Reducing SOFI/ISAAC data means juggling with huge amounts of pixels. A typical jitter template produces 64 frames, using about 256 megabytes of disk space just to store raw frames. For data reduction, it is safer to be able to access about twice the input data size just for temporary storage of intermediate data. In this case, it means that about 800 free megabytes are needed to reduce one single frame set. VLT workstations have been scaled accordingly to allow such heavy processing, but it might not be the case on any user workstation. If you do not have enough RAM on your machine, the process will create its own swap spaces on your disk. You need then a decent workspace available on a local disk to allow processing.
The jitter command uses both memory and available disk space to store intermediate data products such as estimated sky. Much use is done of the mmap() system call, which belongs to the real-time POSIX extensions. This allows to work on virtually any machine no matter how much RAM+swap is available, as long as there is enough free user disk space available.