svn commit: r366697 - head/usr.bin/xinstall

[Alexander Richardson]

On Thu, 15 Oct 2020 at 06:59, Enji Cooper <yaneurabeya at gmail.com> wrote:
>
>
> > On Oct 14, 2020, at 5:28 AM, Alex Richardson <arichardson at 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).

I've used ext4 and afps, but it doesn't matter since there's a fallback.

>         * 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.
There was always fallback code in case mmap fails:
https://github.com/freebsd/freebsd/blob/8349de39d23fc152c3782ee3b9eeab3df4610944/usr.bin/xinstall/xinstall.c#L1253
and https://github.com/freebsd/freebsd/blob/8349de39d23fc152c3782ee3b9eeab3df4610944/usr.bin/xinstall/xinstall.c#L1253
so it is really "should I try to use mmap()".

Alex