System calls on Linux are relatively cheap, though the mitigations for
speculative-execution vulnerabilities have made them more expensive than
they once were. But even cheap system calls add up if one has to make a
large number of them. Thus, developers have been working on ways to avoid
system calls for a long time. Currently under discussion is a pair of ways
to reduce the number of system calls required to read a file’s contents,
one of which is rather simpler than the other.

Source: LWN.net – [$] Two new ways to read a file quickly