Re: scsi-mq V2

From: Sagi Grimberg
Date: Mon Jul 14 2014 - 05:13:44 EST

On 7/8/2014 5:48 PM, Christoph Hellwig wrote:
I've pushed out a new scsi-mq.3 branch, which has been rebased on the
latest core-for-3.17 tree + the "RFC: clean up command setup" series
from June 29th. Robert Elliot found a problem with not fully zeroed
out UNMAP CDBs, which is fixed by the saner discard handling in that

There is a new patch to factor the code from the above series for
blk-mq use, which I've attached below. Besides that the only changes
are minor merge fixups in the main blk-mq usage patch.

Hey Christoph & Co,

I'd like to share some benchmarks I took on this patch set using iSER initiator (+2 pre-submitted performance improvements) vs LIO iSER target.
I ran workloads I think are interesting use-cases (single LUN with 1,2,4 IO threads up to a fully occupied system doing IO to multiple LUNs).
Overall (except 2 strange anomalies) seems that scsi-mq patches (use_blk_mq=N) roughly sustains traditional scsi performance.
On the other hand scsi-mq code path (use_blk_mq=Y) on its own clearly shows better performance (tables below).

At first I too hit the aio issues discussed in this thread and converted to scsi-mq.3-no-rebase for testing (thanks Doug & Rob for raising it).
I must say that for some reason I get very low numbers for writes vs. reads (writes perf stuck at ~20K IOPs per thread), this happens
on 3.16-rc2 even before scsi-mq patches. Did anyone step on this as well or is it just a weird problem I'm having in my setup?
Anyway this is why my benchmarks shows only randread IO pattern (getting familiar numbers). I need to figure out whats wrong
with IO writes - I'll start bisecting on this.

I also reviewed the patch set and at this point, I don't have any comments. So you can add to the series:
Reviewed-by: Sagi Grimberg '<sagig@xxxxxxxxxxxxxxxxxx>' (or Tested-by - whatever you choose).

I want to state that I tested a traditional iSER initiator - no scsi-mq adoption at all.
I started looking into adopting scsi-mq to iSCSI/iSER recently and I must that say the scsi-mq adoption is not so
trivial due to iSCSI session-wide CmdSN/StatSN ordering constraints (can't just use more RDMA channels per connection...)
I'll be on vacation for the next couple of weeks, so I'll start a separate thread to get the community input on this matter.

Results: table entries are KIOPS(CPU%)
3.16-rc2 (scsi-mq patches reverted)
Threads/LUN 1 2 4
1 231(6.5%) 355(18.5%) 337(31.1%)
2 446(13.6%) 673(37.2%) 654(49.8%)
4 594(25%) 960(49.41%) 1165(99.3%)
8 1018(50.3%) 1563(99.6%) 1696(99.9%)
16 1660(86.5%) 1731(99.6%) 1710(100%)

3.16-rc2 (scsi-mq included, use_blk_mq=N)
Threads/LUN 1 2 4
1 231(6.5%) 351(18.5%) 337(31.4%)
2 446(13.6%) 660(37.3%) 647(50%)
4 591(25%) 967(49.7%) 1136(98.1%)
8 1014(52.1%) 1296(100%) 1470(100%)
16 1741(100%) 1761(100%) 1853(100%)

3.16-rc2 (scsi-mq included, use_blk_mq=Y)
Threads/LUN 1 2 4
1 265(6.4%) 465(13.4%) 572(27.9%)
2 507(13.4%) 902(27.8%) 1034(45.9%)
4 697(25%) 1197(49.5%) 1477(98.6%)
8 1257(53.6%) 1856(98.7%) 1906(100%)
16 1991(100%) 2021(100%) 2020(100%)

- IOPs measurements are the average of a 60 seconds runs.
- The CPU measurement is the total usage across all CPUs, In order
to understand per-CPU utilization value should be normalized to 16
- scsi_mq (use_blk_mq=N) has roughly the same performance as
traditional scsi IO path but I see an anomaly in test cases
{8 LUNs, 2/4 threads per LUN}. This may result in NUMA
misalignment for threads/interrupts – requires further
- iSER initiator has no Multi-Queue awareness.

Testing environment:
- Initiator and target systems of 16 (8x2) cores (Hyperthreading
- CPU model: Intel(R) Xeon(R) @ 2.60GHz
- Block Layer settings:
- scheduler=noop
- rq_affinity=1
- add_random=0
- nomerges=1
- Single FDR link between the target and initiator.
- Device model: Mellanox ConnectIB (the numbers are also familiar
with Mellanox ConnectX-3).
- MSIX interrupt vectors were spread across system cores.
- irqbalancer was disabled.
- scsi_host settings:
- cmd_per_lun=32 (default)
- can_queue=113 (default)
- In the multi-LUN test cases, each LUN exposed via different
scsi_host (iSCSI session).

- fio version: 2.0.13
- LIO iSER target (target-pending for-next)
- Null backing devices (NULLIO)
- Upstream based iSER initiator + internal pre-submitted
performance enhancements.

fio configuration:

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