On Tue, Jun 23, 2015 at 7:06 PM, Pravin Shelar <pshe...@nicira.com> wrote: > On Tue, Jun 23, 2015 at 2:51 PM, Jesse Gross <je...@nicira.com> wrote: >> 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. > > ok, Once the DPDK fix is backported to DPDK 2.0, we can remove the workaround.
Oh, I was just providing my justification for agreeing with you. I was considering putting the check for the RSS flag on master to be removing the workaround, so I don't think there is anything to be done beyond what you described. _______________________________________________ dev mailing list dev@openvswitch.org http://openvswitch.org/mailman/listinfo/dev
