On 08/15/2012 05:31 AM, Mathieu Desnoyers wrote: > * 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. I'm guessing that once this static hashtable is stable, a DEFINE_DYNAMIC_HASHTABLE() will get introduced which will evolve into something similar to what Mathieu has pointed out in the urcu. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
