Builtin message passing functions¶
In DGL, message passing is expressed by two APIs:
send(edges, message_func)
for computing the messages along the given edges.recv(nodes, reduce_func)
for collecting the incoming messages, perform aggregation and so on.
Although the twostage abstraction can cover all the models that are defined in the message passing paradigm, it is inefficient due to storing explicit messages. See our blogpost for more details and performance results.
Our solution, also explained in the blogpost, is to fuse the two stages into one kernel so no explicit messages are generated and stored. To achieve this, we recommend using our builtin message/reduce functions so that DGL can analyze and map them to fused dedicated kernels. Here are some examples (in pytorch syntax):
import dgl
import dgl.function as fn
import torch as th
g = ... # create a DGLGraph
g.ndata['h'] = th.randn((g.number_of_nodes(), 10)) # each node has feature size 10
g.edata['w'] = th.randn((g.number_of_edges(), 1)) # each edge has feature size 1
# collect features from source nodes and aggregate them in destination nodes
g.update_all(fn.copy_u('h', 'm'), fn.sum('m', 'h_sum'))
# multiply source node features with edge weights and aggregate them in destination nodes
g.update_all(fn.u_mul_e('h', 'w', 'm'), fn.max('m', 'h_max'))
# compute edge embedding by multiplying source and destination node embeddings
g.apply_edges(fn.u_mul_v('h', 'h', 'w_new'))
fn.copy_u
, fn.u_mul_e
, fn.u_mul_v
are builtin message functions, while fn.sum
and fn.max
are builtin reduce functions. We use u
, v
and e
to represent
source nodes, destination nodes and edges among them, respectively. Hence, copy_u
copies the source
node data as the messages, u_mul_e
multiplies source node features with edge features,
so on and so forth.
To define a unary message function (e.g. copy_u
) requires one input feature name and one output
message name. To define a binary message function (e.g. u_mul_e
) requires
two input feature names and one output message name. During the computation,
the message function will read the data under the given names, perform computation, and return
the output using the output name. For example, the above fn.u_mul_e('h', 'w', 'm')
is
the same as the following userdefined function:
def udf_u_mul_e(edges):
return {'m' : edges.src['h'] * edges.data['w']}
To define a reduce function, one input message name and one output node feature name
need to be specified. For example, the above fn.max('m', 'h_max')
is the same as the
following userdefined function:
def udf_max(nodes):
return {'h_max' : th.max(nodes.mailbox['m'], 1)[0]}
Broadcasting is supported for binary message function, which means the tensor arguments
can be automatically expanded to be of equal sizes. The supported broadcasting semantic
is standard as in numpy’s
and pytorch’s. For unfamiliar
users, we highly suggest reading those documents as broadcasting is very useful. In the
above example, fn.u_mul_e
will perform broadcasted multiplication automatically because
the node feature 'h'
and the edge feature 'w'
are of different, but broadcastable shapes.
All DGL’s builtin functions support both CPU and GPU and backward computation so they
can be used in any autograd system. Also, builtin functions can be used not only in update_all
or apply_edges
as shown in the example, but wherever message/reduce functions are
required (e.g. pull
, push
, send_and_recv
, etc.).
Here is a cheatsheet of all the DGL builtins.
Category  Functions  Memo 

Unary message function  copy_u 

copy_e 

copy_src 
alias of copy_u 

copy_edge 
alias of copy_e 

Binary message function  u_add_v , u_sub_v , u_mul_v , u_div_v 

u_add_e , u_sub_e , u_mul_e , u_div_e 

v_add_u , v_sub_u , v_mul_u , v_div_u 

v_add_e , v_sub_e , v_mul_e , v_div_e 

e_add_u , e_sub_u , e_mul_u , e_div_u 

e_add_v , e_sub_v , e_mul_v , e_div_v 

src_mul_edge 
alias of u_mul_e 

Reduce function  max 

min 

sum 

prod 