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. 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. --nab -- 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
