On Mon, Jun 22, 2015 at 8:08 PM, Pravin Shelar <pshe...@nicira.com> wrote: > On Fri, Jun 19, 2015 at 11:24 AM, Daniele Di Proietto > <diproiet...@vmware.com> wrote: >> >> >> On 18/06/2015 23:57, "Traynor, Kevin" <kevin.tray...@intel.com> wrote: >> >>> >>> >>>> -----Original Message----- >>> >>>> From: dev [mailto:dev-boun...@openvswitch.org] On Behalf Of Daniele Di >>> >>>> Proietto >>> >>>> Sent: Tuesday, June 16, 2015 7:39 PM >>> >>>> To: dev@openvswitch.org >>> >>>> Subject: [ovs-dev] [PATCH] dpif-netdev: Check for PKT_RX_RSS_HASH flag. >>> >>>> >>> >>>> DPDK mbufs contain a valid RSS hash only if PKT_RX_RSS_HASH is >>> >>>> set in 'ol_flags'. Otherwise the hash is garbage and doesn't >>> >>>> relate to the packet. >>> >>>> >>> >>>> This fixes an issue with vhost, which, being a virtual NIC, doesn't >>> >>>> compute the hash. >>> >>>> >>> >>>> Unfortunately the ixgbe vPMD doesn't set the PKT_RX_RSS_HASH, forcing >>> >>>> OVS to compute an hash is software. This has a significant impact on >>> >>>> performance (-30% throughput in a single flow setup) which can be >>> >>>> mitigated in the CPU supports crc32c instructions. >>> >>> >>> >>>As per the other thread on this I'm a bit concerned about the performance >>> >>>drop from this patch, so I did some testing of this and alternative/ >>> >>>complimentary solutions. >>> >>> >>> >>>Here's the options I looked at and some comments: >>> >>>1. This patch in isolation: vhost drops about ~15% vhost-vhost and >>> >>>phy-vhost-phy (because of sw hash) but also there is drops of ~25% for >>> >>>phy-phy and ~15% drop for phy-ivshmem-phy. >>> >>> >>> >>>2. Leave the code as is and let EMC misses happen for vhost rx pkts: >>> >>>I measure this at ~35% drop if missed *everytime* for vhost-vhost. We >>> >>>see in testing that it can also never happen, but this is not realistic. >>> >>>There should be no impact to other DPDK interfaces. >>> >>> >>> >>>3. Add hash reset for packets from vhost: This is another way of forcing >>> >>>the software hash for vhost rx and it is roughly equivalent in performance >>> >>>to 1. for vhost-vhost (~15% drop). While there is a no significant drop >>> >>>for phy-vhost-phy. There should be no impact to other DPDK interfaces. >>> >>> >>> >>>4. Apply this patch and turn off Rx Vectorisation. vhost-vhost will drop >>> >>>~15% as per 1. and there should be nothing significant for phy-vhost-phy. >>> >>>We would lose the 10% gain that rx vectorisation gave us for phy-phy. >>> >>>There should be no impact for dpdkr ports. >>> >>> >>> >>> >>> >>>In terms of not knowing whether the hw hash is valid or not if the flag is >>> >>>not checked, I would have expected the pmd to return an error on config if >>> >>>the hash wasn't supported, but I'm not sure that it does. >>> >>>In the worst case where there was an incorrect hash, it would miss the EMC >>> >>>which is about a 45% drop for phy-phy. I would think it's pretty safe that >>> >>>if we configure it, the hash will be correct but I guess there is a >>> >>>possibility it wouldn't be. >>> >>> >>> >>>Even if it is possible to get a smaller patch to fix the underlying issue >>> >>>in DPDK, it would be in DPDK 2.1 at the earliest meaning the performance >>> >>>would remain low until sometime in August. If it's DPDK 2.2, then it would >>> >>>be sometime in December. This would mean any performance drops would be >>> >>>present in OVS 2.4 and possibly OVS 2.5. >>> >>> >>> >>>Sorry :( but based on the performance drop with this patch in isolation it >>> >>>would be a NAK from me. My preference would be 3 which gives best >>>performance, >>> >>>or 4 which is a bit lower for phy-phy but safer. >>> >>> >>> >>>Kevin. >> >> Thanks for all the testing. I guess it might make sense to stretch our >> interpretation of the API in this case, because it wouldn't affect >> correctness. >> >> Unless there any other objection I'm fine with the 3rd approach. >> > > We can use 3rd approach to fix issue on branch 2.4. Then have patch to > check the PKT_RX_RSS_HASH flag on master. By the time we release > branch 2.5 we will have proper fix in DPDK and performance will bounce > back.
I think this is probably a reasonable compromise. I think it's better to not keep a workaround in for an unbounded amount of time, otherwise we'll forget about it and it will come back to bite us in the future. _______________________________________________ dev mailing list dev@openvswitch.org http://openvswitch.org/mailman/listinfo/dev
