Re: [PATCH] net: increase the maximum of RX/TX descriptors

["Lukáš Šišmiš" <sismis@cesnet.cz>]

On 30. 10. 24 16:20, Stephen Hemminger wrote:
> On Wed, 30 Oct 2024 14:58:40 +0100
> Lukáš Šišmiš <sismis@cesnet.cz> wrote:
>
>> On 29. 10. 24 15:37, Morten Brørup wrote:
>>>> From: Lukas Sismis [mailto:sismis@cesnet.cz]
>>>> Sent: Tuesday, 29 October 2024 13.49
>>>>
>>>> Intel PMDs are capped by default to only 4096 RX/TX descriptors.
>>>> This can be limiting for applications requiring a bigger buffer
>>>> capabilities. The cap prevented the applications to configure
>>>> more descriptors. By bufferring more packets with RX/TX
>>>> descriptors, the applications can better handle the processing
>>>> peaks.
>>>>
>>>> Signed-off-by: Lukas Sismis <sismis@cesnet.cz>
>>>> ---
>>> Seems like a good idea.
>>>
>>> Have the max number of descriptors been checked with the datasheets for all the affected NIC chips?
>>>   
>> I was hoping to get some feedback on this from the Intel folks.
>>
>> But it seems like I can change it only for ixgbe (82599) to 32k
>> (possibly to 64k - 8), others - ice (E810) and i40e (X710) are capped at
>> 8k - 32.
>>
>> I neither have any experience with other drivers nor I have them
>> available to test so I will let it be in the follow-up version of this
>> patch.
>>
>> Lukas
>>
> Having large number of descriptors especially at lower speeds will
> increase buffer bloat. For real life applications, do not want increase
> latency more than 1ms.
>
> 10 Gbps has 7.62Gbps of effective bandwidth due to overhead.
> Rate for 1500 MTU is 7.62Gbs / (1500 * 8) = 635 K pps (i.e 1.5 us per packet)
> A ring of 4096 descriptors can take 6 ms for full size packets.
>
> Be careful, optimizing for 64 byte benchmarks can be disaster in real world.
>
Thanks for the info Stephen, however I am not trying to optimize for 64 
byte benchmarks. The work has been initiated by an IO problem and Intel 
NICs. Suricata IDS worker (1 core per queue) received a burst of packets 
and then sequentially processes them one by one. Well it seems like 
having a 4k buffers it seems to not be enough. NVIDIA NICs allow e.g. 
32k descriptors and it works fine. In the end it worked fine when ixgbe 
descriptors were increased as well. I am not sure why AF-Packet can 
handle this much better than DPDK, AFP doesn't have crazy high number of 
descriptors configured <= 4096, yet it works better. At the moment I 
assume there is an internal buffering in the kernel which allows to 
handle processing spikes.

To give more context here is the forum discussion - 
https://forum.suricata.io/t/high-packet-drop-rate-with-dpdk-compared-to-af-packet-in-suricata-7-0-7/4896