Re: Performance impact in networking data path tests in Linux 5.5 Kernel

[Vincent Guittot]

Hi Rajender,

On Tue, 25 Feb 2020 at 06:46, Rajender M <manir@xxxxxxxxxx> wrote:
>
> As part of VMware's performance regression testing for Linux Kernel upstream
>  releases, when comparing Linux 5.5 kernel against Linux 5.4 kernel, we noticed
> 20% improvement in networking throughput performance at the cost of a 30%
> increase in the CPU utilization.

Thanks for testing and sharing results with us. It's always
interesting to get feedbacks from various tests cases

>
> After performing the bisect between 5.4 and 5.5, we identified the root cause
> of this behaviour to be a scheduling change from Vincent Guittot's
> 2ab4092fc82d ("sched/fair: Spread out tasks evenly when not overloaded").
>
> The impacted testcases are TCP_STREAM SEND & RECV â on both small
> (8K socket & 256B message) & large (64K socket & 16K message) packet sizes.
>
> We backed out Vincent's commit & reran our networking tests and found that
> the performance were similar to 5.4 kernel - improvements in networking tests
> were no more.
>
> In our current network performance testing, we use Intel 10G NIC to evaluate
> all Linux Kernel releases. In order to confirm that the impact is also seen in
> higher bandwidth NIC, we repeated the same test cases with Intel 40G and
> we were able to reproduce the same behaviour - 25% improvements in
> throughput with 10% more CPU consumption.
>
> The overall results indicate that the new scheduler change has introduced
> much better network throughput performance at the cost of incremental
> CPU usage. This can be seen as expected behavior because now the
> TCP streams are evenly spread across all the CPUs and eventually drives
> more network packets, with additional CPU consumption.
>
>
> We have also confirmed this theory by parsing the ESX stats for 5.4 and 5.5
> kernels in a 4vCPU VM running 8 TCP streams - as shown below;
>
> 5.4 kernel:
>   "2132149": {"id": 2132149, "used": 94.37, "ready": 0.01, "cstp": 0.00, "name": "vmx-vcpu-0:rhel7x64-0",
>   "2132151": {"id": 2132151, "used": 0.13, "ready": 0.00, "cstp": 0.00, "name": "vmx-vcpu-1:rhel7x64-0",
>   "2132152": {"id": 2132152, "used": 9.07, "ready": 0.03, "cstp": 0.00, "name": "vmx-vcpu-2:rhel7x64-0",
>   "2132153": {"id": 2132153, "used": 34.77, "ready": 0.01, "cstp": 0.00, "name": "vmx-vcpu-3:rhel7x64-0",
>
> 5.5 kernel:
>   "2132041": {"id": 2132041, "used": 55.70, "ready": 0.01, "cstp": 0.00, "name": "vmx-vcpu-0:rhel7x64-0",
>   "2132043": {"id": 2132043, "used": 47.53, "ready": 0.01, "cstp": 0.00, "name": "vmx-vcpu-1:rhel7x64-0",
>   "2132044": {"id": 2132044, "used": 77.81, "ready": 0.00, "cstp": 0.00, "name": "vmx-vcpu-2:rhel7x64-0",
>   "2132045": {"id": 2132045, "used": 57.11, "ready": 0.02, "cstp": 0.00, "name": "vmx-vcpu-3:rhel7x64-0",
>
> Note, "used %" in above stats for 5.5 kernel is evenly distributed across all vCPUs.
>
> On the whole, this change should be seen as a significant improvement for
> most customers.
>
> Rajender M
> Performance Engineering
> VMware, Inc.
>