What is the role of headroom in DPDK Mbuf？

Question

For a newly allocated mbuf, the area at which the data begins in the message buffer is RTE_PKTMBUF_HEADROOM bytes after the beginning of the buffer, which is cache aligned.

As far as I know, the headroom does not contain any data. So I wonder why there should be a headroom in the mbuf？In addition, the default size of HEADROOM is 128B.I've tried setting the HEADROOM size to something bigger, like 5120B, and it doesn't affect performance. So I want to know what is the purpose of the existence of the Headroom, and is there any theoretical limit to the size of the Headroom?

Answer 1

A DPDK mbuf's headroom is suited for prepending extra data (typically, metadata) to the packet data. Its use cases are PMD-specific. For example, virtio PMD stores struct virtio_net_hdr in it. Other drivers may use this for offload results: checksum validity status, packet classification, hash values, you name it.

As for the upper limit, it depends on the mbuf object's maximum size and on the minimum packet data room, which, in turn, might be PMD-specific. For the majority of use cases, it's OK to just rely on default size.