How to make multiple 'parallel' runs exclusive?

[Jianghui Geng]

Dear colleagues,

Sometime we submit multiple ‘parallel’ runs, such as 

parallel A ::: a b
parallel B ::: c d

We hope B can wait if A have already occupied all CPUs. However, it’s not the 
case. It seems that B does not know the presence of A, and will thus ‘compete’ 
for the CPU resources with A. How can I resolve this problem? I just want B to 
wait until A has released CPUs.

Thanks,
Jianghui