Difference in request send between cpp and python

[xia rui]

Hello, everyone.

I am trying to trace the request sending of gRPC.

In grpc-cpp, when I start to send a request, the client first malloc a buffer, then swap data to it. The tracing log is:

E0816 12:59:37.909413170    6205 slice.cc:309]               grpc_slice_malloc() call grpc_slice_malloc_large(), length: 2605

E0816 12:59:37.909489871    6205 call.cc:1519]               call_start_batch() start

E0816 12:59:37.909508644    6205 call.cc:1637]               call_start_batch(): GRPC_OP_SEND_MESSAGE

E0816 12:59:37.909518989    6205 byte_stream.cc:42]          init SliceBufferByteStream()

E0816 12:59:37.909527288    6205 slice_buffer.cc:208]        grpc_slice_buffer_swap() start

E0816 12:59:37.909535026    6205 slice_buffer.cc:209]        a info: length: 2605, capacity, 8

E0816 12:59:37.909601320    6205 slice_buffer.cc:215]        b info: length: 0, capacity, 8

(1) gprc_slice_malloc() first create new space for send_message

(2) GRPC_OP_SEND_MESSAGE operation create SliceBufferByteStream

(3) SliceBufferByteStream call grpc_slice_buffer_swap() to swap memory into its data.

In grpc-python, when I start to send a request, the client invoked grpc_slice_from_copied_buffer() to copy the payload of the request. The tracing log is:

in operation.pyx.pxi, call grpc_slice_from_copied_buffer(), length: 3905

E0816 13:07:32.242668044    7720 slice.cc:216]               grpc_slice_from_copied_buffer(), length: 3905

E0816 13:07:32.242682086    7720 slice.cc:309]               grpc_slice_malloc() call grpc_slice_malloc_large(), length: 3905

E0816 13:07:32.242703140    7720 call.cc:1519]               call_start_batch() start

E0816 13:07:32.242719473    7720 call.cc:1637]               call_start_batch(): GRPC_OP_SEND_MESSAGE

E0816 13:07:32.242732400    7720 byte_stream.cc:42]          init SliceBufferByteStream()

E0816 13:07:32.242744987    7720 slice_buffer.cc:208]        grpc_slice_buffer_swap() start

E0816 13:07:32.242758298    7720 slice_buffer.cc:209]        a info: length: 3905, capacity, 8

E0816 13:07:32.242856925    7720 slice_buffer.cc:215]        b info: length: 0, capacity, 8

Here, I find that the public API grpc_slice_from_copied_buffer() is called to copy the payload.

I am trying to find the function who invokes "grpc_slice_malloc()" to create a vacant memory for request payload but failed.

I want to know if I miss the grpc_slice_from_copied_buffer() in grpc-cpp?

Best wishes,

Xia Rui

[Juanli Shen]

C++ and Python are wrappers of C core. So they should have behaved similarly after entering C core functions. 

Have you tried gdb to find out who called `grpc_slice_malloc()` here? If the memory size is the same among different runs, you can add a line to check if the run-time parameter is 3905 and set a breakpoint there. Then you will see the stacktrace.