Guide: Benchmarking sendfile()

Navigate to labs/lab-11/guides/benchmarking-sendfile/support and run make to prepare the test file.

sendfile() transfers data between two file descriptors directly within the kernel, bypassing user-space buffers. This process is known as zero-copy. Having established that sendfile() is likely faster than traditional I/O operations, it’s time to put this theory to the test!

The code in server.py creates two threads that behave nearly identically. One listens on port 8081 and handles connections using read() and send(), while the other listens on port 8082 and uses sendfile(). Start the server in one terminal with python server.py. In a second terminal, run benchmark_client.py read-send followed by benchmark_client.py sendfile to compare the performance.

The results below are generic, and your outcomes may vary significantly depending on factors such as disk performance, network interface card (NIC), kernel version, Python version, and system load.

student@os:/.../benchmarking-sendfile/support$ python3 benchmark_client.py read-send
Time taken: 7.175773588009179 seconds

student@os:/.../benchmarking-sendfile/support$ python3 benchmark_client.py sendfile
Time taken: 3.71454380400246 seconds

Using sendfile() halves the number of copies required, reducing it from 4 to 2. This should translate to a roughly halved running time as well, making sendfile() a clear performance improvement over traditional methods.

You can explore another example of zero-copy in practice in this Quiz: Why is cp faster than mmap()-based cp.