1ba045f4_he_GATK___RETIRED___GATK_Forum__Shared_Memory
[Property] Cons [Scatter/Gather] - Requires manual preparation of sliced genomic intervals for processing (if you aren't using GATK-Queue). - For ReadWalkers and other tools that can only be processed by chromosome, leading to load balancing problems (chr1 is much bigger than chr22) - Sensitive to data density variation over the genome. Dividing chr20 processing in 63 1MB chunks leads to 10x differences in completion times due to data pileups around the centromere, for example. - Must wait until all parts of the scatter have completed before gathering, making the process sensitive to farm scheduling problems. If a job will run for M minutes, but waits Z minutes to start on the farm, the entire SG process will not complete for at least M + Z minutes. [Shared Memory] 
- Limited to fewer than 32 processors without significant overhead - Limited to cores physically present on the machine, cannot take advantage of computing cluster resources - Does not work for ReadWalkers (Table Recalibrator, Indel Realigner)
Parallelism with the GATK - RETIRED - GATK-Forum
Shared Memory