From mboxrd@z Thu Jan 1 00:00:00 1970 From: James Bottomley Subject: Re: [LSF/MM TOPIC] iSCSI MQ adoption via MCS discussion Date: Thu, 08 Jan 2015 14:29:02 -0800 Message-ID: <1420756142.11310.9.camel@HansenPartnership.com> References: <54AD5DDD.2090808@dev.mellanox.co.il> <54AD6563.4040603@suse.de> <54ADA777.6090801@cs.wisc.edu> <54AE36CE.8020509@acm.org> <1420755361.2842.16.camel@haakon3.risingtidesystems.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <1420755361.2842.16.camel@haakon3.risingtidesystems.com> Sender: target-devel-owner@vger.kernel.org To: "Nicholas A. Bellinger" Cc: Bart Van Assche , open-iscsi@googlegroups.com, Hannes Reinecke , Sagi Grimberg , lsf-pc@lists.linux-foundation.org, linux-scsi , target-devel List-Id: linux-scsi@vger.kernel.org On Thu, 2015-01-08 at 14:16 -0800, Nicholas A. Bellinger wrote: > On Thu, 2015-01-08 at 08:50 +0100, Bart Van Assche wrote: > > On 01/07/15 22:39, Mike Christie wrote: > > > On 01/07/2015 10:57 AM, Hannes Reinecke wrote: > > >> On 01/07/2015 05:25 PM, Sagi Grimberg wrote: > > >>> Hi everyone, > > >>> > > >>> Now that scsi-mq is fully included, we need an iSCSI initiator = that > > >>> would use it to achieve scalable performance. The need is even = greater > > >>> for iSCSI offload devices and transports that support multiple = HW > > >>> queues. As iSER maintainer I'd like to discuss the way we would= choose > > >>> to implement that in iSCSI. > > >>> > > >>> My measurements show that iSER initiator can scale up to ~2.1M = IOPs > > >>> with multiple sessions but only ~630K IOPs with a single sessio= n where > > >>> the most significant bottleneck the (single) core processing > > >>> completions. > > >>> > > >>> In the existing single connection per session model, given that= command > > >>> ordering must be preserved session-wide, we end up in a serial = command > > >>> execution over a single connection which is basically a single = queue > > >>> model. The best fit seems to be plugging iSCSI MCS as a multi-q= ueued > > >>> scsi LLDD. In this model, a hardware context will have a 1x1 ma= pping > > >>> with an iSCSI connection (TCP socket or a HW queue). > > >>> > > >>> iSCSI MCS and it's role in the presence of dm-multipath layer w= as > > >>> discussed several times in the past decade(s). The basic need f= or MCS is > > >>> implementing a multi-queue data path, so perhaps we may want to= avoid > > >>> doing any type link aggregation or load balancing to not overla= p > > >>> dm-multipath. For example we can implement ERL=3D0 (which is ba= sically the > > >>> scsi-mq ERL) and/or restrict a session to a single portal. > > >>> > > >>> As I see it, the todo's are: > > >>> 1. Getting MCS to work (kernel + user-space) with ERL=3D0 and a > > >>> round-robin connection selection (per scsi command executio= n). > > >>> 2. Plug into scsi-mq - exposing num_connections as nr_hw_queues= and > > >>> using blk-mq based queue (conn) selection. > > >>> 3. Rework iSCSI core locking scheme to avoid session-wide locki= ng > > >>> as much as possible. > > >>> 4. Use blk-mq pre-allocation and tagging facilities. > > >>> > > >>> I've recently started looking into this. I would like the commu= nity to > > >>> agree (or debate) on this scheme and also talk about implementa= tion > > >>> with anyone who is also interested in this. > > >>> > > >> Yes, that's a really good topic. > > >> > > >> I've pondered implementing MC/S for iscsi/TCP but then I've figu= red my > > >> network implementation knowledge doesn't spread that far. > > >> So yeah, a discussion here would be good. > > >> > > >> Mike? Any comments? > > > > > > I have been working under the assumption that people would be ok = with > > > MCS upstream if we are only using it to handle the issue where we= want > > > to do something like have a tcp/iscsi connection per CPU then map= the > > > connection to a blk_mq_hw_ctx. In this more limited MCS implement= ation > > > there would be no iscsi layer code to do something like load bala= nce > > > across ports or transport paths like how dm-multipath does, so th= ere > > > would be no feature/code duplication. For balancing across hctxs,= then > > > the iscsi layer would also leave that up to whatever we end up wi= th in > > > upper layers, so again no feature/code duplication with upper lay= ers. > > > > > > So pretty non controversial I hope :) > > > > > > If people want to add something like round robin connection selec= tion in > > > the iscsi layer, then I think we want to leave that for after the > > > initial merge, so people can argue about that separately. > >=20 > > Hello Sagi and Mike, > >=20 > > I agree with Sagi that adding scsi-mq support in the iSER initiator= =20 > > would help iSER users because that would allow these users to confi= gure=20 > > a single iSER target and use the multiqueue feature instead of havi= ng to=20 > > configure multiple iSER targets to spread the workload over multipl= e=20 > > cpus at the target side. > >=20 > > And I agree with Mike that implementing scsi-mq support in the iSER= =20 > > initiator as multiple independent connections probably is a better=20 > > choice than MC/S. RFC 3720 namely requires that iSCSI numbering is=20 > > session-wide. This means maintaining a single counter for all MC/S=20 > > sessions. Such a counter would be a contention point. I'm afraid th= at=20 > > because of that counter performance on a multi-socket initiator sys= tem=20 > > with a scsi-mq implementation based on MC/S could be worse than wit= h the=20 > > approach with multiple iSER targets. Hence my preference for an app= roach=20 > > based on multiple independent iSER connections instead of MC/S. > >=20 >=20 > The idea that a simple session wide counter for command sequence numb= er > assignment adds such a degree of contention that it renders MC/S at a > performance disadvantage vs. multi-session configurations with all of > the extra multipath logic overhead on top is at best, a naive > proposition. >=20 > On the initiator side for MC/S, literally the only thing that needs t= o > be serialized is the assignment of the command sequence number to > individual non-immediate PDUs. The sending of the outgoing PDUs + > immediate data by the initiator can happen out-of-order, and it's up = to > the target to ensure that the submission of the commands to the devic= e > server is in command sequence number order. >=20 > All of the actual immediate data + R2T -> data-out processing by the > target can also be done out-of-order as well. Right, but what he's saying is that we've taken great pains in the MQ situation to free our issue queues of all entanglements and cross queue locking so they can fly as fast as possible. If we have to assign an in-order sequence number across all the queues, this becomes both a cross CPU bus lock point to ensure atomicity and a sync point to ensure sequencing. Na=C3=AFvely that does look to be a bottleneck which would= n't necessarily be mitigated simply by allowing everything to proceed out o= f order after this point. James