(Note: lists.mozilla.org seems to be having some issues this week according to a few colleagues, so these posts might not all appear. Feel free to email directly if you're concerned).
On May 18, 2012, at 4:17 AM, Alan Ogilvie wrote: > Thank you all for your response so far - clearly this is going to be a > somewhat complicated answer (I didn't believe that it wouldn't) ;-) :) Yup, but we can work it out and de-complexify (new word maybe) it all. > > 3/ - yes, my (and somewhat 'our') hypothesis is that because chunk requests > are made through the host browser, they will be restricted as any other > internet content is - for example, requesting a web page, then all the > assets from the HTML response, is clearly managed by limiting the number of > concurrent 'threads' (I may obviously be using the wrong words here) to > some sort of rule for 'same domain'. If we had more than one player in the > browser instance (maybe on multiple tabs/windows) all requesting chunks > from the same domain, but different 'channels' (ie. different streams), > then they will likely 'fight' each other for spare slots in the concurrent > 'threads'. We're not sure how much of an 'edge case' that is - though I can > believe some of our viewers/listeners having a couple of windows open > during big events (like the Olympics in London coming soon) with HDS > playback in Flash. That's a good example. So, I've done so more poking around today, and this is what I can see in our networking code as far as resources for connections are concerned. At a high level, we have 4 maximums for the number of connections (you can look at these in "about:config" - if you're unfamiliar with that, just type about:config in the address bar and tell Firefox that you'll be careful): network.http.max-connections = 256 network.http.max-connections-per-server = 15 network.http.max-persistent-connections-per-proxy = 8 network.http.max-persistent-connections-per-server = 6 I'm guessing that proxy will matter for those folk watching the games from their workplaces, but there may be other scenarios. And of course, we're not limited to the Olympic games here. These values are configurable, but for the vast majority of users, they won't touch them. Once we get an HTTP request, we create a transaction and determine what to do next based on the following logic in nsHttpConnectionMgr::ProcessNewTransaction (currently found at http://mxr.mozilla.org/mozilla-central/source/netwerk/protocol/http/nsHttpConnectionMgr.cpp#1641). Note that the transaction is created by the content thread and then dispatched to the socket thread for the following: // 0 - If this should use spdy then dispatch it post haste. // 1 - If there is connection pressure then see if we can pipeline this on // a connection of a matching type instead of using a new conn // 2 - If there is an idle connection, use it! // 3 - if class == reval or script and there is an open conn of that type // then pipeline onto shortest pipeline of that class if limits allow // 4 - If we aren't up against our connection limit, // then open a new one // 5 - Try a pipeline if we haven't already - this will be unusual because // it implies a low connection pressure situation where // MakeNewConnection() failed.. that is possible, but unlikely, due to // global limits // 6 - no connection is available - queue it So, deciding what to do with the transaction depends on what protocol it's using (SPDY, HTTP pipelining or non-pipelining), how many connections are free and the type of connection. If the connection is SPDY, the transaction is added as a stream if there is room (max of 100 streams per connection I think). Otherwise the transaction is queued until a slot opens up in the SPDY stream. So there are multiple layers in the decision-making to determine what is a relevant resource (num of sockets, num of streams, etc). Some points: "under pressure" means most of the allowed parallel connections are already used up. "idle connections" are persistent connections, i.e. sockets that are kept open thereby avoiding the DNS and TCP RTT for subsequent requests. Once we figure out what type of connection, the transaction is dispatched. Some things I'm still exploring: - We have one socket thread on which the previous decision about how to add a transaction is processed. I'm not sure if we have more socket threads for reading and writing or how we multiplex all of that - I'll find that out. - There may be a max for the number of requests which can be pipelined. > > I have supplied an example of HDS through Flash which is live on our site > at the moment. This may help you see how it's working in the real world. > I've already poked around to see what happens to the chunks. What have you observed if anything? Steve. _______________________________________________ dev-tech-network mailing list [email protected] https://lists.mozilla.org/listinfo/dev-tech-network
