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I've noticed that with Mountain Lion (running on a mid-2011 iMac w/SSD) that once a download has started in Safari, or iTunes or Finder (AFP), etc. that any subsequent network activity is severely bottlenecked. It's as if the first download is filling my entire pipe and subsequent requests are given a lower (or NO) priority.

This didn't happen under Lion (or any previous OS X cat) and nothing has changed with my broadband Internet connectivity/bandwidth. Is there something I can do to make ML distribute the network bandwidth more 'cooperatively' so that when I'm waiting for some download I can at least browse the web or check my email?!?

This isn't localized to any one app (iTunes, Safari, etc.) it seems to be endemic to the entire system...and only started under Mountain Lion.

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This is atypical behavior - have you made measurements to ensure it's not a bottleneck on the router or upstream? If the pipe were truly full, the next download attempt would time out rather than succeed. If you have some hard data to edit into the question, it might be more clear what is responsible for the behavior you are seeing. –  bmike Sep 13 '12 at 17:19
    
Well, it's good to know it's not normal behavior. It's definitely happening on my machine and has since 10.8.0. Hoped 10.8.1 would fix it. It hasn't. Happy to provide you with more info if you can be more specific about what exactly you'd like to see. I don't have any "hard" data only a very noticeable "bottleneck" when I'm browsing, etc. and I know the system is busy downloading in the background. It's very annoying! –  Meltemi Sep 13 '12 at 17:56
    
anyone? crickets... –  Meltemi Sep 27 '12 at 19:01
    
You could run sysdiagnose on Safari while it's downloading - but this is going to be a tough thing to explain how to profile if you're not familiar with unix process debugging. I suppose you could also check the Activity Monitor - is the CPU busy, what amount if IO is happening to the drive at the time when things are slow. –  bmike Sep 27 '12 at 19:11
    
CPU is at about 3% usage. (8 cores!) –  Meltemi Sep 27 '12 at 22:06

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