In order to speed-up display of film rolls, darktable stores thumbnails in a cache file on disk (primary cache) and loads it into memory at startup. This value controls the cache size in megabytes. It needs a restart if changed (default 256MB).
If activated darktable stores all thumbnails on disk as a secondary cache, and thereby keeping thumbnails accessible in case they get dropped from the primary cache. This needs more disk space but speeds up lighttable view as it avoids reprocessing of thumbnails (default on).
If enabled, darktable write full preview to disk (.cache/darktable/) when evicted from the memory cache. Note that this can take a lot of memory (several gigabytes for 20k images) and darktable will never delete cached images. It's safe though to delete these manually, if you want. Lighttable performance will be increased greatly when zooming image in full preview mode (default off).
This controls how many parallel threads are used to create thumbnails during import. On 32bit systems it is strongly recommended to set this to 1. Needs a restart if changed (default 2).
In order to manage large images on systems with limited memory darktable does tile-wise processing. This variable controls the maximum amount of memory (in MB) a module may use during image processing. Lower values will force memory hungry modules to process an image with increasing number of tiles. Setting this to 0 will omit any limits. Values below 500 will be treated as 500. On a 32bit system you should set this to 500. Needs a restart if changed (default 1500).
If set to a positive, non-zero value, this variable defines the minimum amount of memory (in MB) that darktable should take for a single tile. On a 32bit system you should set this to 8. 64bit systems can live with higher values. Needs a restart if changed (default 16).
darktable can use your GPU to speed up processing significantly. Interface OpenCL requires suitable hardware and matching OpenCL drivers on your system. If one of those is not found the option is greyed out. Can be switched on and off at any time and takes immediate effect (default on).
Defines how preview and full pixelpipe tasks are scheduled on OpenCL enabled systems. “default”: the GPU processes the center view pixelpipe and the CPU processes the preview pipe; “multiple GPUs”: both pixelpipes are processed in parallel on two different GPUs; “very fast GPU”: both pixelpipes are processed sequentially on the GPU.