> On Oct 14, 2020, at 5:28 AM, Alex Richardson <arichard...@freebsd.org> wrote:
>
> Author: arichardson
> Date: Wed Oct 14 12:28:41 2020
> New Revision: 366697
> URL: https://svnweb.freebsd.org/changeset/base/366697
>
> Log:
> install(1): Avoid unncessary fstatfs() calls and use mmap() based on size
>
> According to git blame the trymmap() function was added in 1996 to skip
> mmap() calls for NFS file systems. However, nowadays mmap() should be
> perfectly safe even on NFS. Importantly, onl ufs and cd9660 file systems
> were whitelisted so we don't use mmap() on ZFS. It also prevents the use
> of mmap() when bootstrapping from macOS/Linux since on those systems the
> trymmap() function was always returning zero due to the missing MFSNAMELEN
> define.
>
> This change keeps the trymmap() function but changes it to check whether
> using mmap() can reduce the number of system calls that are required.
> Using mmap() only reduces the number of system calls if we need multiple
> read()
> syscalls, i.e. if the file size is > MAXBSIZE. However, mmap() is more
> expensive
> than read() so this sets the threshold at 4 fewer syscalls. Additionally, for
> larger file size mmap() can significantly increase the number of page faults,
> so avoid it in that case.
>
> It's unclear whether using mmap() is ever faster than a read with an
> appropriate
> buffer size, but this change at least removes two unnecessary system calls
> for every file that is installed.
>
> Reviewed By: markj
> Differential Revision: https://reviews.freebsd.org/D26041
* Has this change been tested out with source filesystems other than
UFS/ZFS? Not all filesystems support mmap(2).
* trymmap(..) seems to be less about computing whether or not the
filesystem should use mmap(2) after this change, but how it should use mmap(2).
Seems like a misnamed function call now.
Cheers,
-Enji
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