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
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