[dpdk-dev] i40e xmit path HW limitation

Zhang, Helin helin.zhang at intel.com
Thu Jul 30 19:33:59 CEST 2015



> -----Original Message-----
> From: Vlad Zolotarov [mailto:vladz at cloudius-systems.com]
> Sent: Thursday, July 30, 2015 9:44 AM
> To: Zhang, Helin; Ananyev, Konstantin
> Cc: dev at dpdk.org
> Subject: Re: i40e xmit path HW limitation
> 
> 
> 
> On 07/30/15 19:10, Zhang, Helin wrote:
> >
> >> -----Original Message-----
> >> From: Vlad Zolotarov [mailto:vladz at cloudius-systems.com]
> >> Sent: Thursday, July 30, 2015 7:58 AM
> >> To: dev at dpdk.org; Ananyev, Konstantin; Zhang, Helin
> >> Subject: RFC: i40e xmit path HW limitation
> >>
> >> Hi, Konstantin, Helin,
> >> there is a documented limitation of xl710 controllers (i40e driver)
> >> which is not handled in any way by a DPDK driver.
> >>   From the datasheet chapter 8.4.1:
> >>
> >> "• A single transmit packet may span up to 8 buffers (up to 8 data
> >> descriptors per packet including both the header and payload buffers).
> >> • The total number of data descriptors for the whole TSO (explained
> >> later on in this chapter) is unlimited as long as each segment within
> >> the TSO obeys the previous rule (up to 8 data descriptors per segment
> >> for both the TSO header and the segment payload buffers)."
> > Yes, I remember the RX side just supports 5 segments per packet receiving.
> > But what's the possible issue you thought about?
> Note that it's a Tx size we are talking about.
> 
> See 30520831f058cd9d75c0f6b360bc5c5ae49b5f27 commit in linux net-next repo.
> If such a cluster arrives and you post it on the HW ring - HW will shut this HW ring
> down permanently. The application will see that it's ring is stuck.
That issue was because of using more than 8 descriptors for a packet for TSO.

> 
> >
> >> This means that, for instance, long cluster with small fragments has to be
> >> linearized before it may be placed on the HW ring.
> > What type of size of the small fragments? Basically 2KB is the default size of
> mbuf of most
> > example applications. 2KB x 8 is bigger than 1.5KB. So it is enough for the
> maximum
> > packet size we supported.
> > If 1KB mbuf is used, don't expect it can transmit more than 8KB size of packet.
> 
> I kinda lost u here. Again, we talk about the Tx side here and buffers
> are not obligatory completely filled. Namely there may be a cluster with
> 15 fragments 100 bytes each.
The root cause is using more than 8 descriptors for a packet. Linux driver can help
on reducing number of descriptors to be used by merging small size of payload
together, right?
It is not for TSO, it is just for packet transmitting. 2 options in my mind:
1. Use should ensure it will not use more than 8 descriptors per packet for transmitting.
2. DPDK driver should try to merge small packet together for such case, like Linux kernel driver.
I prefer to use option 1, users should ensure that in the application or up layer software,
and keep the PMD driver as simple as possible.

But I have a thought that the maximum number of RX/TX descriptor should be able to be
queried somewhere.

Regards,
Helin
> 
> >
> >> In more standard environments like Linux or FreeBSD drivers the solution is
> >> straight forward - call skb_linearize()/m_collapse() corresponding.
> >> In the non-conformist environment like DPDK life is not that easy - there is no
> >> easy way to collapse the cluster into a linear buffer from inside the device
> driver
> >> since device driver doesn't allocate memory in a fast path and utilizes the user
> >> allocated pools only.
> >> Here are two proposals for a solution:
> >>
> >>   1. We may provide a callback that would return a user TRUE if a give
> >>      cluster has to be linearized and it should always be called before
> >>      rte_eth_tx_burst(). Alternatively it may be called from inside the
> >>      rte_eth_tx_burst() and rte_eth_tx_burst() is changed to return some
> >>      error code for a case when one of the clusters it's given has to be
> >>      linearized.
> >>   2. Another option is to allocate a mempool in the driver with the
> >>      elements consuming a single page each (standard 2KB buffers would
> >>      do). Number of elements in the pool should be as Tx ring length
> >>      multiplied by "64KB/(linear data length of the buffer in the pool
> >>      above)". Here I use 64KB as a maximum packet length and not taking
> >>      into an account esoteric things like "Giant" TSO mentioned in the
> >>      spec above. Then we may actually go and linearize the cluster if
> >>      needed on top of the buffers from the pool above, post the buffer
> >>      from the mempool above on the HW ring, link the original cluster to
> >>      that new cluster (using the private data) and release it when the
> >>      send is done.
> >>
> >>
> >> The first is a change in the API and would require from the application some
> >> additional handling (linearization). The second would require some additional
> >> memory but would keep all dirty details inside the driver and would leave the
> >> rest of the code intact.
> >>
> >> Pls., comment.
> >>
> >> thanks,
> >> vlad
> >>



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