Re: [PATCH 02/16] user_ns: use new hashtable implementation

Sasha Levin [off-list ref] writes:

On 08/15/2012 03:08 AM, Eric W. Biederman wrote:

quoted

quoted

I can offer the following: I'll write a small module that will hash 1...10000

quoted

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.

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

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