On 1/8/2015 9:50 AM, 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.
Hey Bart,
IMHO, iSER is an iSCSI extension, so I think the discussion should
focus the solving this in iSCSI level in a way that would apply both
for TCP and RDMA (and offload devices).
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.
Actually I started with that approach, but the independent connections
under a single session (I-T-Nexus) violates the command ordering
requirement. Plus, such a solution is specific to iSER...
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.
So this comment is spot on the pros/cons of the discussion (we might
want to leave something for LSF ;)).
MCS would not allow a completely lockless data-path due to command
ordering. On the other hand implementing some kind of multiple sessions
solution feels somewhat like a mis-fit (at least in my view).
One of my thoughts about how to overcome the contention on commands
sequence numbering was to suggest some kind of negotiable "relaxed
ordering" mode but of course I don't have anything figured out yet.
I had a short discussion on this with Mallikarjun Chadalapaka at SDC-14.
He said that if I show some numbers to back up such a proposal it can
be considered.
Sagi.
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