On Tue, 2015-11-03 at 10:33 -0500, David Miller wrote: > From: KY Srinivasan <k...@microsoft.com> > Date: Tue, 3 Nov 2015 07:59:36 +0000 > > > I have implemented the scheme we had discussed a few weeks ago. In > > this new implementation our driver is NOT requesting addition > > headroom - rndis header and the per packet state is being maintained > > outside of the skb. What I am seeing is that when I have > > LL_MAX_HEADER set to 220 bytes, even though our driver is not using > > the additional head room, I see about a 10% boost in the peak > > performance (about 34 Gbps on a 40Gbps interface). However, when I > > set the LL_MAX_HEADER value to the current default, the peak > > performance drops back to what we currently have (around 31 > > Gbps). In both these cases, there is no reallocation of skb since no > > additional headroom is being requested and yet there is a > > significant difference in performance. I trying to figure out why > > this is the case, your insights will be greatly appreciated. > > It probably has something to do with cache line or data alignment.
This also might be because of a slight change in skb->truesize, and/or a change of amount of payload in skb->head (Increasing LL_MAX_HEADER is reducing amount of payload in skb->head) Can't you run perf tool to get some precise profiling ? Another red flag in you driver xmit is : return (ret == -EAGAIN) ? NETDEV_TX_BUSY : NETDEV_TX_OK; extract from include/linux/netdevice.h * netdev_tx_t (*ndo_start_xmit)(struct sk_buff *skb, * struct net_device *dev); * Called when a packet needs to be transmitted. * Returns NETDEV_TX_OK. Can return NETDEV_TX_BUSY, but you should stop * the queue before that can happen; it's for obsolete devices and weird * corner cases, but the stack really does a non-trivial amount * of useless work if you return NETDEV_TX_BUSY. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
