On Wed, Mar 25, 2026 at 09:19:21AM -0700, Stephen Hemminger wrote: > On Wed, 25 Mar 2026 10:36:56 +0100 > Morten Brørup <[email protected]> wrote: > > > If an application clones packets instead of copying them, it is probably > > for performance reasons. > > If the drivers start copying those clones, it may defeat the performance > > purpose. > > > > <brainstorming> > > Maybe segmentation can be used instead of copying the full packet: > > Make the "copy" packet of two (or more) segments, where the header is > > copied into a new mbuf (where the VLAN tag is added), and the remaining > > part of the packet uses an indirect mbuf referring to the "original" packet > > at the offset after the header. > > </brainstorming> > > > > Furthermore... > > If drivers start copying packets in the Tx function, the Tx queue should > > have its own mbuf pool to allocate these mbufs from. > > Drivers should not steal mbufs from the pools used by the packets being > > transmitted. > > E.g. if a segmented packet has a small mbuf for the first few bytes, > > followed by a large mbuf (from another pool) for the remaining bytes. > > Or if the "original" mbuf comes from a mempool allocated on different CPU > > socket, the "copy" would too. > > > The problem with the Tx function is how backpressure gets handled. > Not sure that it is documented well enough that if a packet is not sent > due to backpressure, the mbuf in the array may still have been replaced.
Most drivers should be able to check for space in a Tx ring, or whatever other backpressure mechanism is being used, before modifying a buffer. /Bruce
