On Tue, Aug 30, 2016 at 09:58 +1000, David Gwynne wrote:
> On Mon, Aug 29, 2016 at 08:30:37PM +0200, Alexander Bluhm wrote:
> > On Mon, Aug 29, 2016 at 07:10:48PM +0200, Mike Belopuhov wrote:
> > > Due to a recent change in -current the socket sending routine
> > > has started producing small data packets crossing memory page
> > > boundary. This is not supported by Xen and kernels with this
> > > change will experience broken bulk TCP transmit behaviour.
> > > We're working on fixing it.
> >
> > For the same reason some old i386 machines from 2006 and 2005 have
> > performance problems when sending data with tcpbench.
> >
> > em 82573E drops to 200 MBit/sec output, 82546GB and 82540EM do only
> > 10 MBit anymore.
> >
> > With the patch below I get 946, 642, 422 MBit/sec output performance
> > over these chips respectively.
> >
> > Don't know wether PAGE_SIZE is the correct fix as I think the problem
> > is more related to the network chip than to the processor's page
> > size.
>
> does this diff help those chips?
>
This diff defeats the purpose of the sosend change by punishing
every other chip not suffering from the aforementioned problem.
Lots of packets from the bulk TCP transfer will have to be
defragmented for no good reason.
> Index: if_em.c
> ===================================================================
> RCS file: /cvs/src/sys/dev/pci/if_em.c,v
> retrieving revision 1.331
> diff -u -p -r1.331 if_em.c
> --- if_em.c 13 Apr 2016 10:34:32 -0000 1.331
> +++ if_em.c 29 Aug 2016 23:52:07 -0000
> @@ -2134,7 +2134,7 @@ em_setup_transmit_structures(struct em_s
> pkt = &sc->sc_tx_pkts_ring[i];
> error = bus_dmamap_create(sc->sc_dmat, MAX_JUMBO_FRAME_SIZE,
> EM_MAX_SCATTER / (sc->pcix_82544 ? 2 : 1),
> - MAX_JUMBO_FRAME_SIZE, 0, BUS_DMA_NOWAIT, &pkt->pkt_map);
> + MAX_JUMBO_FRAME_SIZE, 4096, BUS_DMA_NOWAIT, &pkt->pkt_map);
> if (error != 0) {
> printf("%s: Unable to create TX DMA map\n",
> DEVNAME(sc));
