Sasha Levin <levinsasha928@xxxxxxxxx> writes: > On 08/15/2012 01:52 AM, Eric W. Biederman wrote: >> Sasha Levin <levinsasha928@xxxxxxxxx> writes: >> >>> Switch user_ns to use the new hashtable implementation. This reduces the amount of >>> generic unrelated code in user_ns. >> >> Two concerns here. >> 1) When adding a new entry you recompute the hash where previously that >> was not done. I believe that will slow down adding of new entries. > > I figured that the price for the extra hashing isn't significant since hash_32 > is just a multiplication and a shift. > > I'll modify the code to calculate the key just once. Honestly I don't know either way, but it seemed a shame to give up a common and trivial optimization. >> 2) Using hash_32 for uids is an interesting choice. hash_32 discards >> the low bits. Last I checked for uids the low bits were the bits >> that were most likely to be different and had the most entropy. >> >> I'm not certain how multiplying by the GOLDEN_RATION_PRIME_32 will >> affect things but I would be surprised if it shifted all of the >> randomness from the low bits to the high bits. > > "Is hash_* good enough for our purpose?" - I was actually surprised that no one > raised that question during the RFC and assumed it was because everybody agreed > that it's indeed good enough. > > 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. > If the results of the above will be satisfactory we can avoid the discussion > about which hash function we should really be using. If not, I guess now is a > good time for that :) Yes. A small emperical test sounds good. Eric -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
