On Wed, May 18, 2016 at 07:27:43PM +0530, Jerin Jacob wrote: > To avoid multiple stores on fast path, Ethernet drivers > aggregate the writes to data_off, refcnt, nb_segs and port > to an uint64_t data and write the data in one shot > with uint64_t* at &mbuf->rearm_data address. > > Some of the non-IA platforms have store operation overhead > if the store address is not naturally aligned.This patch > fixes the performance issue on those targets. > > Signed-off-by: Jerin Jacob <jerin.jacob at caviumnetworks.com> > --- > > Tested this patch on IA and non-IA(ThunderX) platforms. > This patch shows 400Kpps/core improvement on ThunderX + ixgbe + vector > environment. > and this patch does not have any overhead on IA platform. > > Have tried an another similar approach by replacing "buf_len" with "pad" > (in this patch context), > Since it has additional overhead on read and then mask to keep "buf_len" > intact, > not much improvement is not shown. > ref: http://dpdk.org/ml/archives/dev/2016-May/038914.html > > --- While this will work and from your tests doesn't seem to have a performance impact, I'm not sure I particularly like it. It's extending out the end of cacheline0 of the mbuf by 16 bytes, though I suppose it's not technically using up any more space of it.
What I'm wondering about though, is do we have any usecases where we need a variable buf_len for packets for RX. These mbufs come directly from a mempool, which is generally understood to be a set of fixed-sized buffers. I realise that this change was made in the past after some discussion, but one of the key points there [at least to my reading] was that - even though nobody actually made a concrete case where they had variable-sized buffers - having support for them made no performance difference. The latter part of that has now changed, and supporting variable-sized mbufs from an mbuf pool has a perf impact. Do we definitely need that functionality, because the easiest fix here is just to move the rxrearm marker back above mbuf_len as it was originally in releases like 1.8? Regards, /Bruce Ref: http://dpdk.org/ml/archives/dev/2014-December/009432.html