Re: [PATCH v2] net: netvsc: Update default VMBus channels

[Erni Sri Satya Vennela]

On Fri, Aug 16, 2024 at 08:52:41AM -0700, Jakub Kicinski wrote:
> On Thu, 15 Aug 2024 19:23:50 +0000 Haiyang Zhang wrote:
> > Your suggestion on netif_get_num_default_rss_queues() is not ignored.
> > We discussed internally on the formula we used for the num_chn, and
> > chose a similar formula for higher number of vCPUs as in 
> > netif_get_num_default_rss_queues().
> > For lower number of vCPUs, we use the same default as Windows guests,
> > because we don't want any potential regression.
> 
> Ideally you'd just use netif_get_num_default_rss_queues()
> but the code is close enough to that, and I don't have enough
> experience with the question of online CPUs vs physical CPUs.
> 
> I would definitely advise you to try this on real workloads.
> While "iperf" looks great with a lot of rings, real workloads
> suffer measurably from having more channels eating up memory
> and generating interrupts.
> 
> But if you're confident with the online_cpus() / 2, that's fine.
> You may be better off coding it up using max:
> 
> 	dev_info->num_chn = max(DIV_ROUND_UP(num_online_cpus(), 2),
> 				VRSS_CHANNEL_DEFAULT);
Due to hyper-threading, #of physical cores = online CPUs/2.
Therefore, netif_get_num_default_rss_queues() returns 
#of physical cores/2 = online CPUs/4.

In my testing, the throughput performance was similar for both the
configurations even for higher SKUs.To utilize lesser CPU resources, 
will be using netif_get_num_default_rss_queues() for the next version 
of the patch.