The basic matrix algorithms appear to fall in the synchronous class in the language of Section 3.4. Correspondingly, one would expect to get good performance on SIMD machines. This is indeed true for matrix multiplication, but it is hard to get good SIMD performance on LU factorization and the more complicated matrix algorithms. Here the algorithm is not fully synchronous. In particular, there are several operations involving row or column operations. These lead to two problems. Firstly, the parallelism is reduced from (for an matrix) to -this is typically a serious problem on SIMD machines, such as the CM-2 or Maspar MP-1,2 which are fine grain and require ``massive parallelism.'' Secondly, the use of pivoting clearly introduces irregularity into the algorithm, which complicates the SIMD implementation. For these reasons, most research on matrix algorithms has concentrated on MIMD multicomputers, such as the hypercube.