"Vonlanthen, Elmar":
> You are right. I think most of the time, the command is not really
> hanging. Normally the remount-duration is ~30 seconds. But I had some
> cases, where the duration was much more. A few minutes up to some hours.
> I think there is a point, after which the command really hangs.
Ok.
> I have some news: The problem happens only on multi-CPU systems. I have
> a Virtualbox System. When I configure only 1 CPU, everything is normal.
> If I change to 2 CPU's, The remount duration is much longer.
My current guess is,
- VFS uses RCU to free the cached inodes.
- fsnotify users SRCU to free its own data corresponding to the inode
asynchronously by the fsnotify_mark kthread.
- during this free operation, "find" adds more and more inodes.
- in order to free the cached inode, the call trace will be
VFS -- aufs -- fsnotify -- aufs -- VFS
- in every "--", RCU *MAY* take unexpectedly long time.
One more important thing is that aufs uses fsnotify heavily when
udba=notify is specified. Probably the number of aufs inodes setting fsnotify
exceeds the limit which the fsnotify module expects.
> How can it be, that a simple read can slow down the remount? A option
> like "noatime" doesn't change anything. "coo" is set to "none". Is there
> another option which I can try? See my test results below.
"noatime" for aufs is meaningless since all the timestamp in aufs
follows the branch fs.
"coo" has no meaning at all. The feature is already dropped.
> Anfortunately this patch doesn't change the duration. :-(
Ok, thanks for testing.
> My test is now a little bit simpler:
> # mount / -o remount,udba=3Dnotify; find /usr -type f &>/dev/null; time
> mount / -o remount,udba=3Dreval
How much RAM does your system install?
And how many files does /usr have?
If you run "time find ..." in four cases,
udba=(notify and udba=reval) x d_delete patch(on and off)
will they show any differences?
J. R. Okajima
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