Question about the sndbuf of the tap interface with vhost-net

[Harold Huang]

 Sorry. The performance tested by iperf is degraded from 4.5 Gbps to
750Mbps per flow.

Harold Huang <baymaxhuang at gmail.com> 于2022年2月23日周三 21:13写道：
>
> I see in dpdk virtio-user driver, the TUNSETSNDBUF is initialized with
> INT_MAX, see: https://github.com/DPDK/dpdk/blob/main/drivers/net/virtio/virtio_user/vhost_kernel_tap.c#L169
> It is ok because tap driver uses it to support tx baching, see this
> patch: https://github.com/torvalds/linux/commit/0a0be13b8fe2cac11da2063fb03f0f39359b3069
>
> But in tun_xdp_one, napi is not supported and I want to user napi in
> tun_get_user to enable gro. As I result, I change the sndbuf to a
> value such as 212992 in /proc/sys/net/core/wmem_default. But the
> performance tested by iperf is greatly degraded, from 4.5 Gbps to
> 750Gbps per flow. I see the the iperf server consume 100% cpu core,
> which should be the bottleneck of the this test. The perf top result
> of iperf server cpu core is as follows:
>
> '''
> Samples: 72  of event 'cycles', 4000 Hz, Event count (approx.):
> 22685278 lost: 0/0 drop: 0/0
> Overhead  Shared O  Symbol
>   59.86%  [kernel]  [k] report_bug
>   20.66%  [kernel]  [k] module_find_bug
>    6.51%  [kernel]  [k] common_interrupt
>    2.82%  [kernel]  [k] __slab_free
>    1.48%  [kernel]  [k] copy_user_enhanced_fast_string
>    1.44%  [kernel]  [k] __skb_datagram_iter
>    1.42%  [kernel]  [k] notifier_call_chain
>    1.41%  [kernel]  [k] irq_work_run_list
>    1.41%  [kernel]  [k] update_irq_load_avg
>    1.41%  [kernel]  [k] task_tick_fair
>    1.41%  [kernel]  [k] cmp_ex_search
>    0.16%  [kernel]  [k] __ghes_peek_estatus.isra.12
>    0.02%  [kernel]  [k] acpi_os_read_memory
>    0.00%  [kernel]  [k] native_apic_mem_write
> '''
> I am not clear about the test result. Can we change the sndbuf size in
> dpdk? Is any way to enable vhost_net to use napi without changing the
> tun kernel driver?