Recovery Algorithms
Primary rollback and induced rollback.
Primary rollback : When an error is detected at Dk(Pj),
1. Let the global matrix be Sp.
2. Pj is rolled back to the most recent checkpoint Cq prior to Dk-2(Pj).
3. For every i, (0 <= i <= N-1), i<>j,
if (Sp[i,j] < Sp[j,j] then rollback message is sent to Pi.