From: Nick Piggin <npiggin@kernel.dk>
To: Nick Piggin <npiggin@kernel.dk>, linux-mm@kvack.org
Cc: Dave Chinner <david@fromorbit.com>,
linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [patch 31/35] fs: icache per-zone inode LRU
Date: Wed, 20 Oct 2010 14:12:56 +1100 [thread overview]
Message-ID: <20101020031256.GA4095@amd> (raw)
In-Reply-To: <20101020023556.GC3740@amd>
Gah. Try again.
On Wed, Oct 20, 2010 at 01:35:56PM +1100, Nick Piggin wrote:
> [I should have cc'ed this one to linux-mm as well, so I quote your
> reply in full here]
>
> On Tue, Oct 19, 2010 at 11:38:52PM +1100, Dave Chinner wrote:
> > On Tue, Oct 19, 2010 at 02:42:47PM +1100, npiggin@kernel.dk wrote:
> > > Per-zone LRUs and shrinkers for inode cache.
> >
> > Regardless of whether this is the right way to scale or not, I don't
> > like the fact that this moves the cache LRUs into the memory
> > management structures, and expands the use of MM specific structures
> > throughout the code.
>
> The zone structure really is the basic unit of memory abstraction
> in the whole zoned VM concept (which covers different properties
> of both physical address and NUMA cost).
>
> The zone contains structures for memory management that aren't
> otherwise directly related to one another. Generic page waitqueues,
> page allocator structures, pagecache reclaim structures, memory model
> data, and various statistics.
>
> Structures to reclaim inodes from a particular zone belong in the
> zone struct as much as those to reclaim pagecache or anonymous
> memory from that zone too. It actually fits far better in here than
> globally, because all our allocation/reclaiming/watermarks etc is
> driven per-zone.
>
> The structure is not frequent -- a couple per NUMA node.
>
>
> > It ties the cache implementation to the current
> > VM implementation. That, IMO, goes against all the principle of
> > modularisation at the source code level, and it means we have to tie
> > all shrinker implemenations to the current internal implementation
> > of the VM. I don't think that is wise thing to do because of the
> > dependencies and impedance mismatches it introduces.
>
> It's very fundamental. We allocate memory from, and have to reclaim
> memory from -- zones. Memory reclaim is driven based on how the VM
> wants to reclaim memory: nothing you can do to avoid some linkage
> between the two.
>
> Look at it this way. The dumb global shrinker is also tied to an
> MM implementation detail, but that detail in fact does *not* match
> the reality of the MM, and so it has all these problems interacting
> with real reclaim.
>
> What problems? OK, on an N zone system (assuming equal zones and
> even distribution of objects around memory), then if there is a shortage
> on a particular zone, slabs from _all_ zones are reclaimed. We reclaim
> a factor of N too many objects. In a NUMA situation, we also touch
> remote memory with a chance (N-1)/N.
>
> As number of nodes grow beyond 2, this quickly goes down hill.
>
> In summary, there needs to be some knowledge of how MM reclaims memory
> in memory reclaim shrinkers -- simply can't do a good implementation
> without that. If the zone concept changes, the MM gets turned upside
> down and all those assumptions would need to be revisited anyway.
>
>
> > As an example: XFS inodes to be reclaimed are simply tagged in a
> > radix tree so the shrinker can reclaim inodes in optimal IO order
> > rather strict LRU order. It simply does not match a zone-based
>
> This is another problem, similar to what we have in pagecache. In
> the pagecache, we need to clean pages in optimal IO order, but we
> still reclaim them according to some LRU order.
>
> If you reclaim them in optimal IO order, cache efficiency will go
> down because you sacrifice recency/frequency information. If you
> IO in reclaim order, IO efficiency goes down. The solution is to
> decouple them with like writeout versus reclaim.
>
> But anyway, that's kind of an "aside": inode caches are reclaimed
> in LRU, IO-suboptimal order today anyway. Per-zone LRU doesn't
> change that in the slightest.
>
> > shrinker implementation in any way, shape or form, nor does it's
> > inherent parallelism match that of the way shrinkers are called.
> >
> > Any change in shrinker infrastructure needs to be able to handle
> > these sorts of impedance mismatches between the VM and the cache
> > subsystem. The current API doesn't handle this very well, either,
> > so it's something that we need to fix so that scalability is easy
> > for everyone.
> >
> > Anyway, my main point is that tying the LRU and shrinker scaling to
> > the implementation of the VM is a one-off solution that doesn't work
> > for generic infrastructure.
>
> No it isn't. It worked for the pagecache, and it works for dcache.
>
>
> > Other subsystems need the same
> > large-machine scaling treatment, and there's no way we should be
> > tying them all into the struct zone. It needs further abstraction.
>
> An abstraction? Other than the zone? What do you suggest? Invent
> something that the VM has no concept of and try to use that?
>
> No. The zone is the right thing to base it on.
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next prev parent reply other threads:[~2010-10-20 3:12 UTC|newest]
Thread overview: 70+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-10-19 3:42 [patch 00/35] my inode scaling series for review npiggin
2010-10-19 3:42 ` [patch 01/35] bit_spinlock: add required includes npiggin
2010-10-19 3:42 ` [patch 02/35] kernel: add bl_list npiggin
2010-10-19 3:42 ` [patch 03/35] mm: implement per-zone shrinker npiggin
2010-10-19 4:49 ` KOSAKI Motohiro
2010-10-19 5:33 ` Nick Piggin
2010-10-19 5:40 ` KOSAKI Motohiro
2010-10-19 3:42 ` [patch 04/35] vfs: convert inode and dentry caches to " npiggin
2010-10-19 3:42 ` [patch 05/35] fs: icache lock s_inodes list npiggin
2010-10-19 3:42 ` [patch 06/35] fs: icache lock inode hash npiggin
2010-10-19 3:42 ` [patch 07/35] fs: icache lock i_state npiggin
2010-10-19 10:47 ` Miklos Szeredi
2010-10-19 17:06 ` Peter Zijlstra
2010-10-19 3:42 ` [patch 08/35] fs: icache lock i_count npiggin
2010-10-19 10:16 ` Boaz Harrosh
2010-10-20 2:14 ` Nick Piggin
2010-10-19 3:42 ` [patch 09/35] fs: icache lock lru/writeback lists npiggin
2010-10-19 3:42 ` [patch 10/35] fs: icache atomic inodes_stat npiggin
2010-10-19 3:42 ` [patch 11/35] fs: icache lock inode state npiggin
2010-10-19 3:42 ` [patch 12/35] fs: inode atomic last_ino, iunique lock npiggin
2010-10-19 3:42 ` [patch 13/35] fs: icache remove inode_lock npiggin
2010-10-19 3:42 ` [patch 14/35] fs: icache factor hash lock into functions npiggin
2010-10-19 3:42 ` [patch 15/35] fs: icache per-bucket inode hash locks npiggin
2010-10-19 3:42 ` [patch 16/35] fs: icache lazy inode lru npiggin
2010-10-19 3:42 ` [patch 17/35] fs: icache RCU free inodes npiggin
2010-10-19 3:42 ` [patch 18/35] fs: avoid inode RCU freeing for pseudo fs npiggin
2010-10-19 3:42 ` [patch 19/35] fs: icache remove redundant i_sb_list umount locking npiggin
2010-10-20 12:46 ` Al Viro
2010-10-20 13:03 ` Nick Piggin
2010-10-20 13:27 ` Al Viro
2010-10-19 3:42 ` [patch 20/35] fs: icache rcu walk for i_sb_list npiggin
2010-10-19 3:42 ` [patch 21/35] fs: icache per-cpu nr_inodes, non-atomic nr_unused counters npiggin
2010-10-19 3:42 ` [patch 22/35] fs: icache per-cpu last_ino allocator npiggin
2010-10-19 3:42 ` [patch 23/35] fs: icache use per-CPU lists and locks for sb inode lists npiggin
2010-10-19 15:33 ` Miklos Szeredi
2010-10-20 2:37 ` Nick Piggin
2010-10-19 3:42 ` [patch 24/35] fs: icache use RCU to avoid locking in hash lookups npiggin
2010-10-19 3:42 ` [patch 25/35] fs: icache reduce some locking overheads npiggin
2010-10-19 3:42 ` [patch 26/35] fs: icache alloc anonymous inode allocation npiggin
2010-10-19 15:50 ` Miklos Szeredi
2010-10-20 2:38 ` Nick Piggin
2010-10-19 16:33 ` Christoph Hellwig
2010-10-20 3:07 ` Nick Piggin
2010-10-19 3:42 ` [patch 27/35] fs: icache split IO and LRU lists npiggin
2010-10-19 16:12 ` Miklos Szeredi
2010-10-20 2:41 ` Nick Piggin
2010-10-19 3:42 ` [patch 28/35] fs: icache split writeback and lru locks npiggin
2010-10-19 3:42 ` [patch 29/35] fs: icache per-bdi writeback list locking npiggin
2010-10-19 3:42 ` [patch 30/35] fs: icache lazy LRU avoid LRU locking after IO operation npiggin
2010-10-19 3:42 ` [patch 31/35] fs: icache per-zone inode LRU npiggin
2010-10-19 12:38 ` Dave Chinner
2010-10-20 2:35 ` Nick Piggin
2010-10-20 3:12 ` Nick Piggin [this message]
2010-10-20 9:43 ` Dave Chinner
2010-10-20 10:02 ` Nick Piggin
2010-10-20 3:14 ` KOSAKI Motohiro
2010-10-20 3:20 ` Nick Piggin
2010-10-20 3:29 ` KOSAKI Motohiro
2010-10-20 10:19 ` Dave Chinner
2010-10-20 10:41 ` Nick Piggin
2010-10-19 3:42 ` [patch 32/35] fs: icache minimise I_FREEING latency npiggin
2010-10-19 3:42 ` [patch 33/35] fs: icache introduce inode_get/inode_get_ilock npiggin
2010-10-19 10:17 ` Boaz Harrosh
2010-10-20 2:17 ` Nick Piggin
2010-10-19 3:42 ` [patch 34/35] fs: inode rename i_count to i_refs npiggin
2010-10-19 3:42 ` [patch 35/35] fs: icache document more lock orders npiggin
2010-10-19 16:22 ` [patch 00/35] my inode scaling series for review Christoph Hellwig
2010-10-20 3:05 ` Nick Piggin
2010-10-20 13:14 ` Al Viro
2010-10-20 13:59 ` Nick Piggin
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