igb: Improve tiny packets reception performance on low frequency CPU

Update RDT register a little bit often, so the RX descriptors are made
to the NIC chip on a more regularly base:
The RDT register is updated after certain amount of RX descriptors are
added to the hardware RX ring.  The default value of the amount of RX
descriptors are 32.  This value could be further tuned by per-device
sysctl node hw.igbX.rxY_wreg.

The default value improves tiny packets reception performance w/ 82576
on AMD970@800Mhz under interrupt mode for single stream (1.28Mpps ->
1.48Mpps) and it does not increase CPU usage on AMD970@3500Mhz (CPU
usage stays @36%).

This commit does _not_ seem to affect the tiny packet reception
performance when the workload are evenly distributed to all CPUs.

diff --git a/sys/dev/netif/igb/if_igb.c b/sys/dev/netif/igb/if_igb.c
index 844494c..09a1c1d 100644 (file)
--- a/sys/dev/netif/igb/if_igb.c
+++ b/sys/dev/netif/igb/if_igb.c
@@ -173,6 +173,7 @@ static void igb_init_tx_ring(struct igb_tx_ring *);
 static int     igb_init_rx_ring(struct igb_rx_ring *);
 static int     igb_newbuf(struct igb_rx_ring *, int, boolean_t);
 static int     igb_encap(struct igb_tx_ring *, struct mbuf **, int *, int *);
+static void    igb_rx_refresh(struct igb_rx_ring *, int);
 
 static void    igb_stop(struct igb_softc *);
 static void    igb_init(void *);
@@ -1542,13 +1543,20 @@ igb_add_sysctl(struct igb_softc *sc)
        SYSCTL_ADD_INT(&sc->sysctl_ctx, SYSCTL_CHILDREN(sc->sysctl_tree),
            OID_AUTO, "rss_debug", CTLFLAG_RW, &sc->rss_debug, 0,
            "RSS debug level");
+#endif
        for (i = 0; i < sc->rx_ring_cnt; ++i) {
+#ifdef IGB_RSS_DEBUG
                ksnprintf(node, sizeof(node), "rx%d_pkt", i);
                SYSCTL_ADD_ULONG(&sc->sysctl_ctx,
                    SYSCTL_CHILDREN(sc->sysctl_tree), OID_AUTO, node,
                    CTLFLAG_RW, &sc->rx_rings[i].rx_packets, "RXed packets");
-       }
 #endif
+               ksnprintf(node, sizeof(node), "rx%d_wreg", i);
+               SYSCTL_ADD_INT(&sc->sysctl_ctx,
+                   SYSCTL_CHILDREN(sc->sysctl_tree), OID_AUTO, node,
+                   CTLFLAG_RW, &sc->rx_rings[i].rx_wreg, 0,
+                   "# of segments before write to hardare register");
+       }
 }
 
 static int
@@ -2084,6 +2092,12 @@ igb_create_rx_ring(struct igb_rx_ring *rxr)
                        return error;
                }
        }
+
+       /*
+        * Initialize various watermark
+        */
+       rxr->rx_wreg = 32;
+
        return 0;
 }
 
@@ -2414,12 +2428,20 @@ igb_init_rx_unit(struct igb_softc *sc)
 }
 
 static void
+igb_rx_refresh(struct igb_rx_ring *rxr, int i)
+{
+       if (--i < 0)
+               i = rxr->num_rx_desc - 1;
+       E1000_WRITE_REG(&rxr->sc->hw, E1000_RDT(rxr->me), i);
+}
+
+static void
 igb_rxeof(struct igb_rx_ring *rxr, int count)
 {
        struct ifnet *ifp = &rxr->sc->arpcom.ac_if;
        union e1000_adv_rx_desc *cur;
        uint32_t staterr;
-       int i;
+       int i, ncoll = 0;
 
        i = rxr->next_to_check;
        cur = &rxr->rx_base[i];
@@ -2438,6 +2460,7 @@ igb_rxeof(struct igb_rx_ring *rxr, int count)
                if (eop)
                        --count;
 
+               ++ncoll;
                if ((staterr & E1000_RXDEXT_ERR_FRAME_ERR_MASK) == 0 &&
                    !rxr->discard) {
                        struct mbuf *mp = rxbuf->m_head;
@@ -2526,14 +2549,18 @@ discard:
                if (++i == rxr->num_rx_desc)
                        i = 0;
 
+               if (ncoll > rxr->rx_wreg) {
+                       igb_rx_refresh(rxr, i);
+                       ncoll = 0;
+               }
+
                cur = &rxr->rx_base[i];
                staterr = le32toh(cur->wb.upper.status_error);
        }
        rxr->next_to_check = i;
 
-       if (--i < 0)
-               i = rxr->num_rx_desc - 1;
-       E1000_WRITE_REG(&rxr->sc->hw, E1000_RDT(rxr->me), i);
+       if (ncoll > 0)
+               igb_rx_refresh(rxr, i);
 }
 
 

diff --git a/sys/dev/netif/igb/if_igb.h b/sys/dev/netif/igb/if_igb.h
index dd62415..792a413 100644 (file)
--- a/sys/dev/netif/igb/if_igb.h
+++ b/sys/dev/netif/igb/if_igb.h
@@ -265,6 +265,7 @@ struct igb_rx_ring {
         */
        struct mbuf             *fmp;
        struct mbuf             *lmp;
+       int                     rx_wreg;
 
        /* Soft stats */
        u_long                  rx_packets;