On Fri, May 28, 2010 at 03:19:24PM +1000, Nick Piggin wrote: > 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. We don't get a single shrinker call - we get batches of them fo each shrinker. The shrinker determines how many objects to scan in the cache, and that only changes for each shrinker to call based on the the shrinker->seeks and the number of objects in the cache. Once the number is decided, then the shrinker gets batches of reclaim to operate on. Fundamentally, the current shrinker will ask for the same percentage of each cache to be scanned - if it decides to scan 20% of the dentry cache, it will also decide to scan 20% of the inode cache Hence what the inode shrinker is doing is scanning 20% of the inodes freed by the dcache shrinker. In rough numbers, say we have 100k dentries, and the shrinker calculates it needs to scan 20% of the caches to reclaim them, the current code will end up with: unused dentries unused inodes before 100k 0 after dentry 80k 20k after inode 80k 16k So we get 20k dentries freed and 4k inodes freed on that shrink_slab pass. To contrast this against the code I proposed, I'll make a couple of simplicfications to avoid hurting my brain. That is, I'll assume SHRINK_BATCH=100 (rather than 128) and forgetting about rounding errors. With this, the algorithm I encoded gives roughly the following for a 20% object reclaim: number of batches = 20k / 100 = 200 Unused dentries+inodes dentries inodes before 100k 100k 0 batch 1 100k 99900 100 batch 2 100k 99800 200 .... batch 10 100k 99000 1000 batch 20 99990 98010 1990 batch 30 99980 97030 2950 batch 50 99910 95100 4810 batch 60 99860 94150 5710 ..... batch 200 98100 81900 16200 And so (roughly) we see that the number of inodes being reclaim per set of 10 batches roughly equals the (batch number - 10). Hence over 200 batches, we can expect to see roughly 190 + 180 + ... + 10 inodes reclaimed. That is 1900 inodes. Similarly for dentries, we get roughly 1000 + 990 + 980 + ... 810 dentries reclaimed - 18,100 in total. In other words, we have roughly 18k dentries and 1.9k inodes reclaimed for the code I wrote new algorithm. That does mean it initially attempts to reclaim dentries faster than the current code, but as the number of unused inodes increases, this comes back to parity with the current code and we end up with a 1:1 reclaim ratio. This is good behaviour - dentries are cheap to reconstruct from the inode cache, and we should hold onto the inode cache as much as possible. i.e. we should reclaim them more aggressively only if there is sustained pressure on the superblock and that is what the above algorithm does. > > 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. Multipliers are not self-balancing, and generally just amplify any imbalance an algorithm tends towards. The vfs_cache_pressure multiplier is a shining example of this kind of utterly useless knob... > > > 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. Sure, but the shrinkers are just a simple mechanism for implementing VM policy decisions. IMO reclaim policy decisions should not be pushed down and replicated in every one of these reclaim mechanisms. > But shrinkers are very subsystem specific. And as such should concentrate on getting their subsystem reclaim correct, not have to worry about implementing VM policy calculations... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>