[ovs-dev] [PATCH v5 01/14] netdev-dpdk: fix mbuf sizing

Ian Stokes ian.stokes at intel.com
Thu Jul 12 16:07:41 UTC 2018

On 7/12/2018 4:40 PM, Lam, Tiago wrote:
> On 12/07/2018 14:37, Ian Stokes wrote:
>> On 7/11/2018 7:23 PM, Tiago Lam wrote:
>>> From: Mark Kavanagh <mark.b.kavanagh at intel.com>
>>> There are numerous factors that must be considered when calculating
>>> the size of an mbuf:
>>> - the data portion of the mbuf must be sized in accordance With Rx
>>>     buffer alignment (typically 1024B). So, for example, in order to
>>>     successfully receive and capture a 1500B packet, mbufs with a
>>>     data portion of size 2048B must be used.
>>> - in OvS, the elements that comprise an mbuf are:
>>>     * the dp packet, which includes a struct rte mbuf (704B)
>>>     * packet data (aligned to 1k, as previously described)
>>>     * RTE_PKTMBUF_TAILROOM (typically 0)
>>> Some PMDs require that the total mbuf size (i.e. the total sum of all
>>> of the above-listed components' lengths) is cache-aligned. To satisfy
>>> this requirement, it may be necessary to round up the total mbuf size
>>> with respect to cacheline size. In doing so, it's possible that the
>>> dp_packet's data portion is inadvertently increased in size, such that
>>> it no longer adheres to Rx buffer alignment. Consequently, the
>>> following property of the mbuf no longer holds true:
>>>       mbuf.data_len == mbuf.buf_len - mbuf.data_off
>>> This creates a problem in the case of multi-segment mbufs, where that
>>> assumption is assumed to be true for all but the final segment in an
>>> mbuf chain. Resolve this issue by adjusting the size of the mbuf's
>>> private data portion, as opposed to the packet data portion when
>>> aligning mbuf size to cachelines.
>> Hi Tiago,
>> with this patch I still don't see mbuf.data_len == mbuf.buf_len -
>> mbuf.data_off to be true.
>> I've tested with both Jumbo frames and non jumbo packets by examining
>> the mbufs on both tx and rx. mbuf.data_len is always smaller than
>> mbuf.buf_len - mbuf.data_off.
>> Maybe I've missed something here?
> Thanks for looking into this, Ian.
> `mbuf.data_len == mbuf.buf_len - mbuf.data_off` isn't always true.
> Actually, `mbuf.data_len <= mbuf.buf_len - mbuf.data_off` would be a
> better representation.
> If there's a chain of mbufs that are linked together, the expectation is
> that `mbuf.data_len == mbuf.buf_len - mbuf.data_off` holds true for all
> of them, except maybe for the last in the chain, since there may not be
> enough data to fill the whole mbuf.
> So, for non jumbo frames I would expect `data_len < mbuf_len -
> data_off`, but for jumbo frames I'd expect that to happen only on the
> last mbuf in the chain, and in the rest we should see `data_len ==
> mbuf_len - data_off` hold true. Is that what you're seeing here?

I had tested this with jumbo frames (MTU 9000) with just this patch 
applied, in that case I was seeing 'data_len < mbuf_len - data_off'.

Is this because its 1 mbuf for the entire 9000, if it was segmented then 
the segments would be equal besides the final mbuf.


> Regards,
> Tiago.

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