bge: Improve tiny packets transmission performance on low frequency CPU

Update TX HOST_PROD register a little bit often; mainly to improve
timeliness of packets transmission:
The TX HOST_PROD register is updated after certain amount of TX
descriptors are added to the hardware TX ring.  The default value of the
amount of TX descriptors are 16.  This value could be further tuned by
per-device sysctl node hw.bgeX.tx_wreg.

The default value improves tiny packets transmission performance w/ 5721
on AMD970@800Mhz (770Kpps -> 800Kpps) and it does not increase CPU usage
on AMD970@3500Mhz (CPU usage stays @20%, 5721 could only do 810Kpps).

NOTE: Tuning hw.bgeX.tx_coal_bds to 32 could make packets transmission
performance w/ 5721 on AMD970@800Mhz to be 810Kpps, however, this shows
no performance improvement on commonly used CPU frequency like 2200Mhz
and 3500Mhz.  And this tuning could burden systems not using polling(4).

diff --git a/sys/dev/netif/bge/if_bge.c b/sys/dev/netif/bge/if_bge.c
index 5df80d2..bd357a9 100644 (file)
--- a/sys/dev/netif/bge/if_bge.c
+++ b/sys/dev/netif/bge/if_bge.c
@@ -303,7 +303,9 @@ static void bge_stats_update(struct bge_softc *);
 static void    bge_stats_update_regs(struct bge_softc *);
 static struct mbuf *
                bge_defrag_shortdma(struct mbuf *);
-static int     bge_encap(struct bge_softc *, struct mbuf **, uint32_t *);
+static int     bge_encap(struct bge_softc *, struct mbuf **,
+                   uint32_t *, int *);
+static void    bge_xmit(struct bge_softc *, uint32_t);
 static int     bge_setup_tso(struct bge_softc *, struct mbuf **,
                    uint16_t *, uint16_t *);
 
@@ -2321,6 +2323,7 @@ bge_attach(device_t dev)
                sc->bge_rx_coal_bds_int = BGE_RX_COAL_BDS_MIN;
                sc->bge_tx_coal_bds_int = BGE_TX_COAL_BDS_MIN;
        }
+       sc->bge_tx_wreg = 16;
 
        /* Set up TX spare and reserved descriptor count */
        if (sc->bge_flags & BGE_FLAG_TSO) {
@@ -2491,6 +2494,13 @@ bge_attach(device_t dev)
                        CTLTYPE_INT | CTLFLAG_RW,
                        sc, 0, bge_sysctl_tx_coal_bds, "I",
                        "Transmit max coalesced BD count.");
+
+       SYSCTL_ADD_INT(&sc->bge_sysctl_ctx,
+                      SYSCTL_CHILDREN(sc->bge_sysctl_tree),
+                      OID_AUTO, "tx_wreg", CTLFLAG_RW,
+                      &sc->bge_tx_wreg, 0,
+                      "# of segments before writing to hardware register");
+
        if (sc->bge_flags & BGE_FLAG_PCIE) {
                /*
                 * A common design characteristic for many Broadcom
@@ -3343,7 +3353,8 @@ bge_stats_update(struct bge_softc *sc)
  * pointers to descriptors.
  */
 static int
-bge_encap(struct bge_softc *sc, struct mbuf **m_head0, uint32_t *txidx)
+bge_encap(struct bge_softc *sc, struct mbuf **m_head0, uint32_t *txidx,
+    int *segs_used)
 {
        struct bge_tx_bd *d = NULL, *last_d;
        uint16_t csum_flags = 0, mss = 0;
@@ -3420,6 +3431,7 @@ bge_encap(struct bge_softc *sc, struct mbuf **m_head0, uint32_t *txidx)
                        m_head0, segs, maxsegs, &nsegs, BUS_DMA_NOWAIT);
        if (error)
                goto back;
+       *segs_used += nsegs;
 
        m_head = *m_head0;
        bus_dmamap_sync(sc->bge_cdata.bge_tx_mtag, map, BUS_DMASYNC_PREWRITE);
@@ -3470,6 +3482,16 @@ back:
        return error;
 }
 
+static void
+bge_xmit(struct bge_softc *sc, uint32_t prodidx)
+{
+       /* Transmit */
+       bge_writembx(sc, BGE_MBX_TX_HOST_PROD0_LO, prodidx);
+       /* 5700 b2 errata */
+       if (sc->bge_chiprev == BGE_CHIPREV_5700_BX)
+               bge_writembx(sc, BGE_MBX_TX_HOST_PROD0_LO, prodidx);
+}
+
 /*
  * Main transmit routine. To avoid having to do mbuf copies, we put pointers
  * to the mbuf data regions directly in the transmit descriptors.
@@ -3480,14 +3502,13 @@ bge_start(struct ifnet *ifp)
        struct bge_softc *sc = ifp->if_softc;
        struct mbuf *m_head = NULL;
        uint32_t prodidx;
-       int need_trans;
+       int nsegs = 0;
 
        if ((ifp->if_flags & (IFF_RUNNING | IFF_OACTIVE)) != IFF_RUNNING)
                return;
 
        prodidx = sc->bge_tx_prodidx;
 
-       need_trans = 0;
        while (sc->bge_cdata.bge_tx_chain[prodidx] == NULL) {
                m_head = ifq_dequeue(&ifp->if_snd, NULL);
                if (m_head == NULL)
@@ -3534,31 +3555,28 @@ bge_start(struct ifnet *ifp)
                 * don't have room, set the OACTIVE flag and wait
                 * for the NIC to drain the ring.
                 */
-               if (bge_encap(sc, &m_head, &prodidx)) {
+               if (bge_encap(sc, &m_head, &prodidx, &nsegs)) {
                        ifp->if_flags |= IFF_OACTIVE;
                        ifp->if_oerrors++;
                        break;
                }
-               need_trans = 1;
 
-               ETHER_BPF_MTAP(ifp, m_head);
-       }
+               if (nsegs >= sc->bge_tx_wreg) {
+                       bge_xmit(sc, prodidx);
+                       nsegs = 0;
+               }
 
-       if (!need_trans)
-               return;
+               ETHER_BPF_MTAP(ifp, m_head);
 
-       /* Transmit */
-       bge_writembx(sc, BGE_MBX_TX_HOST_PROD0_LO, prodidx);
-       /* 5700 b2 errata */
-       if (sc->bge_chiprev == BGE_CHIPREV_5700_BX)
-               bge_writembx(sc, BGE_MBX_TX_HOST_PROD0_LO, prodidx);
+               /*
+                * Set a timeout in case the chip goes out to lunch.
+                */
+               ifp->if_timer = 5;
+       }
 
+       if (nsegs > 0)
+               bge_xmit(sc, prodidx);
        sc->bge_tx_prodidx = prodidx;
-
-       /*
-        * Set a timeout in case the chip goes out to lunch.
-        */
-       ifp->if_timer = 5;
 }
 
 static void

diff --git a/sys/dev/netif/bge/if_bgevar.h b/sys/dev/netif/bge/if_bgevar.h
index a676b6f..a1a6f44 100644 (file)
--- a/sys/dev/netif/bge/if_bgevar.h
+++ b/sys/dev/netif/bge/if_bgevar.h
@@ -252,6 +252,7 @@ struct bge_softc {
        uint32_t                bge_rx_coal_bds_int;
        uint32_t                bge_tx_coal_bds_int;
        uint32_t                bge_tx_prodidx;
+       int                     bge_tx_wreg;
        uint32_t                bge_tx_buf_ratio;
        uint32_t                bge_mi_mode;
        int                     bge_force_defrag;