Re: [libvirt] Raising limits for our RPC messages

On Thu, May 04, 2017 at 01:39:17PM +0200, Michal Privoznik wrote: [snip] I guess it is helpful to consider the reason why we have a limit in the first place. Originally the RPC message buffer was statically allocated on the stack :-) We fixed that, but then the struct containing the buffer was still fully allocated in te struct. We fixed that too, and the RPC message struct dynamically grows the buffer as needed. At this point, the limit really just exists to avoid accidental / deliberate DoS attack on server / client by making unreasonably large requests. Whether we have 1 large message or 3 fragmented messages, we're going to end up serializing the RPC top the messages at the same time for sake of simplicity, so our peak memory usage is going to be the same in both cases. So I agree, that fragmenting messages is just adding complexity without a real win. So I think it is reasonable to simply increase the buffer size as we have done before. That said, we should bear this problem in mind before adding more "bulk query" APIs, as it isn't sensible to carry on increasing RPC message size forever. As somepoint we have to consider that serializing 100's of MB of data into an RPC message is an inherantly inefficient design, and consider alternative API designs without this. For bulk stats query we could do something totally radical and have the facility for the client to send us a shared memory region which we asynchronously populate with stats, avoiding the RPC layer entirely. Obviously only works for local connections, but I get the impression that most mgmt apps have a node-local agent talking to libvirtd anyway. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|