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Message-ID: <2797ce03-a52a-459a-a1a1-7591233f51cd@grimberg.me> Date: Thu, 4 Jul 2024 11:40:33 +0300 From: Sagi Grimberg <sagi@...mberg.me> To: Ping Gan <jacky_gam_2001@....com>, hch@....de, kch@...dia.com, linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org Cc: ping.gan@...l.com Subject: Re: [PATCH 0/2] nvmet: support polling task for RDMA and TCP On 7/4/24 11:10, Ping Gan wrote: >> On 02/07/2024 13:02, Ping Gan wrote: >>>> On 01/07/2024 10:42, Ping Gan wrote: >>>>>> Hey Ping Gan, >>>>>> >>>>>> >>>>>> On 26/06/2024 11:28, Ping Gan wrote: >>>>>>> When running nvmf on SMP platform, current nvme target's RDMA and >>>>>>> TCP use kworker to handle IO. But if there is other high workload >>>>>>> in the system(eg: on kubernetes), the competition between the >>>>>>> kworker and other workload is very radical. And since the kworker >>>>>>> is scheduled by OS randomly, it's difficult to control OS >>>>>>> resource >>>>>>> and also tune the performance. If target support to use delicated >>>>>>> polling task to handle IO, it's useful to control OS resource and >>>>>>> gain good performance. So it makes sense to add polling task in >>>>>>> rdma-rdma and rdma-tcp modules. >>>>>> This is NOT the way to go here. >>>>>> >>>>>> Both rdma and tcp are driven from workqueue context, which are >>>>>> bound >>>>>> workqueues. >>>>>> >>>>>> So there are two ways to go here: >>>>>> 1. Add generic port cpuset and use that to direct traffic to the >>>>>> appropriate set of cores >>>>>> (i.e. select an appropriate comp_vector for rdma and add an >>>>>> appropriate >>>>>> steering rule >>>>>> for tcp). >>>>>> 2. Add options to rdma/tcp to use UNBOUND workqueues, and allow >>>>>> users >>>>>> to >>>>>> control >>>>>> these UNBOUND workqueues cpumask via sysfs. >>>>>> >>>>>> (2) will not control interrupts to steer to other workloads cpus, >>>>>> but >>>>>> the handlers may >>>>>> run on a set of dedicated cpus. >>>>>> >>>>>> (1) is a better solution, but harder to implement. >>>>>> >>>>>> You also should look into nvmet-fc as well (and nvmet-loop for >>>>>> that >>>>>> matter). >>>>> hi Sagi Grimberg, >>>>> Thanks for your reply, actually we had tried the first advice you >>>>> suggested, but we found the performance was poor when using spdk >>>>> as initiator. >>>> I suggest that you focus on that instead of what you proposed. >>>> What is the source of your poor performance? >>> Before these patches, we had used linux's RPS to forward the packets >>> to a fixed cpu set for nvmet-tcp. But when did that we can still not >>> cancel the competition between softirq and workqueue since nvme >>> target's >>> kworker cpu core bind on socket's cpu which is from skb. Besides that >>> we found workqueue's wait latency was very high even we enabled >>> polling >>> on nvmet-tcp by module parameter idle_poll_period_usecs. So when >>> initiator >>> is polling mode, the target of workqueue is the bottleneck. Below is >>> work's wait latency trace log of our test on our cluster(per node >>> uses >>> 4 numas 96 cores, 192G memory, one dual ports mellanox CX4LX(25Gbps X >>> 2) >>> ethernet adapter and randrw 1M IO size) by RPS to 6 cpu cores. And >>> system's CPU and memory were used about 80%. >> I'd try a simple unbound CPU case, steer packets to say cores [0-5] >> and >> assign >> the cpumask of the unbound workqueue to cores [6-11]. > Okay, thanks for your guide. > >>> ogden-brown:~ #/usr/share/bcc/tools/wqlat -T -w nvmet_tcp_wq 1 2 >>> 01:06:59 >>> usecs : count distribution >>> 0 -> 1 : 0 | | >>> 2 -> 3 : 0 | | >>> 4 -> 7 : 0 | | >>> 8 -> 15 : 3 | | >>> 16 -> 31 : 10 | | >>> 32 -> 63 : 3 | | >>> 64 -> 127 : 2 | | >>> 128 -> 255 : 0 | | >>> 256 -> 511 : 5 | | >>> 512 -> 1023 : 12 | | >>> 1024 -> 2047 : 26 |* | >>> 2048 -> 4095 : 34 |* | >>> 4096 -> 8191 : 350 |************ | >>> 8192 -> 16383 : 625 |******************************| >>> 16384 -> 32767 : 244 |********* | >>> 32768 -> 65535 : 39 |* | >>> >>> 01:07:00 >>> usecs : count distribution >>> 0 -> 1 : 1 | | >>> 2 -> 3 : 0 | | >>> 4 -> 7 : 4 | | >>> 8 -> 15 : 3 | | >>> 16 -> 31 : 8 | | >>> 32 -> 63 : 10 | | >>> 64 -> 127 : 3 | | >>> 128 -> 255 : 6 | | >>> 256 -> 511 : 8 | | >>> 512 -> 1023 : 20 |* | >>> 1024 -> 2047 : 19 |* | >>> 2048 -> 4095 : 57 |** | >>> 4096 -> 8191 : 325 |**************** | >>> 8192 -> 16383 : 647 |******************************| >>> 16384 -> 32767 : 228 |*********** | >>> 32768 -> 65535 : 43 |** | >>> 65536 -> 131071 : 1 | | >>> >>> And the bandwidth of a node is only 3100MB. While we used the patch >>> and enable 6 polling task, the bandwidth can be 4000MB. It's a good >>> improvement. >> I think you will see similar performance with unbound workqueue and >> rps. > Yes, I remodified the nvmet-tcp/nvmet-rdma code for supporting unbound > workqueue, and in same prerequisites of above to run test, and compared > the result of unbound workqueue and polling mode task. And I got a good > performance for unbound workqueue. For unbound workqueue TCP we got > 3850M/node, it's almost equal to polling task. And also tested > nvmet-rdma > we get 5100M/node for unbound workqueue RDMA versus 5600M for polling > task, > seems the diff is very small. Anyway, your advice is good. I'm a bit surprised that you see ~10% delta here. I would look into what is the root-cause of this difference. If indeed the load is high, the overhead of the workqueue mgmt should be negligible. I'm assuming you used IB_POLL_UNBOUND_WORKQUEUE ? > Do you think > we > should submit the unbound workqueue patches for nvmet-tcp and nvmet-rdma > to upstream nvmet? For nvmet-tcp, I think there is merit to split socket processing from napi context. For nvmet-rdma I think the only difference is if you have multiple CQs assigned with the same comp_vector. How many queues do you have in your test? > BTW I have another question: Will nvmet of upstream have the plan to > support > polling queue when doing submit_bio in future? No plans that I know of. Don't have a coherent idea of how that would work.
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