On Thu, 2015-01-08 at 14:57 -0800, Nicholas A. Bellinger wrote: > On Thu, 2015-01-08 at 14:29 -0800, James Bottomley wrote: > > 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 session > > > > >>> 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-queued > > > > >>> scsi LLDD. In this model, a hardware context will have a 1x1 mapping > > > > >>> with an iSCSI connection (TCP socket or a HW queue). > > > > >>> > > > > >>> iSCSI MCS and it's role in the presence of dm-multipath layer was > > > > >>> discussed several times in the past decade(s). The basic need for > > > > >>> 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 overlap > > > > >>> dm-multipath. For example we can implement ERL=0 (which is > > > > >>> basically 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=0 and a > > > > >>> round-robin connection selection (per scsi command execution). > > > > >>> 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 locking > > > > >>> as much as possible. > > > > >>> 4. Use blk-mq pre-allocation and tagging facilities. > > > > >>> > > > > >>> I've recently started looking into this. I would like the community > > > > >>> to > > > > >>> agree (or debate) on this scheme and also talk about implementation > > > > >>> 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 figured > > > > >> 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 implementation > > > > > there would be no iscsi layer code to do something like load balance > > > > > across ports or transport paths like how dm-multipath does, so there > > > > > would be no feature/code duplication. For balancing across hctxs, then > > > > > the iscsi layer would also leave that up to whatever we end up with in > > > > > upper layers, so again no feature/code duplication with upper layers. > > > > > > > > > > So pretty non controversial I hope :) > > > > > > > > > > If people want to add something like round robin connection selection > > > > > in > > > > > the iscsi layer, then I think we want to leave that for after the > > > > > initial merge, so people can argue about that separately. > > > > > > > > Hello Sagi and Mike, > > > > > > > > I agree with Sagi that adding scsi-mq support in the iSER initiator > > > > would help iSER users because that would allow these users to configure > > > > a single iSER target and use the multiqueue feature instead of having > > > > to > > > > configure multiple iSER targets to spread the workload over multiple > > > > cpus at the target side. > > > > > > > > And I agree with Mike that implementing scsi-mq support in the iSER > > > > initiator as multiple independent connections probably is a better > > > > choice than MC/S. RFC 3720 namely requires that iSCSI numbering is > > > > session-wide. This means maintaining a single counter for all MC/S > > > > sessions. Such a counter would be a contention point. I'm afraid that > > > > because of that counter performance on a multi-socket initiator system > > > > with a scsi-mq implementation based on MC/S could be worse than with > > > > the > > > > approach with multiple iSER targets. Hence my preference for an > > > > approach > > > > based on multiple independent iSER connections instead of MC/S. > > > > > > > > > > The idea that a simple session wide counter for command sequence number > > > 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. > > > > > > On the initiator side for MC/S, literally the only thing that needs to > > > 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 device > > > server is in command sequence number order. > > > > > > 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. Naïvely that does look to be a bottleneck which wouldn't > > necessarily be mitigated simply by allowing everything to proceed out of > > order after this point. > > > > The point is that a simple session wide counter for command sequence > number assignment is significantly less overhead than all of the > overhead associated with running a full multipath stack atop multiple > sessions.
I don't see how that's relevant to issue speed, which was the measure we were using: The layers above are just a hopper. As long as they're loaded, the MQ lower layer can issue at full speed. So as long as the multipath hopper is efficient enough to keep the queues loaded there's no speed degradation. The problem with a sequence point inside the MQ issue layer is that it can cause a stall that reduces the issue speed. so the counter sequence point causes a degraded issue speed over the multipath hopper approach above even if the multipath approach has a higher CPU overhead. Now, if the system is close to 100% cpu already, *then* the multipath overhead will try to take CPU power we don't have and cause a stall, but it's only in the flat out CPU case. > Not to mention that our iSCSI/iSER initiator is already taking a session > wide lock when sending outgoing PDUs, so adding a session wide counter > isn't adding any additional synchronization overhead vs. what's already > in place. I'll leave it up to the iSER people to decide whether they're redoing this as part of the MQ work. James -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
