I don't see any fundamental reason GCM should be more efficient than a properly written application sleeping in a poll or a thread making a blocking read from the notification socket. It's more a matter of apps being idiotic or maliciously doing other stuff in the background.
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The reason it's more efficient is that it's a single connection with highly optimized rare polling based on a good heuristics for tuning the frequency of the polling. If the apps all use the same polling interval and coordinate it with flexible alarms it wouldn't be that bad.
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poll does not have a frequency of polling unless you misuse it by passing a timeout. It sleeps until there's data.
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The issue is that connections often break in practice without notifying you, so you need to regularly wake up and send some data both to keep the connection from being killed and to make sure that it hasn't died yet. It's particularly bad with most mobile data connections.
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poll should return with an exceptional condition if the socket dies. If it doesn't this is a problem the OS should solve; it's trivial to do if you can see the connection go down/up again.
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Connections die pretty quickly if you aren't sending anything through them on mobile networks and you don't receive reliable notice of that. OS doesn't know about it either and the device is generally asleep when this happens. Need to decide how often to wake to keep them going.
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Is this NAT-hell only? In principle it shouldn't happen on a real IPv6 network, which we'll hopefully be able to assume in a few years...
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An IPv6 network without stateful firewalls shouldn't have this issue, but stateful firewalls will still end up dropping old / inactive connections. There's also just a whole lot of broken stuff. I think there are a lot of pure IPv6 mobile networks already and it still happens.
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Why would you put stateful firewalls on an IPv6 network with millions of nodes? Do they just *want* to burn money and spew CO2?
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I don't know what they are doing or why, but carriers love dropping connections. *shrug*
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en.wikipedia.org/wiki/Carrier-g is definitely the main issue, but IPv6 definitely isn't 100% reliable either. The code really needs to be designed to actively figure this out.
Shouldn't TCP keepalive at the kernel level be able to handle it?
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The device goes to sleep though and that's not going to wake it up. The only way it could work is if it was offloaded to hardware that kept running. It's totally possible that exists, I don't know.
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