From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Sasha Levin <levinsasha928@gmail.com>
Cc: Tejun Heo <tj@kernel.org>,
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
Date: Tue, 28 Aug 2012 19:00:50 -0400 [thread overview]
Message-ID: <20120828230050.GA3337@Krystal> (raw)
In-Reply-To: <20120828115638.GC23818@Krystal>
* 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
next prev parent reply other threads:[~2012-08-28 23:00 UTC|newest]
Thread overview: 67+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-08-22 2:26 [PATCH v3 00/17] generic hashtable implementation Sasha Levin
2012-08-22 2:26 ` [PATCH v3 01/17] hashtable: introduce a small and naive hashtable Sasha Levin
2012-08-22 18:01 ` Tejun Heo
2012-08-22 23:54 ` Ryan Mallon
2012-08-23 0:24 ` Sasha Levin
2012-08-23 20:04 ` Tejun Heo
2012-08-24 19:47 ` Sasha Levin
2012-08-24 19:59 ` Tejun Heo
2012-08-24 20:11 ` Sasha Levin
2012-08-24 20:33 ` Tejun Heo
2012-08-24 20:53 ` Sasha Levin
2012-08-24 21:23 ` Tejun Heo
2012-08-24 22:59 ` Sasha Levin
2012-08-24 23:07 ` Tejun Heo
2012-08-25 4:24 ` Mathieu Desnoyers
2012-08-28 9:56 ` Sasha Levin
2012-08-28 10:11 ` Mathieu Desnoyers
2012-08-28 11:27 ` Sasha Levin
2012-08-28 11:56 ` Mathieu Desnoyers
2012-08-28 23:00 ` Mathieu Desnoyers [this message]
2012-09-04 15:35 ` Steven Rostedt
2012-09-04 16:30 ` Pedro Alves
2012-09-04 16:40 ` Pedro Alves
2012-09-04 17:01 ` Mathieu Desnoyers
2012-09-06 13:53 ` Sasha Levin
2012-09-06 14:19 ` Pedro Alves
2012-09-06 14:33 ` Mathieu Desnoyers
2012-09-06 14:36 ` David Laight
2012-09-06 14:55 ` Josh Triplett
2012-09-06 15:11 ` Steven Rostedt
2012-09-06 15:49 ` Sasha Levin
2012-09-06 16:00 ` Steven Rostedt
2012-09-06 16:21 ` Sasha Levin
2012-09-06 16:50 ` Mathieu Desnoyers
2012-09-06 17:01 ` Sasha Levin
2012-09-06 17:02 ` Mathieu Desnoyers
2012-09-06 17:15 ` Steven Rostedt
2012-09-04 17:17 ` Steven Rostedt
2012-09-04 17:21 ` Pedro Alves
2012-09-04 20:59 ` Steven Rostedt
2012-09-04 21:51 ` Pedro Alves
2012-09-04 22:41 ` Steven Rostedt
2012-09-04 22:58 ` Pedro Alves
2012-09-04 23:27 ` Steven Rostedt
2012-09-04 16:32 ` Mathieu Desnoyers
2012-08-22 2:26 ` [PATCH v3 02/17] userns: use new hashtable implementation Sasha Levin
2012-08-22 2:26 ` [PATCH v3 03/17] mm,ksm: " Sasha Levin
2012-08-22 2:26 ` [PATCH v3 04/17] workqueue: " Sasha Levin
2012-08-22 18:05 ` Tejun Heo
2012-08-22 2:27 ` [PATCH v3 05/17] mm/huge_memory: " Sasha Levin
2012-08-22 2:27 ` [PATCH v3 06/17] tracepoint: " Sasha Levin
2012-08-22 2:27 ` [PATCH v3 07/17] net,9p: " Sasha Levin
2012-08-22 2:27 ` [PATCH v3 08/17] block,elevator: " Sasha Levin
2012-08-22 2:27 ` [PATCH v3 09/17] SUNRPC/cache: " Sasha Levin
2012-08-22 2:27 ` [PATCH v3 10/17] dlm: " Sasha Levin
2012-08-22 2:27 ` [PATCH v3 11/17] net,l2tp: " Sasha Levin
2012-08-22 2:27 ` [PATCH v3 12/17] dm: " Sasha Levin
2012-08-22 2:27 ` [PATCH v3 13/17] lockd: " Sasha Levin
2012-08-22 11:47 ` J. Bruce Fields
2012-08-22 12:13 ` Sasha Levin
2012-08-22 13:12 ` J. Bruce Fields
2012-08-22 13:22 ` Mathieu Desnoyers
2012-08-22 17:32 ` Sasha Levin
2012-08-22 2:27 ` [PATCH v3 14/17] net,rds: " Sasha Levin
2012-08-22 2:27 ` [PATCH v3 15/17] openvswitch: " Sasha Levin
2012-08-22 2:27 ` [PATCH v3 16/17] tracing output: " Sasha Levin
2012-08-22 2:27 ` [PATCH v3 17/17] SUNRPC: use new hashtable implementation in auth Sasha Levin
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20120828230050.GA3337@Krystal \
--to=mathieu.desnoyers@efficios.com \
--cc=Trond.Myklebust@netapp.com \
--cc=aarcange@redhat.com \
--cc=agk@redhat.com \
--cc=akpm@linux-foundation.org \
--cc=axboe@kernel.dk \
--cc=bfields@fieldses.org \
--cc=ccaulfie@redhat.com \
--cc=davem@davemloft.net \
--cc=dev@openvswitch.org \
--cc=dm-devel@redhat.com \
--cc=ebiederm@xmission.com \
--cc=edumazet@google.com \
--cc=ejt@redhat.com \
--cc=eric.dumazet@gmail.com \
--cc=ericvh@gmail.com \
--cc=fweisbec@gmail.com \
--cc=jesse@nicira.com \
--cc=josh@joshtriplett.org \
--cc=levinsasha928@gmail.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=linux-nfs@vger.kernel.org \
--cc=lw@cn.fujitsu.com \
--cc=mingo@elte.hu \
--cc=neilb@suse.de \
--cc=netdev@vger.kernel.org \
--cc=paul.gortmaker@windriver.com \
--cc=rds-devel@oss.oracle.com \
--cc=rostedt@goodmis.org \
--cc=snitzer@redhat.com \
--cc=teigland@redhat.com \
--cc=tj@kernel.org \
--cc=torvalds@linux-foundation.org \
--cc=venkat.x.venkatsubra@oracle.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).