[ovs-dev] [PATCH] afxdp: Reduce afxdp's batch size to match kernel's xdp batch size

[Yifeng Sun]

William reported that there is iperf TCP issue between two afxdp ports:

[  3] local 10.1.1.2 port 40384 connected with 10.1.1.1 port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0- 1.0 sec  17.0 MBytes   143 Mbits/sec
[  3]  1.0- 2.0 sec  9.62 MBytes  80.7 Mbits/sec
[  3]  2.0- 3.0 sec  6.75 MBytes  56.6 Mbits/sec
[  3]  3.0- 4.0 sec  11.0 MBytes  92.3 Mbits/sec
[  3]  5.0- 6.0 sec  0.00 Bytes  0.00 bits/sec
[  3]  6.0- 7.0 sec  0.00 Bytes  0.00 bits/sec
[  3]  7.0- 8.0 sec  0.00 Bytes  0.00 bits/sec
[  3]  8.0- 9.0 sec  0.00 Bytes  0.00 bits/sec
[  3]  9.0-10.0 sec  0.00 Bytes  0.00 bits/sec
[  3] 10.0-11.0 sec  0.00 Bytes  0.00 bits/sec

The reason is, currently, netdev-afxdp's batch size is 32 while kernel's
xdp batch size is only 16. This can result in exhausting of sock wmem if
netdev-afxdp keeps sending large number of packets. Later on, when ARP
expires at one side of TCP connection, ARP packets can be delayed or
even dropped because sock wmen is already full.

This patch fixes this issue by reducing netdev-afxdp's batch size so
as to match kernel's xdp batch size. Now iperf TCP works correctly.

[  3] local 10.1.1.2 port 57770 connected with 10.1.1.1 port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0- 1.0 sec   262 MBytes  2.20 Gbits/sec
[  3]  1.0- 2.0 sec   299 MBytes  2.51 Gbits/sec
[  3]  2.0- 3.0 sec   271 MBytes  2.27 Gbits/sec
[  3]  3.0- 4.0 sec   247 MBytes  2.07 Gbits/sec
[  3]  4.0- 5.0 sec   290 MBytes  2.43 Gbits/sec
[  3]  5.0- 6.0 sec   292 MBytes  2.45 Gbits/sec
[  3]  6.0- 7.0 sec   223 MBytes  1.87 Gbits/sec
[  3]  7.0- 8.0 sec   243 MBytes  2.04 Gbits/sec
[  3]  8.0- 9.0 sec   234 MBytes  1.97 Gbits/sec
[  3]  9.0-10.0 sec   238 MBytes  2.00 Gbits/sec

Reported-by: William Tu <u9012063 at gmail.com>
Reported-at: https://mail.openvswitch.org/pipermail/ovs-dev/2019-November/365076.html
Signed-off-by: Yifeng Sun <pkusunyifeng at gmail.com>
---
 lib/netdev-afxdp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/netdev-afxdp.c b/lib/netdev-afxdp.c
index 58365ed483e3..38bbbeb055cc 100644
--- a/lib/netdev-afxdp.c
+++ b/lib/netdev-afxdp.c
@@ -82,7 +82,7 @@ static struct vlog_rate_limit rl = VLOG_RATE_LIMIT_INIT(5, 20);
  * enough for most corner cases.
  */
 #define NUM_FRAMES          (4 * (PROD_NUM_DESCS + CONS_NUM_DESCS))
-#define BATCH_SIZE          NETDEV_MAX_BURST
+#define BATCH_SIZE          16
 
 BUILD_ASSERT_DECL(IS_POW2(NUM_FRAMES));
 BUILD_ASSERT_DECL(PROD_NUM_DESCS == CONS_NUM_DESCS);
-- 
2.7.4