Author :
Andy Pan
Date :
2024-01-14 21:24:37
Hash :55899c2f Message :Use read/write instead of readv/writev for single buffer
The `readv`/`writev` functions are designed for scattered I/O optimally,
their logic in the kernel is more sophisticated, compared to read/write,
which includes extra on-stack `iovec` in the kernel space, importing `iovec`
array from user space to kernel space, reading/writing with `iov_iter`, etc.
As a result, using `readv`/`writev` on single-segment `iovec` will fall into
the special branch in the kernel where it is imported as `ITER_UBUF` differed
from `ITER_IOVEC` for multiple-segments `iovec`.
Thus, it is just not worth calling `readv`/`writev` for single-segment `iovec`,
we should use `read`/`write` instead, to save it from going through the
sophisticated yet unnecessary kernel code path, circumvent a waste of
kernel on-stack memory, copying `iovec` between user space and kernel space, etc.
Andy Pan