From mboxrd@z Thu Jan 1 00:00:00 1970 From: ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org (Eric W. Biederman) Subject: Re: [PATCH 02/16] user_ns: use new hashtable implementation Date: Tue, 14 Aug 2012 20:13:01 -0700 Message-ID: <87obmchmpu.fsf@xmission.com> References: <1344961490-4068-1-git-send-email-levinsasha928@gmail.com> <1344961490-4068-3-git-send-email-levinsasha928@gmail.com> <87txw5hw0s.fsf@xmission.com> <502AF184.4010907@gmail.com> <87393phshy.fsf@xmission.com> <502AFCD5.6070104@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: snitzer-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, neilb-l3A5Bk7waGM@public.gmane.org, fweisbec-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org, Trond.Myklebust-HgOvQuBEEgTQT0dZR+AlfA@public.gmane.org, bfields-uC3wQj2KruNg9hUCZPvPmw@public.gmane.org, paul.gortmaker-CWA4WttNNZF54TAoqtyWWQ@public.gmane.org, dm-devel-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, agk-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, aarcange-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, rds-devel-N0ozoZBvEnrZJqsBc5GL+g@public.gmane.org, eric.dumazet-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org, venkat.x.venkatsubra-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org, ccaulfie-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, mingo-X9Un+BFzKDI@public.gmane.org, dev-yBygre7rU0TnMu66kgdUjQ@public.gmane.org, ericvh-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org, josh-iaAMLnmF4UmaiuxdJuQwMA@public.gmane.org, rostedt-nx8X9YLhiw1AfugRpC6u6w@public.gmane.org, lw-BthXqXjhjHXQFUHtdCDX3A@public.gmane.org, mathieu.desnoyers-vg+e7yoeK/dWk0Htik3J/w@public.gmane.org, axboe-tSWWG44O7X1aa/9Udqfwiw@public.gmane.org, linux-nfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, edumazet-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, netdev-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, ejt-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org, teigland-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org, torvalds-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org, davem-fT/PcQaiUtIeIZ0/mPfg9Q@public.gmane.org To: Sasha Levin Return-path: In-Reply-To: <502AFCD5.6070104-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> (Sasha Levin's message of "Wed, 15 Aug 2012 03:35:17 +0200") List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: dev-bounces-yBygre7rU0TnMu66kgdUjQ@public.gmane.org Errors-To: dev-bounces-yBygre7rU0TnMu66kgdUjQ@public.gmane.org List-Id: netdev.vger.kernel.org Sasha Levin 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. 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