[dpdk-dev] Weird 2 KB MBUF data room requirement

[Bruce Richardson]

On Fri, Jul 10, 2020 at 10:21:40AM +0200, Morten Brørup wrote:
> Dear Ethernet PMD developers,
> 
> According to rte_mbuf_core.h, RTE_MBUF_DEFAULT_DATAROOM is 2048 bytes because some NICs need at least 2 KB buffer to receive standard Ethernet frames without splitting them into multiple segments.
> 
> This is a serious waste of memory, considering that standard Ethernet frames are max 1518 bytes.
> 
> How wide spread is this limitation... is it common or a rare exception?
> 
> Where is it documented which NICs suffer from this limitation?
> 
> Do any Intel NICs suffer from this limitation?
> 
> 
> NB: We are targeting an MBUF total size (incl. memzone element overhead) of 2^N, and this limitation would increase our MBUF total size to 4 KB.
> 
> 
> Med venlig hilsen / kind regards
> - Morten Brørup
> 

AFAIK: the NICs supported by the ixgbe driver only allow the size to be
specified in KB granularity.

However, it may be safe to have a driver modification whereby anything over
1600 bytes is considered as 2KB if jumbo frame support is disabled. I don't
think anyone has actually looked into doing so though, or if there are
other hidden gotchas about attempting to do so.

/Bruce