On Tue, Sep 25, 2018 at 10:18 AM Han Zhou <[email protected]> wrote: > > > > On Thu, Sep 20, 2018 at 4:43 PM Ben Pfaff <[email protected]> wrote: > > > > On Thu, Sep 13, 2018 at 12:28:27PM -0700, Han Zhou wrote: > > > 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). > > > > It sounds like reducing the size is essential, because you say that the > > sheer quantity of data is 5 minutes worth of raw bandwidth. Let's go > > through the options there. > > > > 1.1, using BSON instead of JSON, won't help sizewise. See > > http://bsonspec.org/faq.html. > > > > 1.2 would change the OVN architecture, so I don't think it's a good > > idea. > > > > 1.3, incremental data transfer, is an idea that Andy Zhou explored a > > little bit before he left. There is some description of the approach I > > suggested in ovn/TODO.rst: > > > > * Reducing startup time. > > > > As-is, if ovsdb-server restarts, every client will fetch a fresh copy of > > the part of the database that it cares about. With hundreds of clients, > > this could cause heavy CPU load on ovsdb-server and use excessive network > > bandwidth. It would be better to allow incremental updates even across > > connection loss. One way might be to use "Difference Digests" as described > > in Epstein et al., "What's the Difference? Efficient Set Reconciliation > > Without Prior Context". (I'm not yet aware of previous non-academic use of > > this technique.) > > > > When Andy left VMware, the project got dropped, but it could be picked > > up again. > > > > There are other ways to implement incremental data transfer, too. > > > > Scaling out ovsdb-server is a good idea too, but I think it's probably > > less important for this particular case than reducing bandwidth > > requirements > > > > 2.1, multithreading, is also something that Andy explored; again, the > > project would have to be resumed or restarted. > > Thanks Ben! It seems 1.3 (incremental data transfer) is the most effective approach to solve this problem. > I had a brief study on the "Difference Digests" paper. For my understanding it is particularly useful when there is no prior context. However, in OVSDB use case, especially in this OVN DB restart scenario, we do have the context about the last data received from the server. I think it would be more efficient (no full data scanning and encoding) and maybe simpler to implement based on the append-only (in most cases, for the part of data that hasn't been compressed yet) nature of OVSDB. Here is what I have in mind: > With some more study, here are some more details added inline:
> - We need a versioning mechanism. For each transaction record in OVSDB file, it needs a unique version. The hash string may be used for this purpose, so that the file format doesn't need to be changed. If we allow the file format to be changed, it may be better to have a version number that is sequentially increased. > For standalone format, the hash value can be used as unique version id. (what's the chance of the 10 bytes hash value having a conflict?) For clustered DB format, the eid is perfect for this purpose. (Sequentially increasing seems not really necessary, since we only need to keep a small amount of history transactions for the DB restart scenario) > - We can include the latest version number in every OVSDB notification from server to client, and the client IDL records the version. > > - When a client reconnects to server, it can request to get only the changes after “last version”. > > - When a server starts up, it reads the DB file and keeps track of the “version” for last N (e.g. 100) transactions, and maintains the changes in memory of that N transactions. The current implementation keeps track in monitors for the transactions that haven't been flushed to all clients. We can extend this by keeping track of extra N previous transactions. Now for the DB restart scenario, the previous N transactions are read from DB file. Currently, during reading the DB file, there is no monitoring connected yet, so replaying transactions will not trigger monitor data population. We can create a fake monitor for all tables beforehand so that we can reuse the code to populate monitor data while replaying DB file transactions. When a real monitor is created, it copies and converts the data in the fake monitor to its own table according to the monitor criteria. Flushing the data for the fake monitor can be done so that maximumly M transactions will be kept in the monitor (because there is no real client to consume the fake monitor data). > > - When a new client asks data after a given “version”, if the version is among the N transactions, the server sends the data afterwards to the client. If the given version is not found (e.g. when client reconnect, there is already lot of changes happened in server and the old changes were already flushed out, or a DB compression has been performed thus the transaction data is gone), server can: > - Option1: return an error telling the client the version is not available, and client can re-request the whole snapshot > - Option2: server directly send the whole snapshot, with a flag indicating this is the whole snapshot instead of the delta > Option2 seems better. As to the OVSDB protocol change, since we need to add a version id to every update notification, it would be better to have a new method "monitor_cond_since": "method": "monitor_cond_since" "params": [<db-name>, <json-value>, <monitor-cond-requests>, <latest_version_id>] "id": <nonnull-json-value> <latest_version_id> is the version id that identifies the latest data the client already has. Everything else is same as "monitor_cond" method. The response object has the following members: "result": [<found>, <latest_version_id>, <table-updates2>] "error": null "id": same "id" as request <found> is a boolean value that tells if the <latest_version_id> requested by client is found in history or not. If true, the data after that version up to current is sent. Otherwise, all data is sent. <latest_version_id> is the version id that identifies the latest change involved in this response, so that client can keep track. Following changes will be notified to client using "update3" method: "method": "update3" "params": [<json-value>, <latest_version_id>, <table-updates2>] "id": null Similar as the response to "monitor_cond_since", <latest_version_id> is added in the update3 method. > - Client will not destroy the old copy of data, unless the requested version is not available and it has to reinitialize with the whole DB. > > This is less general than the Difference Digests approach, but I think it is sufficient and more relevant for OVSDB use case. I am sure there will be details need more consideration, e.g. OVSDB protocol update, caching in server, etc., but do you think this is a right direction? Ben, please suggest if this is reasonable so that I could go ahead with a POC, or please let me know if you see obvious problems/pitfalls. Thanks, Han
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