Re: [PATCH v1] ixgbe_pmd: forbid tx_rs_thresh above 1 for all NICs but 82598

[Avi Kivity <avi@cloudius-systems.com>]

On 08/25/2015 10:16 PM, Zhang, Helin wrote:
>
>> -----Original Message-----
>> From: Vlad Zolotarov [mailto:vladz@cloudius-systems.com]
>> Sent: Tuesday, August 25, 2015 11:53 AM
>> To: Zhang, Helin
>> Cc: Lu, Wenzhuo; dev@dpdk.org
>> Subject: Re: [dpdk-dev] [PATCH v1] ixgbe_pmd: forbid tx_rs_thresh above 1 for
>> all NICs but 82598
>>
>>
>>
>> On 08/25/15 21:43, Zhang, Helin wrote:
>>> Hi Vlad
>>>
>>> I think this could possibly be the root cause of your TX hang issue.
>>> Please try to limit the number to 8 or less, and then see if the issue
>>> will still be there or not?
>>>
>> Helin, the issue has been seen on x540 devices. Pls., see a chapter
>> 7.2.1.1 of x540 devices spec:
>>
>> A packet (or multiple packets in transmit segmentation) can span any number of
>> buffers (and their descriptors) up to a limit of 40 minus WTHRESH minus 2 (see
>> Section 7.2.3.3 for Tx Ring details and section Section 7.2.3.5.1 for WTHRESH
>> details). For best performance it is recommended to minimize the number of
>> buffers as possible.
>>
>> Could u, pls., clarify why do u think that the maximum number of data buffers is
>> limited by 8?
> OK, i40e hardware is 8, so I'd assume x540 could have a similar one. Yes, in your case,
> the limit could be around 38, right?
> Could you help to make sure there is no packet to be transmitted uses more than
> 38 descriptors?
> I heard that there is a similar hang issue on X710 if using more than 8 descriptors for
> a single packet. I am wondering if the issue is similar on x540.
>
>

I believe that the ixgbe Linux driver does not limit packets to 8 
fragments, so apparently the hardware is capable.