Re: ext4 extent status tree LRU locking

[Zheng Liu <gnehzuil.liu@gmail.com>]

On Fri, Jun 14, 2013 at 10:02:15AM -0400, Theodore Ts'o wrote:
> On Fri, Jun 14, 2013 at 10:09:40PM +0800, Zheng Liu wrote:
> > This commit tries to fix this problem.  Now a new member called
> > i_touch_when is added into ext4_inode_info to record the last access
> > time for an inode.  Meanwhile we never need to keep a proper in-order
> > LRU list.  So this can avoid to burns some CPU time.  When we try to
> > reclaim some entries from extent status tree, we use list_sort() to get
> > a proper in-order list.  Then we traverse this list to discard some
> > entries.
> 
> I think this approach is very sound.  I have one reservation, however.
> If there are a large number of inodes on this list, list_sort() could
> burn a lot of CPU time, especially if the shrinker is called a very
> large number of times in a short time period (i.e., when the system is
> thrashing, or when one or more memory containers are under a large
> amount of memory pressure).
> 
> I have a suggestion for how to address this: Keep a timestamp of when
> the list last has been sorted in struct ext4_super_info.  When
> iterating over the list, looking for a candidate inode, if inode's
> i_touch_when is greater than the last time sorted, skip the inode.  If
> there are no inodes left in the list, or more than some threshold
> inodes have been skipped, only then resort the list (and update the
> timestamp in the ext4_super_info structure).
> 
> What do you think?

Thanks for your suggestion.  I fully agree with you.  What I concern is
how to define this threshold.  A fixed value is very simple but too
inflexible.  If this value is too big, this lru list could never be
sorted.  So my proposal is that we set this value accroding to the
number of used inodes.  For example, if the number of used inodes is
10,000, this threshold could be set to 10 (10,000 x 0.1%).  Meanwhile,
we need to provide a sysfs interface to let user set this value.  Does
it make sense?

Thanks,
                                                - Zheng