From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752532Ab2H1L1J (ORCPT ); Tue, 28 Aug 2012 07:27:09 -0400 Received: from mail-bk0-f46.google.com ([209.85.214.46]:45629 "EHLO mail-bk0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751637Ab2H1L1H (ORCPT ); Tue, 28 Aug 2012 07:27:07 -0400 Message-ID: <503CAB1E.5010408@gmail.com> Date: Tue, 28 Aug 2012 13:27:26 +0200 From: Sasha Levin User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:14.0) Gecko/20120826 Thunderbird/14.0 MIME-Version: 1.0 To: Mathieu Desnoyers 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 References: <5037DA47.9010306@gmail.com> <20120824195941.GC21325@google.com> <5037E00B.6090606@gmail.com> <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> In-Reply-To: <20120828101148.GA21683@Krystal> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. 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. Thanks, Sasha > Thanks, > > Mathieu >