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

Lam, Tiago tiago.lam at intel.com
Thu Jul 12 15:40:59 UTC 2018

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?


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