We've developed a fairly sophisticated android tablet application which relies on tablets communicating peer to peer over wifi. We're using straightforward java TCP/IP sockets to do this with our own application level handshaking and protocols, including reconnection when connections break etc. All this is working well on even the flakiest wifi so we have the product running in beta sites now.
However there is one problem that is eluding us so far. Under some circumstances (unknown, but they tend to occur in patches temporally) we are finding socket read or write times suddenly in the 10 second to 150 second range when they are normally in the 100ms range. This can go on for many minutes, followed by equally mysteriously going away. While it is going on we'll also see a corresponding increase in turnround time for UDP packets (pings) addressed to the relevant tablets as if the android wifi has somehow gone to sleep. The app is still in the foreground and is running normally except for slow socket reads and writes which we can measure either side of the socket calls (the thread blocks of course). We've eliminated threading/queue problems as the cause - we now have a basic two thread test program which just sends slightly lumpy generated traffic from one device to another and echoes it back. We're not sending large amounts of traffic - typically a few Kb every few minutes in a burst, and sometimes nothing for periods up to half an hour. Occasionally there is a file download but mostly at startup time. The slowness is unrelated to whether the connection has been recently used (apart from the expected 'wakeup time' if the connection or device has been dormant for a while). Occasional slowness won't hurt us, but unfortunately once it starts it tends to be sticky for a time. Then it goes away without any intervention on our part. When it does manifest (it only happens on some sites) then it may happen between once and a few times a day. It may not happen for weeks. One theory that we have is that android doesn't like long term open socket connections between devices. It would make sense that it might be optimised for short-lived connections like HTTP uses. We are trying a strategy where we close the connection if it is quiet and reopen on demand to see whether that makes a difference. It's a long test cycle though as we have to have a setup running for several days before we can realistically assess whether there's been any improvement as a result of a change. We have many other things on our list to try out but it's taking a long time because of the long cycle. We are using a mix of Android 5 and 6 devices (mostly 6 now - whatever they upgrade to in the normal course), and a wide range of wifi routers (Netgear, Ubiquiti, OpenMesh, Technicolor to name a few). All exhibit the same problem, while we can have Windows machines merrily talking to each other over the very same wifi network at the same time without seeing any problems at all. The routers differ mostly in how often they just drop connections (whereupon we simply reconnect and carry on). We're mostly using Samsung kit as we've found that to be robust, performant and reliable. Wireshark shows an increased number of resent packets when we have the two halves of the test program running on android and windows devices and the slowness manifests. It will usually afflict multiple android devices at about the same time i.e. if one is seeing it then often they all will be. But not necessarily all. Top level question - has anybody else come across problems similar to this with android networking? We get the impression that not many other people are using android like this. We don't expect TCP/IP over wifi to be reliable in any sense especially out in the real world of cheap and cheerful wifi, but this particular problem hurts us as we need responsive communications for certain operations and the stickiness of the problem makes retry ineffective. There are some architectural changes we could make to the design of our apps (more distributed, less client/server) but that's a big move just to get round a very specific problem. -- You received this message because you are subscribed to the Google Groups "Android Developers" group. To unsubscribe from this group and stop receiving emails from it, send an email to android-developers+unsubscr...@googlegroups.com. To post to this group, send email to android-developers@googlegroups.com. Visit this group at https://groups.google.com/group/android-developers. To view this discussion on the web visit https://groups.google.com/d/msgid/android-developers/8620f014-27bb-48da-812a-40c80a51ea1d%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
