code: private static int getDataRows(Chunk[] chunks, Frame f, int[] chunksIds, int cols) {
          double totalRows = 0;
          if(null != chunks) {
              for (Chunk ch : chunks) {
                  totalRows += ch.len();
              }
          } else {
              for(int chunkId : chunksIds) {
                  totalRows += f.anyVec().chunkLen(chunkId);
              }
          }
          return (int) Math.ceil(totalRows * cols / SPARSE_MATRIX_DIM);
      }
docs: FIXME this and the other method should subtract rows where response is 0