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what exactly does "himem" do?

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#1
dimiboy

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hi.
sorry for my noob question but what exactly does it do?
as far as i understood it disables the limit of the phone so it is able to use all its internal memory and not half... on the other hand it was said in the forum here that if i want to disable the himem i have to to reflash the rom because of a wifi driver... can anyone explain me more? i did a search here and didnt find...

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#2
axboe

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hi.
sorry for my noob question but what exactly does it do?
as far as i understood it disables the limit of the phone so it is able to use all its internal memory and not half... on the other hand it was said in the forum here that if i want to disable the himem i have to to reflash the rom because of a wifi driver... can anyone explain me more? i did a search here and didnt find...


Technically, highmem in the kernel refers to pages of memory that don't have a permanent virtual address mapping. The ARM architecture historically didn't have support for this, which is why the stock ROM with the 2.6.29 kernel didn't support that memory bank of the board. Why they decided to design it like this, I have no idea.

The reason you have to update the wifi kernel module when you disable himem, is because you disable himem by flashing a kernel without support for himem. The kernel and modules need to be closely matched, so they always need to be updated together.

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#3
dimiboy

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so... it means if i want to stop using himem i can just restore a nandroid backup?
and will the himem affect somehow the battery? i mean will it drain faster because of all that memory available?

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#4
axboe

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so... it means if i want to stop using himem i can just restore a nandroid backup?
and will the himem affect somehow the battery? i mean will it drain faster because of all that memory available?


You can always just nand restore, yes. High-mem will not negatively impact the battery life.

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#5
dimiboy

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so are there any down sides at all with himem?

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#6
axboe

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so are there any down sides at all with himem?


No. It sometimes exposes bugs in drivers, I think that's especially true for arch specific drivers on ARM, where highmem wasn't available prior to 2.6.32 (or .31, I forget). On generic drivers, we ironed out those bugs way back in the day when highmem was initially introduced as a concept. That happened when 32-bit x86 machines suddenly grew more than 1G of memory. But once those are ironed out, there are no downsides. It would be better if the memory would reside in low memory, as there's some overhead associated with mapping/unmapping such pages in the kernel. But if the choice is between having that memory as highmem or not at all, there's no contest :-)

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#7
dimiboy

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well thats strange...
i applied the superboot with himem and nothing changed! how can it be? is there a way to see somewhere if actually i succeeded or something?

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#8
aaronpaws

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well thats strange...
i applied the superboot with himem and nothing changed! how can it be? is there a way to see somewhere if actually i succeeded or something?

Run advanced task killer and check the amount of ram you have. Before applying himem with 7-8 apps open I would have around 30mb's free. Now, afterward, I have around 140-180mb's free.

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#9
dimiboy

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can it be possible that i have a different advanced task killer? it doesnt show anywhere nothing about ram...
oh and is there a way to increase the space availible for installing apps? but without using apps2sd?

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