Re: [dpdk-dev] [PATCH 1/3] net/af_xdp: Increase max batch size to 512

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 3/3/2021 3:07 PM, Loftus, Ciara wrote:
>>
>> On 2/24/2021 11:18 AM, Ciara Loftus wrote:
>>> Prior to this the max size was 32 which was unnecessarily
>>> small.
>>
>> Can you please describe the impact? Why changed from 32 to 512?
>> I assume this is to improve the performance but can you please explicitly
>> document it in the commit log?
> 
> Indeed - improved performance due to bulk operations and fewer ring accesses and syscalls.
> The value 512 was arbitrary. I will change this to the default ring size as defined by libbpf (2048) in v2.
> Will update the commit log with this info.
> 
>>
>>> Also enforce the max batch size for TX for both
>>> copy and zero copy modes. Prior to this only copy mode
>>> enforced the max size.
>>>
>>
>> By enforcing, the PMD ignores the user provided burst value if it is more than
>> PMS supported MAX, and this ignoring is done in silent. Also there is no way
>> to
>> discover this MAX value without checking the code.
>>
>> Overall, why this max values are required at all? After quick check I can see
>> they are used for some bulk operations, which I assume can be eliminated,
>> what
>> do you think?
> 
> We need to size some arrays at compile time with this max value.
> 
> Instead of removing the bulk operations which may impact performance, how about taking an approach where we split batches that are > 2048 into smaller batches and still handle all the packets instead of discarding those > 2048. Something like what's done in ixgbe for example:
> http://code.dpdk.org/dpdk/v21.02/source/drivers/net/ixgbe/ixgbe_rxtx.c#L318
>` 

If there is no reasonable way to eliminate the fix sized arrays, above 
suggestion looks good.

Thanks,
ferruh