Return-Path: linux-nfs-owner@vger.kernel.org Received: from mail-ie0-f174.google.com ([209.85.223.174]:33139 "EHLO mail-ie0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752276Ab2J2Rf4 (ORCPT ); Mon, 29 Oct 2012 13:35:56 -0400 MIME-Version: 1.0 In-Reply-To: <20121029155957.GB18834@Krystal> References: <1351450948-15618-1-git-send-email-levinsasha928@gmail.com> <1351450948-15618-15-git-send-email-levinsasha928@gmail.com> <20121029132931.GC16391@Krystal> <20121029155957.GB18834@Krystal> From: Sasha Levin Date: Mon, 29 Oct 2012 13:35:35 -0400 Message-ID: Subject: Re: [PATCH v7 15/16] openvswitch: use new hashtable implementation To: Mathieu Desnoyers Cc: torvalds@linux-foundation.org, tj@kernel.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 Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-nfs-owner@vger.kernel.org List-ID: On Mon, Oct 29, 2012 at 11:59 AM, Mathieu Desnoyers wrote: > * Sasha Levin (levinsasha928@gmail.com) wrote: >> Hi Mathieu, >> >> On Mon, Oct 29, 2012 at 9:29 AM, Mathieu Desnoyers >> wrote: >> > * Sasha Levin (levinsasha928@gmail.com) wrote: >> > [...] >> >> -static struct hlist_head *hash_bucket(struct net *net, const char *name) >> >> -{ >> >> - unsigned int hash = jhash(name, strlen(name), (unsigned long) net); >> >> - return &dev_table[hash & (VPORT_HASH_BUCKETS - 1)]; >> >> -} >> >> - >> >> /** >> >> * ovs_vport_locate - find a port that has already been created >> >> * >> >> @@ -84,13 +76,12 @@ static struct hlist_head *hash_bucket(struct net *net, const char *name) >> >> */ >> >> struct vport *ovs_vport_locate(struct net *net, const char *name) >> >> { >> >> - struct hlist_head *bucket = hash_bucket(net, name); >> >> struct vport *vport; >> >> struct hlist_node *node; >> >> + int key = full_name_hash(name, strlen(name)); >> >> >> >> - hlist_for_each_entry_rcu(vport, node, bucket, hash_node) >> >> - if (!strcmp(name, vport->ops->get_name(vport)) && >> >> - net_eq(ovs_dp_get_net(vport->dp), net)) >> >> + hash_for_each_possible_rcu(dev_table, vport, node, hash_node, key) >> > >> > Is applying hash_32() on top of full_name_hash() needed and expected ? >> >> Since this was pointed out in several of the patches, I'll answer it >> just once here. >> >> I've intentionally "allowed" double hashing with hash_32 to keep the >> code simple. >> >> hash_32() is pretty simple and gcc optimizes it to be almost nothing, >> so doing that costs us a multiplication and a shift. On the other >> hand, we benefit from keeping our code simple - how would we avoid >> doing this double hash? adding a different hashtable function for >> strings? or a new function for already hashed keys? I think we benefit >> a lot from having to mul/shr instead of adding extra lines of code >> here. > > This could be done, as I pointed out in another email within this > thread, by changing the "key" argument from add/for_each_possible to an > expected "hash" value, and let the caller invoke hash_32() if they want. > I doubt this would add a significant amount of complexity for users of > this API, but would allow much more flexibility to choose hash > functions. Most callers do need to do the hashing though, so why add an additional step for all callers instead of doing another hash_32 for the ones that don't really need it? Another question is why do you need flexibility? I think that simplicity wins over flexibility here. Thanks, Sasha