Date:	Thu, 24 Jan 2013 10:35:30 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Theodore Ts'o <tytso@....edu>
Cc:	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	gnehzuil.liu@...il.com, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] fs: allow for fs-specific objects to be pruned as part
 of pruning inodes

On Wed, Jan 23, 2013 at 11:34:21AM -0500, Theodore Ts'o wrote:
> On Thu, Jan 24, 2013 at 12:32:31AM +1100, Dave Chinner wrote:
> > Doesn't work. Shrinkers run concurrently on the same context, so a
> > shrinker can be running on multiple CPUs and hence "interfere" with
> > each other. i.e. A shrinker call on CPU 2 could see a reduction in
> > a cache as a result of the same shrinker running on CPU 1 in the
> > same context, and that would mean the shrinker on CPU 2 doesn't do
> > the work it was asked (and needs) to do to reclaim memory.
> 
> Hmm, I had assumed that a fs would only have a single prune_super()
> running at a time.  So you're telling me that was a bad assumption....

Yes.

> > It also seems somewhat incompatible with the proposed memcg/NUMA
> > aware shrinker infrastructure, where the shrinker has much more
> > fine-grained context for operation than the current infrastructure.
> > This seems to assume that there is global context relationship
> > between inode cache and the fs specific cache.
> 
> Can you point me at a mail archive with this proposed memcg-aware
> shrinker?  I was noticing that that at time moment we're not doing any
> shrinking at all on a per-memcg basis, and was reflecting on what a
> mess that could cause....  I agree that's a problem that needs fixing,
> although it seems fundamentally, hard, especially given that we
> currently account for memcg memory usage on a per-page basis, and a
> single object owned by a different memcg could prevent a page which
> was originally allocated (and hence charged) to the first memcg....

http://oss.sgi.com/archives/xfs/2012-11/msg00643.html

The posting is for numa aware LRUs and shrinkers, and the discussion
follows on how to build memcg awareness on top of that generic
LRU/shrinker infrastructure

> > In your proposed use case, the ext4 extent cache size has no direct
> > relationship to the size of the VFS inode cache - the can both
> > change size independently and not impact the balance of the system
> > as long as the hot objects are kept in their respective caches when
> > under memory pressure.
> > 
> > i.e. the superblock fscache shrinker callout is the wrong thing to
> > use here asit doesn't model the relationship between objects at all
> > well. A separate shrinker instance for the extent cache is a much
> > better match....
> 
> Yeah, that was Zheng's original implementation.  My concern was that
> could cause the extent cache to get charged twice.  It would get hit
> one time when we shrank the number of inodes, since the extent cache
> currently does not have a lifetime independent of inodes (rather they
> are linked to the inode via a tree structure), and then if we had a
> separate extent cache shrinker, they would get reduced a second time.

The decision of how much to shrink a cache is made at the time the
shrinker is invoked, not for each call to the shrinker function. The
number to scan from each cache is based on a fixed value, and hence
all caches are put under the same pressure. The amount of objects to
scan is therefore dependent on the relative difference in the number
of objects in each cache. Hence if we remove objects from cache B
while scanning cache A, the shrinker for cache B will see less
objects in the cache and apply less pressure (i.e. scan less).

However, what you have to consider is that the micro-level
behaviour of a single shrinker call is not important. Shrinkers
often run at thousands of scan cycles per second, and so it's the
macro-level behaviour that results from the interactions of multiple
shrinkers that determines the system balance under memory pressure.
Design and tune for macro-level behaviour, not what seems right for
a single shrinker scan call...

> The reason why we need the second shrinker, of course, is because of
> the issue you raised; we could have some files which are heavily
> fragmented, and hence would have many more extent cache objects, and
> so we can't just rely on shrinking the inode cache to keep the growth
> of the extent caches in check in a high memory pressure situation.
> 
> Hmm....  this is going to require more thought.  Do you have any
> sugestions about what might be a better strategy?

In general, the shrinker mechanism balances separate caches pretty
well, so I'd just use a standard shrinker first. Observe the
behaviour under different workloads to see if the standard cache
balancing causes problems. If you see obvious high level imbalances
or performance problems then you need to start considering "special"
solutions.

The coarse knob the shrinkers have to affect this balance is the
"seeks" parameter of the shrinker. That tells the shrinker the
relative cost of replacing the object in the cache, and so has a
high level bias on the pressure the infrastructure places on the
cache. What you need to decide is whether the cost of replacing
objects is more or less expensive than the cost of replaing an inode
in cache, and bias from there.

The filesystem caches also have another "big hammer" knob in the
form of the /proc/sys/vm/vfs_cache_pressure sysctl. This makes the
caches look larger or smaller w.r.t. the page cache and hence biases
reclaim towards or away from the VFS caches. YOu can use this method
in individual shrinkers to cause the shrinker infrastructure to have
different reclaim characterisitics. Hence if you don't want to
reclaim from a cache, then just tell the shrinker it's size is
zero. (FWIW, the changed API in the above patch set makes this
biasing technique much easier and more reliable.)

I guess what I'm trying to say is just use a standard, stand-alone
shrinker and see how it behaves under real world conditions before
trying anything fancy. Often they "just work". :)

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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