Demo: Moving Work into the Kernel with sendfile()

Two servers send the same 10 MB file to a client. One reads each chunk into a user buffer and sends it out; the other hands the whole file to the kernel with one sendfile().

Run

Two terminals. Build and make the test file first:

cd ../../../../02-os-types-interface-full/demos/04-optimize-os-interface/sendfile
make
./create_file.sh

In the first terminal, start one server:

./server_sendfile        # or: ./server_write

In the second, run the client a few times:

./client

The sendfile() server is two to three times faster.

Ask

• server_write reads each chunk into user space and writes it straight back out. How many times is each byte copied?
• sendfile() never brings the data into user space. What two costs does that remove?
• If the server had to encrypt the file before sending, which of the two techniques would still apply?