Hacker News Reader

Comment by rnnr | Hacker News Reader

Memory mapped files also known as sections in VMS/NT have just two advantages:

  * Fewer context switches among user space / kernel syscalls

  * No need to copy _modified_ data into the swap. The behavior for read only data doesn't change

That's it, nothing miraculous about it.

View on HN

No, it also shares them between multiple runs of the same process, and it reuses the pages that were in your file cache anyway.

> * Fewer context switches among user space / kernel syscalls

This is not necessarily true. There's lots of cases where it's actually slower.

> Fewer context switches among user space / kernel syscalls

Note that each page fault to read an mmaped page is also a context switch from user space into the kernel. The kernel entry/exit paths for syscalls and page faults have much in common.

Mmaped pages can in principle use fault-ahead to reduce the number of page faults when sequential access is detected. An equivalent reduction is available for read syscalls by reading larger blocks at a time.

In practice, mmap files are faster in some scenarios compared with read syscalls and slower in others. Same with O_DIRECT reads, those are faster in some scenarios and slower in others.

Reply on news.ycombinator.com