From: Jesper Dangaard Brouer <brouer@redhat.com>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: brouer@redhat.com,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
netdev@vger.kernel.org, Jamal Hadi Salim <jhs@mojatatu.com>
Subject: Re: Queue with wait-free enqueue, blocking dequeue, splice
Date: Mon, 20 Oct 2014 16:02:37 +0200 [thread overview]
Message-ID: <20141020160237.302aa17c@redhat.com> (raw)
In-Reply-To: <412768308.11171.1413632892841.JavaMail.zimbra@efficios.com>
On Sat, 18 Oct 2014 11:48:12 +0000 (UTC) Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
> Following our LPC discussion on lock-free queue algorithms
> for qdisc, here is some info on the wfcqueue implementation
> found in Userspace RCU. See http://urcu.so for info and
> git repository.
Thank for following up on our very interesting discussions.
I've started with the more simple variant "urcu/static/wfqueue.h" to
understand the concepts. And I'm now reading wfcqueue.h, which I guess
it replacing wfqueue.h.
> Here is the wfcqueue ported to the Linux kernel I sent last
> year as RFC:
> https://lkml.org/lkml/2013/3/14/289
>
> I'm very interested to learn if it fits well for your
> use-case,
Does this wfcqueue API support bulk dequeue? (A feature needed for the
lock-less qdisc implementation, else it cannot compete with our new
bulk dequeue strategy).
AFAIK your queue implementation is a CAS-based, Wait-Free on enqueue,
but Lock-Free on dequeue with the potential for waiting/blocking on
a enqueue processes.
I'm not 100% sure, that we want this behavior for the qdisc system.
I can certainly use the wfcq_empty() check, but I guess I need to
maintain a separate counter to maintain the qdisc limit, right?
(I would use the approximate/split counter API percpu_counter to keep
this scalable, and wfcq_empty() would provide an accurate empty check)
I think, we/I should start micro benchmarking the different approaches.
As our time budget is only 67.2ns
http://netoptimizer.blogspot.dk/2014/05/the-calculations-10gbits-wirespeed.html
(or bulking tricks artificially "increase" this budget)
The motivation behind this lockless qdisc is, the current qdisc locking
cost 48ns, see slide 9/16 titled "Qdisc locking is nasty":
http://people.netfilter.org/hawk/presentations/LinuxPlumbers2014/performance_tx_qdisc_bulk_LPC2014.pdf
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Sr. Network Kernel Developer at Red Hat
Author of http://www.iptv-analyzer.org
LinkedIn: http://www.linkedin.com/in/brouer
next prev parent reply other threads:[~2014-10-20 14:03 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <1311316954.11157.1413631325000.JavaMail.zimbra@efficios.com>
2014-10-18 11:48 ` Queue with wait-free enqueue, blocking dequeue, splice Mathieu Desnoyers
2014-10-20 14:02 ` Jesper Dangaard Brouer [this message]
2014-10-21 0:04 ` Mathieu Desnoyers
2014-10-21 11:48 ` Jesper Dangaard Brouer
2014-10-21 12:02 ` Mathieu Desnoyers
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