Return-Path: linux-nfs-owner@vger.kernel.org Received: from mail.openrapids.net ([64.15.138.104]:37919 "EHLO blackscsi.openrapids.net" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751149Ab2H1XAy (ORCPT ); Tue, 28 Aug 2012 19:00:54 -0400 Date: Tue, 28 Aug 2012 19:00:50 -0400 From: Mathieu Desnoyers To: Sasha Levin Cc: Tejun Heo , torvalds@linux-foundation.org, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, paul.gortmaker@windriver.com, davem@davemloft.net, rostedt@goodmis.org, mingo@elte.hu, ebiederm@xmission.com, aarcange@redhat.com, ericvh@gmail.com, netdev@vger.kernel.org, josh@joshtriplett.org, eric.dumazet@gmail.com, axboe@kernel.dk, agk@redhat.com, dm-devel@redhat.com, neilb@suse.de, ccaulfie@redhat.com, teigland@redhat.com, Trond.Myklebust@netapp.com, bfields@fieldses.org, fweisbec@gmail.com, jesse@nicira.com, venkat.x.venkatsubra@oracle.com, ejt@redhat.com, snitzer@redhat.com, edumazet@google.com, linux-nfs@vger.kernel.org, dev@openvswitch.org, rds-devel@oss.oracle.com, lw@cn.fujitsu.com Subject: Re: [PATCH v3 01/17] hashtable: introduce a small and naive hashtable Message-ID: <20120828230050.GA3337@Krystal> References: <20120824203332.GF21325@google.com> <5037E9D9.9000605@gmail.com> <20120824212348.GK21325@google.com> <5038074D.300@gmail.com> <20120824230740.GN21325@google.com> <20120825042419.GA27240@Krystal> <503C95E4.3010000@gmail.com> <20120828101148.GA21683@Krystal> <503CAB1E.5010408@gmail.com> <20120828115638.GC23818@Krystal> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20120828115638.GC23818@Krystal> Sender: linux-nfs-owner@vger.kernel.org List-ID: * Mathieu Desnoyers (mathieu.desnoyers@efficios.com) wrote: > * Sasha Levin (levinsasha928@gmail.com) wrote: > > On 08/28/2012 12:11 PM, Mathieu Desnoyers wrote: > > > * Sasha Levin (levinsasha928@gmail.com) wrote: > > >> On 08/25/2012 06:24 AM, Mathieu Desnoyers wrote: > > >>> * Tejun Heo (tj@kernel.org) wrote: > > >>>> Hello, > > >>>> > > >>>> On Sat, Aug 25, 2012 at 12:59:25AM +0200, Sasha Levin wrote: > > >>>>> Thats the thing, the amount of things of things you can do with a given bucket > > >>>>> is very limited. You can't add entries to any point besides the head (without > > >>>>> walking the entire list). > > >>>> > > >>>> Kinda my point. We already have all the hlist*() interface to deal > > >>>> with such cases. Having something which is evidently the trivial > > >>>> hlist hashtable and advertises as such in the interface can be > > >>>> helpful. I think we need that more than we need anything fancy. > > >>>> > > >>>> Heh, this is a debate about which one is less insignificant. I can > > >>>> see your point. I'd really like to hear what others think on this. > > >>>> > > >>>> Guys, do we want something which is evidently trivial hlist hashtable > > >>>> which can use hlist_*() API directly or do we want something better > > >>>> encapsulated? > > >>> > > >>> My 2 cents, FWIW: I think this specific effort should target a trivially > > >>> understandable API and implementation, for use-cases where one would be > > >>> tempted to reimplement his own trivial hash table anyway. So here > > >>> exposing hlist internals, with which kernel developers are already > > >>> familiar, seems like a good approach in my opinion, because hiding stuff > > >>> behind new abstraction might make the target users go away. > > >>> > > >>> Then, as we see the need, we can eventually merge a more elaborate hash > > >>> table with poneys and whatnot, but I would expect that the trivial hash > > >>> table implementation would still be useful. There are of course very > > >>> compelling reasons to use a more featureful hash table: automatic > > >>> resize, RT-aware updates, scalable updates, etc... but I see a purpose > > >>> for a trivial implementation. Its primary strong points being: > > >>> > > >>> - it's trivially understandable, so anyone how want to be really sure > > >>> they won't end up debugging the hash table instead of their > > >>> work-in-progress code can have a full understanding of it, > > >>> - it has few dependencies, which makes it easier to understand and > > >>> easier to use in some contexts (e.g. early boot). > > >>> > > >>> So I'm in favor of not overdoing the abstraction for this trivial hash > > >>> table, and honestly I would rather prefer that this trivial hash table > > >>> stays trivial. A more elaborate hash table should probably come as a > > >>> separate API. > > >>> > > >>> Thanks, > > >>> > > >>> Mathieu > > >>> > > >> > > >> Alright, let's keep it simple then. > > >> > > >> I do want to keep the hash_for_each[rcu,safe] family though. > > > > > > Just a thought: if the API offered by the simple hash table focus on > > > providing a mechanism to find the hash bucket to which belongs the hash > > > chain containing the key looked up, and then expects the user to use the > > > hlist API to iterate on the chain (with or without the hlist _rcu > > > variant), then it might seem consistent that a helper providing > > > iteration over the entire table would actually just provide iteration on > > > all buckets, and let the user call the hlist for each iterator for each > > > node within the bucket, e.g.: > > > > > > struct hlist_head *head; > > > struct hlist_node *pos; > > > > > > hash_for_each_bucket(ht, head) { > > > hlist_for_each(pos, head) { > > > ... > > > } > > > } > > > > > > That way you only have to provide one single macro > > > (hash_for_each_bucket), and rely on the already existing: > > > > > > - hlist_for_each_entry > > > - hlist_for_each_safe > > > - hlist_for_each_entry_rcu > > > - hlist_for_each_safe_rcu > > > ..... > > > > > > and various flavors that can appear in the future without duplicating > > > this API. So you won't even have to create _rcu, _safe, nor _safe_rcu > > > versions of the hash_for_each_bucket macro. > > > > > > Thoughts ? > > > > In my opinion, the downside here is that it'll require 2 function calls and 2 > > levels of nesting for a simple hash iteration. > > Those are macros, not functions. No function call is required. But I see > your point about nesting. > > > > > hash_for_each_bucket() will always be followed by an iteration of that > > bucket, so splitting a hash_for_each() which does both into 2 > > different functions which will almost always must be called in that > > given order sounds unintuitive to me. > > > > It's also just 3 different possible iterators: > > > > - hlist_for_each_entry > > - hlist_for_each_entry_safe > > - hlist_for_each_entry_rcu > > > > So I think that it's a good price to pay - 2 extra macro definitions > > in the header to save a macro call + nesting level in each place that > > uses a hashtable. > > I must admin I don't care that much one way or another. Looking again at: +#define hash_for_each_size(name, bits, bkt, node, obj, member) \ + for (bkt = 0; bkt < HASH_SIZE(bits); bkt++) \ + hlist_for_each_entry(obj, node, &name[bkt], member) you will notice that a "break" or "continue" in the inner loop will not affect the outer loop, which is certainly not what the programmer would expect! I advise strongly against creating such error-prone construct. Thanks, Mathieu > > Thanks, > > Mathieu > > > > > > > Thanks, > > Sasha > > > > > Thanks, > > > > > > Mathieu > > > > > > > -- > Mathieu Desnoyers > Operating System Efficiency R&D Consultant > EfficiOS Inc. > http://www.efficios.com -- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com