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. _______________________________________________ dev mailing list dev@openvswitch.org http://openvswitch.org/mailman/listinfo/dev
