referred to as the ‘inner thread count’.

The thread count logic is controlled by lenskit.util.parallel.proc_count(), and works as follows:

  • If LK_NUM_PROCS is an integer, the batch functions will use the specified number of processes, and with 1 inner thread.

  • If LK_NUM_PROCS is a comma-separated pair of integers (e.g. 8,4), the batch functions will use the first number for the process count and the second number as the inner thread count. This overrides NUMBA_NUM_THREADS, unless it is larger than NUMBA_NUM_THREADS.

  • If LK_NUM_PROCS is not set, the batch functions use half the number of cores as the process count and 2 as the inner thread count (unless NUMBA_NUM_THREADS is set to 1 in the environment).

Other Notes

  • Batch parallelism disables TensorFlow GPUs in the worker threads. This is fine, because GPUs are most useful for model training; multiple worker processes competing for the GPU causes problems.