In scalability test with ovn-scale-test, ovsdb-server SB load is not a problem at least with 1k HVs. However, if we restart the ovsdb-server, depending on the number of HVs and scale of logical objects, e.g. the number of logical ports, ovsdb-server of SB become an obvious bottleneck.
In our test with 1k HVs and 20k logical ports (200 lport * 100 lswitches connected by one single logical router). Restarting ovsdb-server of SB resulted in 100% CPU of ovsdb-server for more than 1 hour. All HVs (and northd) are reconnecting and resyncing the big amount of data at the same time. Considering the amount of data and json rpc cost, this is not surprising. At this scale, SB ovsdb-server process has RES 303848KB before restart. It is likely a big proportion of this size is SB DB data that is going to be transferred to all 1,001 clients, which is about 300GB. With a 10Gbps NIC, even the pure network transmission would take ~5 minutes. Considering the actual size of JSON RPC would be much bigger than the raw data, and the processing cost of the single thread ovsdb-server, 1 hour is reasonable. In addition to the CPU cost of ovsdb-server, the memory consumption could also be a problem. Since all clients are syncing data from it, probably due to the buffering, RES increases quickly, spiked to 10G at some point. After all the syncing finished, the RES is back to the similar size as before restart. The client side (ovn-controller, northd) were also seeing memory spike - it is a huge JSON RPC for the new snapshot of the whole DB to be downloaded, so it is just buffered until the whole message is received - RES peaked at the doubled size of its original size, and then went back to the original size after the first round of processing of the new snapshot. This means for deploying OVN, this memory spike should be considered for the SB DB restart scenario, especially the central node. Here is some of my brainstorming of how could we improve on this (very rough ones at this stage). There are two directions: 1) reducing the size of data to be transferred. 2) scaling out ovsdb-server. 1) Reducing the size of data to be transferred. 1.1) Using BSON instead of JSON. It could reduce the size of data, but not sure yet how much it could help since most of the data are strings. It might be even worse since the bottleneck is not yet the network bandwidth but processing power of ovsdb-server. 1.2) Move northd processing to HVs - only relevant NB data needs to be transfered, which is much smaller than the SB DB because there is no logical flows. However, this would lead to more processing load on ovn-controller on HVs. Also, it is a big/huge architecture change. 1.3) Incremental data transfer. The way IDL works is like a cache. Now when connection reset the cache has to be rebuilt. But if we know the version the current snapshot, even when connection is reset, the client can still communicate with the newly started server to tell the difference of the current data and the new data, so that only the delta is transferred, as if the server is not restarted at all. 2) Scaling out the ovsdb-server. 2.1) Currently ovsdb-server is single threaded, so that single thread has to take care of transmission to all clients with 100% CPU. If it is mutli-threaded, more cores can be utilized to make this much faster. 2.2) Using ovsdb cluster. This feature is supported already but I haven't tested it in this scenario yet. If everything works as expected, there can be 3 - 5 servers sharing the load, so the transfer should be completed 3 - 5 times faster than it is right now. However, this is a limit of how many nodes there can be in a cluster, so the problem can be alleviated but may still be a problem if the data size goes bigger. 2.3) Using readonly copies of ovsdb replications. If ovn-controller connects to readonly copies, we can deploy a big number of ovsdb-servers of SB, which replicates from a common source - the read/write one populated by ovn-northd. It can be a multi-layered (2 - 3 layer is big enough) tree structure, so that each server only serves a small number of clients. However, today there are some scenarios requires ovn-controller to write data to SB, such as dynamic mac-binding (neighbor table populating), nb_cfg sync feature, etc. These ideas are not mutual exclusive, and the order is random just according to my thought process. I think most of them are worth to try, but not sure about priority (except that 1.2 is almost out of question since I don't think it is a good idea to do any architecture level change at this phase). Among the ideas, I think 1.3) 2.1) and 2.3) are the ones that should have the best result (if they can be implemented with reasonable effort). Any comments/suggestions are welcome!! Thanks, Han
_______________________________________________ discuss mailing list [email protected] https://mail.openvswitch.org/mailman/listinfo/ovs-discuss
