What causes high wired memory usage?

Nokaoi · Feb 26, 2012

I have an app from the app store called system status. At times when my iPad 2 16gb is running crappy I check the memory usage and the wired memory usage is very high yet nothing else is running.

I just do a hard reset and it resolves the issue. But, I'm wondering if it is a bigger problem. Is there an underlying issue causing this to happen? Should I do a restore?

Any help will be greatly appreciated.

psonice · Feb 27, 2012

There is no issue, and the memory usage is supposed to be high when the iPad is working normally. In fact you could say the issue is the system status app, it's reporting numbers that don't have a lot of meaning and failing to explain what they're supposed to be telling you.

My suggestion: delete the app as it's pretty much pointless, and stop worrying about it. If the iPad is running slow, give it a few minutes - it usually sorts itself out. If not, reboot it.

(And if you really want to know why the memory numbers are pointless: The iPad keeps apps that aren't currently active (but that you've recently used) in memory. If you go to use them again, they start instantly and continue from where you left off. That fills up the memory of course. If you open another app that needs it, the iPad wakes up those 'sleeping' apps and tells them to shut down - that lets them shut down cleanly, and frees up the memory quickly, but it can cause a little slowdown while it happens - then it should settle. This way you get good multitasking, good battery life, and apps almost always get the memory they need, the only downside is a little lag when you start a new app now and then.)

chakrava · Feb 29, 2012

There is an issue with memory eating apps like some games. Playing a game like Infinity Blade (I or II) almost always requires a hard reset to make sure enough memory is available to run the game.

For most other apps this isn't an issue and the memory allocation as described by psonice is fine. I can check e-mail, calendars, web browse, etc. with no problem, but when I want to play a better game I'm stuck having to cycle through a hard reset or purchasing a memory application to try to free it up otherwise.

But from a gamer's perspective (admittedly a niche) the iOS memory management is pretty terrible.

Coukos34 · Feb 29, 2012

Chakrava, I'd have to disagree with you there. I have owned each iPhone and iPad (other then the 1st gen iPhone), and have never (ever) had to clear memory or give the device a hard reset to run (any) game (including Infinity Blade). I honestly don't understand this behavior, as I see it mentioned in game app reviews constantly. What are you guys doing to your devices to have this happen anyways? I just don't get it...It's like people that get viruses on their computers....how do you manage this??? I'm sorry, but mem management seems to work pretty well in iOS, as it is designed to release apps from mem as needed. Thus, I'm not sure why any device would ever need a restart, when the proper mem is released automatically anyway. If an application needs all the mem available that an iOS device has, then all app data is released, and it goes on it's way.Sorry for the rant, but I'm just totally sick of listening to this drive. This crap about apps not running without a reboot is just weak

psonice · Mar 1, 2012

Agreed - I've never needed to clear memory either, and have also owned a *lot* of iOS devices back to the first iPhone (I pretty much *have* to buy every new device as a developer, for testing).

I played infinity blade a lot too, eventually completing it and beating that robot thing at the end. The only "issue" was that it'd load a bit slower if memory was low, and it'd be a little jerky initially as background apps were still shutting down. That settled fast though and didn't affect gaming - and it was still a lot more efficient than waiting for a hard reboot or manually shutting apps down!

blackNBUK · Mar 1, 2012

psonice said:
There is no issue, and the memory usage is supposed to be high when the iPad is working normally. In fact you could say the issue is the system status app, it's reporting numbers that don't have a lot of meaning and failing to explain what they're supposed to be telling you.

My suggestion: delete the app as it's pretty much pointless, and stop worrying about it. If the iPad is running slow, give it a few minutes - it usually sorts itself out. If not, reboot it.

(And if you really want to know why the memory numbers are pointless: The iPad keeps apps that aren't currently active (but that you've recently used) in memory. If you go to use them again, they start instantly and continue from where you left off. That fills up the memory of course. If you open another app that needs it, the iPad wakes up those 'sleeping' apps and tells them to shut down - that lets them shut down cleanly, and frees up the memory quickly, but it can cause a little slowdown while it happens - then it should settle. This way you get good multitasking, good battery life, and apps almost always get the memory they need, the only downside is a little lag when you start a new app now and then.)

Normally I would completely agree with you but the OP did specifically mention that wired memory usage was high. On OSX at least this refers to memory that is in use by the kernel/OS and can't be swapped out to disk. I don't know for certain but I would be somewhat surprised if this memory could be reclaimed in the same way as the memory used by normal apps. Maybe the OP is hitting a bug in iOS that is causing the kernel to grab wired memory and not give it back. Over time that could reduce the memory available to apps so much that the iPad starts running badly.

psonice · Mar 1, 2012

blackNBUK said:
Normally I would completely agree with you but the OP did specifically mention that wired memory usage was high. On OSX at least this refers to memory that is in use by the kernel/OS and can't be swapped out to disk. I don't know for certain but I would be somewhat surprised if this memory could be reclaimed in the same way as the memory used by normal apps. Maybe the OP is hitting a bug in iOS that is causing the kernel to grab wired memory and not give it back. Over time that could reduce the memory available to apps so much that the iPad starts running badly.

iOS doesn't manage memory the same as OS X. It doesn't use virtual memory and swap out to disk at all. In this case, as it's low on memory, it would work like this:

1. He opens infinity blade, which asks for a whole load of RAM.
2. The OS checks, and sees that it wants 10x more RAM than is available.
3. It checks which apps are currently open in the background, picks the one that hasn't been used for the longest, and tells it to quit.
4. That background app 'wakes up', saves its state (so if you were in the middle of a game it resumes where you left off), and quits.
5. The OS passes the freed up RAM to infinity blade.
6. If there's still not enough free, go back to 3 and repeat.

So basically it keeps killing old apps until there's enough memory left. If you watched it, you'd probably see the wired memory stay at 90%+ the whole time, yet a large game can still load into memory.

The only real downside is that the game might run poorly to begin with, because it's still loading bits into memory and other apps are silently shutting down in the background. Give it a little time though and it's back to normal.

blackNBUK · Mar 2, 2012

psonice said:
iOS doesn't manage memory the same as OS X. It doesn't use virtual memory and swap out to disk at all. In this case, as it's low on memory, it would work like this:

1. He opens infinity blade, which asks for a whole load of RAM.
2. The OS checks, and sees that it wants 10x more RAM than is available.
3. It checks which apps are currently open in the background, picks the one that hasn't been used for the longest, and tells it to quit.
4. That background app 'wakes up', saves its state (so if you were in the middle of a game it resumes where you left off), and quits.
5. The OS passes the freed up RAM to infinity blade.
6. If there's still not enough free, go back to 3 and repeat.

So basically it keeps killing old apps until there's enough memory left. If you watched it, you'd probably see the wired memory stay at 90%+ the whole time, yet a large game can still load into memory.

The only real downside is that the game might run poorly to begin with, because it's still loading bits into memory and other apps are silently shutting down in the background. Give it a little time though and it's back to normal.

I thought iOS still separated memory into 4 separate pools as OSX does:

Wired - Used by the OS/kernel internally.
Active - Used by applications (both built-in and third-party)
Free - Empty memory ready to be used.
Inactive - Recently used memory that is now unused. Can either be reused if the data is needed again or freed.

I agree that iOS doesn't use swap space and will quit apps to build up enough free memory for a new app to run. However I would have thought quitting apps would only move memory from the active to the free pools. If the OS grabs memory for it's own internal use and (because of a bug) doesn't give it back then I would have also thought that the wired usage would increase and therefore reduce the amount of memory available for apps. Quitting apps wouldn't help in this situation because the memory isn't actually being used by an app.

Of course this theory surposes that there is a bug in the OS that causes wired memory to not be freed properly. I've never seen anything like that in my use of iOS but it would provide an explanation for what the OP appears to be seeing.

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What causes high wired memory usage?

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