Re: NetBSD 5.1 TCP performance issue (lots of ACK)

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

On Fri, Oct 28, 2011 at 10:27:56AM -0500, David Young wrote:
> > Here is an updated patch. The key point to avoid the receive errors is
> > to do another BUS_DMASYNC after reading wrx_status, before reading the
> > other values to avoid reading e.g. len before status gets updated.
> > The errors were because of 0-len receive descriptors.
> 
> Good catch!  Question, though:
> 
> > Index: sys/dev/pci/if_wm.c
> > ===================================================================
> > RCS file: /cvsroot/src/sys/dev/pci/if_wm.c,v
> > retrieving revision 1.162.4.15
> > diff -u -p -u -r1.162.4.15 if_wm.c
> > --- sys/dev/pci/if_wm.c     7 Mar 2011 04:14:19 -0000       1.162.4.15
> > +++ sys/dev/pci/if_wm.c     28 Oct 2011 14:03:33 -0000
> > @@ -2879,11 +2907,7 @@ wm_rxintr(struct wm_softc *sc)
> >                 device_xname(sc->sc_dev), i));
> >  
> >             WM_CDRXSYNC(sc, i, BUS_DMASYNC_POSTREAD|BUS_DMASYNC_POSTWRITE);
> > -
> >             status = sc->sc_rxdescs[i].wrx_status;
> > -           errors = sc->sc_rxdescs[i].wrx_errors;
> > -           len = le16toh(sc->sc_rxdescs[i].wrx_len);
> > -           vlantag = sc->sc_rxdescs[i].wrx_special;
> >  
> >             if ((status & WRX_ST_DD) == 0) {
> >                     /*
> > @@ -2892,6 +2916,14 @@ wm_rxintr(struct wm_softc *sc)
> >                     WM_CDRXSYNC(sc, i, BUS_DMASYNC_PREREAD);
> >                     break;
> >             }
> 
> Should
> 
> >                     WM_CDRXSYNC(sc, i, BUS_DMASYNC_PREREAD);
> 
> move above 
> 
> >             if ((status & WRX_ST_DD) == 0) {
> 
> ?

I don't think so: if WRX_ST_DD is not set, we won't read anything more frm
this descriptor so there's no need to sync it again.

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--