On Sun, Aug 14, 2011 at 07:13:48PM +0400, Glauber Costa wrote: > Hello, > > This series is just like v2, except it addresses > Eric's comments regarding percpu variables. > > Let me know if there are further comments, and > I'll promply address them as well. Otherwise, > I feel this is ready for inclusion Just out of couriousity, one thing I've noticed about dentries is that in general at any given point in time most dentries are unused. Under the workloads I'm testing, even when I have a million cached dentries, I only have roughly 7,000 accounted as used. That is, most of the dentries in the system are on a LRU and accounted in sb->s_nr_dentry_unused of their owner superblock. So rather than introduce a bunch of new infrastructure to track the number of dentries allocated, why not simply limit the number of dentries allowed on the LRU? We already track that, and the shrinker already operates on the LRU, so we don't really need any new infrastructure. The limiting can be lazily - we don't need to limit the growth of dentries until we start to run out of memory. If the superblock shrinker is aware of the limits, then when it gets called by memory reclaim it can do all the work of reducing the number of items on the LRU down to the threshold at that time. IOWs, the limit has no impact on performance until memory is scarce, at which time memory reclaim enforces the limits on LRU size and clean up happens automatically. This also avoids all the problems of setting a limit lower than the number of active dentries required for the workload (i.e. avoids spurious ENOMEM errors trying to allocate dentries), allows overcommitment when memory is plentiful (which will benefit performance) but it brings the caches back to defined limits when memory is not plentiful (which solves the problem you are having). That seems like a much better solution to me, because it doesn't impact the normal working of workloads in the containers and avoids having to work out what the correct minimum size of the cache for each workload is. It's much easier to work out how many extra cached dentries you want the container to be able to have when memory is scarce... What do you think? Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
