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Date: Thu, 19 Feb 2009 23:43:19 +0100 From: Lennert Buytenhek <buytenh@...tstofly.org> To: David Miller <davem@...emloft.net> Cc: jeff@...zik.org, netdev@...r.kernel.org, nico@....org Subject: Re: [PATCH 0/6] mv643xx_eth updates On Sun, Feb 15, 2009 at 11:46:30PM -0800, David Miller wrote: > > Two things here: > > - Exporting some functionality that mostly already existed in the > > driver (interrupt coalescing, etc) to userland via ethtool. > > - Add Kconfig-configurable LRO support. > > Applied to net-next-2.6, but I think making LRO support > Kconfig'able is a terrible precedent to set. > > We have an ethtool knob, people can turn it off. > > And furthermore, if it is worse in some cases that is > a bug that we should fix rather than ignore while hiding > behind some Kconfig option. > > Please reconsider... Hi David, Sorry for being slow with responding to this. I tried a routing test on an ARM board that uses mv643xx_eth on an almost vanilla (some arch/arm changes) 2.6.29-rc3 for each of these three cases: 1. LRO disabled in Kconfig 2. LRO enabled in Kconfig, !NETIF_F_LRO 3. LRO enabled in Kconfig, NETIF_F_LRO I'm injecting 300kpps on one interface (lan1), and routing the packets to another interface (lan2). lan[1234] are virtual (DSA) netdevs that are tagged over eth0. I'm reading sw (linux netdev) and hw (switch chip / CPU ethernet MAC) rx and tx counters on the eth0 and lan[1234] inter- faces every second and reporting them with a vmstat-like program. Columns are: - lan1 hardware rx counter - eth0 hardware rx counter - [...] - eth interrupts/sec (0 because we stay in polling mode) - # of elapsed msec since the last line LRO disabled: -l1hwrx -e0hwrx -e0swrx -l1swrx -l2swtx -e0swtx -e0hwtx -l2hwtx | -ethint ---ms 301753 300129 230784 232064 233344 230784 230784 233341 | 0 1000 298476 300102 231040 229760 228480 231040 231040 228476 | 0 1000 300069 300070 230784 230784 232064 230784 230784 232069 | 0 1000 300108 300113 231168 231168 231168 231168 231168 231164 | 0 1000 300016 300014 229888 229888 229760 229888 229888 229765 | 0 1000 300056 300058 231040 231040 229888 231040 231040 229887 | 0 1000 300161 300153 230912 230912 232192 230912 230912 232196 | 0 1000 300082 300082 231168 231168 229888 231168 231168 229885 | 0 1000 300069 300075 230912 230912 232064 230912 230912 232057 | 0 1000 300108 300107 231040 231040 232192 231040 231040 232197 | 0 1000 300134 300137 230912 230912 228608 230912 230912 228603 | 0 1000 300056 300057 231040 231040 231040 231040 231040 231048 | 0 1000 300056 300045 230784 230784 231936 230784 230784 231927 | 0 1000 LRO enabled, !NETIF_F_LRO: -l1hwrx -e0hwrx -e0swrx -l1swrx -l2swtx -e0swtx -e0hwtx -l2hwtx | -ethint ---ms 300135 300147 221696 221696 221696 221696 221696 221696 | 0 1000 300134 300131 221568 221568 221568 221568 221568 221568 | 0 1000 300121 300116 221568 221568 221568 221568 221568 221568 | 0 1000 300069 300077 221568 221568 222720 221568 221568 222720 | 0 1000 300160 300157 221696 221696 223104 221696 221696 223104 | 0 1000 300148 300146 221696 221696 220416 221696 221696 220416 | 0 1000 300043 300037 221568 221568 220288 221568 221568 220288 | 0 1000 300042 300045 221568 221568 222720 221568 221568 222720 | 0 1000 300135 300134 221696 221696 221824 221696 221696 221824 | 0 1000 300173 300180 221696 221696 220416 221696 221696 220416 | 0 1000 300161 300162 221696 221696 221696 221696 221696 221696 | 0 1000 300043 300034 221568 221568 221568 221568 221568 221568 | 0 1000 300108 300112 221568 221568 221568 221568 221568 221568 | 0 1000 300095 300089 221568 221568 221568 221568 221568 221568 | 0 1000 300160 300161 221696 221696 221696 221696 221696 221696 | 0 1000 300161 300173 221696 221696 221696 221696 221696 221696 | 0 1000 300029 300020 221568 221568 221568 221568 221568 221568 | 0 1000 300161 300163 221568 221568 221568 221568 221568 221568 | 0 1000 LRO enabled, NETIF_F_LRO: -l1hwrx -e0hwrx -e0swrx -l1swrx -l2swtx -e0swtx -e0hwtx -l2hwtx | -ethint ---ms 300159 300154 217088 217088 217088 217088 217088 217091 | 0 1000 300135 300141 217216 217216 219520 217216 217216 219510 | 0 1000 300055 300058 216960 216960 214656 216960 216960 214656 | 0 1000 300147 300151 217344 217344 219648 217344 217344 219657 | 0 1000 300055 300052 217088 217088 214784 217088 217088 214780 | 0 1000 300148 300145 217344 217344 217344 217344 217344 217346 | 0 1000 300029 300027 216960 216960 216960 216960 216960 216960 | 0 1000 300016 300021 217216 217216 217216 217216 217216 217211 | 0 1000 300043 300040 216960 216960 216960 216960 216960 216966 | 0 1000 300082 300084 217344 217344 217344 217344 217344 217337 | 0 1000 300186 300177 217088 217088 217216 217088 217088 217215 | 0 1000 So we take a ~10kpps hit by enabling LRO in Kconfig but without NETIF_F_LRO set (from ~231 kpps to ~221 kpps), and then another ~4kpps hit by setting NETIF_F_LRO (to ~217 kpps). Since this is a 1.2 GHz CPU, the hit of the former is then about 230 cycles per packet (which is probably mostly extra cache misses), and that of the latter about another 100 cycles per packet (a handful of extra function calls and some tests). On one hand, quite some effort has gone into optimising mv643xx_eth routing throughput (from its original 150kpps-ish (don't remember exactly) in 2.6.25-ish on this board to ~280 kpps now with some more patches that are not (yet) in mainline), which has often yielded slow progress at a rate of only a couple kpps per change, and in that regard it feels like a waste to just throw 10kpps away by always enabling LRO. On the other hand, having the Kconfig option kind of fails the goal of having a driver that automatically does the right thing in various environments. What do you think? thanks, Lennert -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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