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authorMichal Schmidt <mschmidt@redhat.com>2012-09-09 13:55:26 +0000
committerDavid S. Miller <davem@davemloft.net>2012-09-19 19:19:07 -0400
commitaee77e4accbeb2c86b1d294cd84fec4a12dde3bd (patch)
treeb65e72ce9c5c3735b512b5cb34056cd838f99897 /drivers/net/ethernet/realtek
parent6b102865e7ba9ff1e3c49c32c7187bb427d91798 (diff)
downloadlinux-aee77e4accbeb2c86b1d294cd84fec4a12dde3bd.tar.gz
r8169: use unlimited DMA burst for TX
The r8169 driver currently limits the DMA burst for TX to 1024 bytes. I have a box where this prevents the interface from using the gigabit line to its full potential. This patch solves the problem by setting TX_DMA_BURST to unlimited. The box has an ASRock B75M motherboard with on-board RTL8168evl/8111evl (XID 0c900880). TSO is enabled. I used netperf (TCP_STREAM test) to measure the dependency of TX throughput on MTU. I did it for three different values of TX_DMA_BURST ('5'=512, '6'=1024, '7'=unlimited). This chart shows the results: http://michich.fedorapeople.org/r8169/r8169-effects-of-TX_DMA_BURST.png Interesting points: - With the current DMA burst limit (1024): - at the default MTU=1500 I get only 842 Mbit/s. - when going from small MTU, the performance rises monotonically with increasing MTU only up to a peak at MTU=1076 (908 MBit/s). Then there's a sudden drop to 762 MBit/s from which the throughput rises monotonically again with further MTU increases. - With a smaller DMA burst limit (512): - there's a similar peak at MTU=1076 and another one at MTU=564. - With unlimited DMA burst: - at the default MTU=1500 I get nice 940 Mbit/s. - the throughput rises monotonically with increasing MTU with no strange peaks. Notice that the peaks occur at MTU sizes that are multiples of the DMA burst limit plus 52. Why 52? Because: 20 (IP header) + 20 (TCP header) + 12 (TCP options) = 52 The Realtek-provided r8168 driver (v8.032.00) uses unlimited TX DMA burst too, except for CFG_METHOD_1 where the TX DMA burst is set to 512 bytes. CFG_METHOD_1 appears to be the oldest MAC version of "RTL8168B/8111B", i.e. RTL_GIGA_MAC_VER_11 in r8169. Not sure if this MAC version really needs the smaller burst limit, or if any other versions have similar requirements. Signed-off-by: Michal Schmidt <mschmidt@redhat.com> Acked-by: Francois Romieu <romieu@fr.zoreil.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'drivers/net/ethernet/realtek')
-rw-r--r--drivers/net/ethernet/realtek/r8169.c2
1 files changed, 1 insertions, 1 deletions
diff --git a/drivers/net/ethernet/realtek/r8169.c b/drivers/net/ethernet/realtek/r8169.c
index 0c96604e624..8b4e0a93f45 100644
--- a/drivers/net/ethernet/realtek/r8169.c
+++ b/drivers/net/ethernet/realtek/r8169.c
@@ -77,7 +77,7 @@
static const int multicast_filter_limit = 32;
#define MAX_READ_REQUEST_SHIFT 12
-#define TX_DMA_BURST 6 /* Maximum PCI burst, '6' is 1024 */
+#define TX_DMA_BURST 7 /* Maximum PCI burst, '7' is unlimited */
#define SafeMtu 0x1c20 /* ... actually life sucks beyond ~7k */
#define InterFrameGap 0x03 /* 3 means InterFrameGap = the shortest one */