Re: [PATCH 3/5] superblock: introduce per-sb cache shrinker infrastructure

Nick Piggin
2010-05-28T05:17:07Z

On Fri, May 28, 2010 at 08:40:34AM +1000, Dave Chinner wrote:
> On Thu, May 27, 2010 at 04:35:23PM +1000, Nick Piggin wrote:
> > But we can think of inodes that are only in use by unused (and aged)
> > dentries as effectively unused themselves. So this sequence under
> > estimates how many inodes to scan. This could bias pressure against
> > dcache I'd think, especially considering inodes are far larger than
> > dentries. Maybe require 2 passes to get the inodes unused inthe
> > first pass.
>
> It's self-balancing - it trends towards an equal number of unused
> dentries and inodes in the caches. Yes, it will tear down more
> dentries at first, but we need to do that to be able to reclaim
> inodes.

But then it doesn't scan enough inodes on the inode pass.

> Ås reclaim progresses the propotion of inodes increases, so
> the amount of inodes reclaimed increases.
>
> Basically this is a recognition that the important cache for
> avoiding IO is the inode cache, not he dentry cache. Once the inode

You can bias against the dcache using multipliers.

> cache is freed that we need to do IO to repopulate it, but
> rebuilding dentries fromteh inode cache only costs CPU time. Hence
> under light reclaim, inodes are mostly left in cache but we free up
> memory that only costs CPU to rebuild. Under heavy, sustained
> reclaim, we trend towards freeing equal amounts of objects from both
> caches.

I don't know if you've got numbers or patterns to justify that.
My point is that things should stay as close to the old code as
possible without good reason.

> This is pretty much what the current code attempts to do - free a
> lot of dentries, then free a smaller amount of the inodes that were
> used by the freed dentries. Once again it is a direct encoding of
> what is currently an implicit design feature - it makes it *obvious*
> how we are trying to balance the caches.

With your patches, if there are no inodes free you would need to take
2 passes at freeing the dentry cache. My suggestion is closer to the
current code.

> Another reason for this is that the calculation changes again to
> allow filesystem caches to modiy this proportioning in the next
> patch....
>
> FWIW, this also makes workloads that generate hundreds of thousands
> of never-to-be-used again negative dentries free dcache memory really
> quickly on memory pressure...

That would still be the case because used inodes aren't getting their
dentries freed so little inode scanning will occur.

>
> > Part of the problem is the funny shrinker API.
> >
> > The right way to do it is to change the shrinker API so that it passes
> > down the lru_pages and scanned into the callback. From there, the
> > shrinkers can calculate the appropriate ratio of objects to scan.
> > No need for 2-call scheme, no need for shrinker->seeks, and the
> > ability to calculate an appropriate ratio first for dcache, and *then*
> > for icache.
>
> My only concern about this is that exposes the inner workings of the
> shrinker and mm subsystem to code that simply doesn't need to know
> about it.

It's just providing a ratio. The shrinkers allready know they are
scanning based on a ratio of pagecache scanned.

> > A helper of course can do the calculation (considering that every
> > driver and their dog will do the wrong thing if we let them :)).
> >
> > unsigned long shrinker_scan(unsigned long lru_pages,
> > unsigned long lru_scanned,
> > unsigned long nr_objects,
> > unsigned long scan_ratio)
> > {
> > unsigned long long tmp = nr_objects;
> >
> > tmp *= lru_scanned * 100;
> > do_div(tmp, (lru_pages * scan_ratio) + 1);
> >
> > return (unsigned long)tmp;
> > }
> >
> > Then the shrinker callback will go:
> > sb->s_nr_dentry_scan += shrinker_scan(lru_pages, lru_scanned,
> > sb->s_nr_dentry_unused,
> > vfs_cache_pressure * SEEKS_PER_DENTRY);
> > if (sb->s_nr_dentry_scan > SHRINK_BATCH)
> > prune_dcache()
> >
> > sb->s_nr_inode_scan += shrinker_scan(lru_pages, lru_scanned,
> > sb->s_nr_inodes_unused,
> > vfs_cache_pressure * SEEKS_PER_INODE);
> > ...
> >
> > What do you think of that? Seeing as we're changing the shrinker API
> > anyway, I'd think it is high time to do somthing like this.
>
> Ignoring the dcache/icache reclaim ratio issues, I'd prefer a two

Well if it is an issue, it should be changed in a different patch
I think (with numbers).

> call API that matches the current behaviour, leaving the caclulation
> of how much to reclaim in shrink_slab(). Encoding it this way makes
> it more difficult to change the high level behaviour e.g. if we want
> to modify the amount of slab reclaim based on reclaim priority, we'd
> have to cahnge every shrinker instead of just shrink_slab().

We can modifiy the ratios before calling if needed, or have a default
ratio define to multiply with as well.

But shrinkers are very subsystem specific.