* Eric W. Biederman (ebiederm@xxxxxxxxxxxx) wrote: > Sasha Levin <levinsasha928@xxxxxxxxx> writes: > > > On 08/15/2012 03:08 AM, Eric W. Biederman wrote: > >>> I can offer the following: I'll write a small module that will hash 1...10000 > >>> > into a hashtable which uses 7 bits (just like user_ns) and post the distribution > >>> > we'll get. > >> That won't hurt. I think 1-100 then 1000-1100 may actually be more > >> representative. Not that I would mind seeing the larger range. > >> Especially since I am in the process of encouraging the use of more > >> uids. > >> > > > > Alrighty, the results are in (numbers are objects in bucket): > > > > For the 0...10000 range: > > > > Average: 78.125 > > Std dev: 1.4197704151 > > Min: 75 > > Max: 80 > > > > > > For the 1...100 range: > > > > Average: 0.78125 > > Std dev: 0.5164613088 > > Min: 0 > > Max: 2 > > > > > > For the 1000...1100 range: > > > > Average: 0.7890625 > > Std dev: 0.4964812206 > > Min: 0 > > Max: 2 > > > > > > Looks like hash_32 is pretty good with small numbers. > > Yes hash_32 seems reasonable for the uid hash. With those long hash > chains I wouldn't like to be on a machine with 10,000 processes with > each with a different uid, and a processes calling setuid in the fast > path. > > The uid hash that we are playing with is one that I sort of wish that > the hash table could grow in size, so that we could scale up better. Hi Eric, If you want to try out something that has more features than a basic hash table, already exists and is available for you to play with, you might want to have a look at the RCU lock-free resizable hash table. It's initially done in userspace, but shares the same RCU semantic as the kernel, and has chunk-based kernel-friendly index backends (thanks to Lai Jiangshan), very useful to integrate with the kernel page allocator. It has the following properties that might make this container a good fit for uid hashing: - Real-time friendly lookups: Lookups are RCU and wait-free. - Fast and real-time friendly updates: Use cmpxchg for update, and RCU to deal with ABA. - Resize (expand/shrink) for each power of two size, performed concurrently with ongoing updates and lookups. - Has add_unique (uniquify), add_replace, and also duplicate semantics. - Provide uniqueness guarantees for RCU traversals of the hash table with respect to add_unique and add_replace. So if you are looking for a fast, RT-friendly, resizable hash table to play with, you might want to have a look at the userspace RCU implementation, which now features this hash table: https://lttng.org/urcu See urcu/rculfhash.h for the API. Best regards, Mathieu > > Aw well. Most of the time we only have a very small number of uids > in play, so it doesn't matter at this point. > > Eric > -- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
