Date: Sat, 21 Mar 2015 20:49:00 +1100 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Xin LI <delphij@freebsd.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r280308 - head/sys/fs/devfs Message-ID: <20150321200221.K1846@besplex.bde.org> In-Reply-To: <201503210114.t2L1ECcB075556@svn.freebsd.org> References: <201503210114.t2L1ECcB075556@svn.freebsd.org>
next in thread | previous in thread | raw e-mail | index | archive | help
On Sat, 21 Mar 2015, Xin LI wrote:
> Log:
> Disable timestamping on devfs read/write operations by default.
>
> Currently we update timestamps unconditionally when doing read or
> write operations. This may slow things down on hardware where
> reading timestamps is expensive (e.g. HPET, because of the default
> vfs.timestamp_precision setting is nanosecond now) with limited
> benefit.
>
> A new sysctl variable, vfs.devfs.dotimes is added, which can be
> set to non-zero value when the old behavior is desirable.
I don't like this. It defaults to non-POSIX-conformant behaviour...
The slowness is mostly from no delayed update of times in devfs.
Switching vfs.timestamp_precision to a hardware timecounter would
have been even more expensive for regular files if file systems
didn't have delayed updates. The assumption that vfs_timestamp()
doesn't use a slow timecounter was so often satisfied that no one
missed devfs also not supporting mounting with -noatime.
Delayed updates are even easier to implement for devfs than for disk
file systems the times never need to be written to disk. A slow update
is still wasteful for atimes, but not as bad as for disk file systems
since it doesn't trigger a slower sync to disk.
> ...
> Modified: head/sys/fs/devfs/devfs_vnops.c
> ==============================================================================
> --- head/sys/fs/devfs/devfs_vnops.c Sat Mar 21 00:21:30 2015 (r280307)
> +++ head/sys/fs/devfs/devfs_vnops.c Sat Mar 21 01:14:11 2015 (r280308)
> ...
> @@ -1700,7 +1708,8 @@ devfs_write_f(struct file *fp, struct ui
> resid = uio->uio_resid;
>
> error = dsw->d_write(dev, uio, ioflag);
> - if (uio->uio_resid != resid || (error == 0 && resid != 0)) {
> + if (devfs_dotimes &&
> + (uio->uio_resid != resid || (error == 0 && resid != 0))) {
> vfs_timestamp(&dev->si_ctime);
> dev->si_mtime = dev->si_ctime;
> }
An old bug is evident in the diff. Writing shouldn't change the ctime.
Bruce
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20150321200221.K1846>