increase lowmem value for ppc

Marco Stornelli marco.stornelli at
Thu Jul 17 22:05:56 EST 2008

Ruksen INANIR ha scritto:
> Thanks for your comments.
> I tried the highmem option but one of the device drivers does not 
> support highmem, so it did not work. can try to look at it, it could be only some GFP flag.

> the only choice for me is to increase the lowmem to 1520 MB. the maximum 
> lowmem value i reached succesfully is 1503 MB. but any value more than 
> 1503 results kernel not able to load kernel modules.

Mmmm....have you tried a stress test? It sounds like a bit strange that 
the kernel works well with this mapping but without other modification.

> Upgrading to a 2.6.x kernel may solve the problem, but it needs a lot of 
> effort. Does 2.6.x kernel support 2G physical memory?
It's the same. Some architecture have 36-bit addressing, but I don't 
know if the latest kernel has the support for this feature for powerpc.
> Marco Stornelli wrote:
>> Ruksen INANIR ha scritto:
>>> Is there any side effects of changing the address splitting? 
>> Yes, in this way the application address space is smaller than normal. 
>> Some big applications (for example same dbms) might not work. Usually 
>> with 2G/2G most of the applications work well.
>>> What should i change for 2G/2G splitting? should i shift the kernel 
>>> start?
>>> Thanks
>> Yes in advanced options of the kernel menu, but sometimes it's not 
>> easy.  Sometimes ago I had to do the same thing but I have to change 
>> the kernel code because some operations were hard-coded. Be careful 
>> because it's not an easy operation. However I'd suggest you to use the 
>> highmem, the memory performance are worse than direct memory mapping, 
>> but usually it's not a problem because the overhead is low.
>>> Marco Stornelli wrote:
>>>> Marco Stornelli ha scritto:
>>>>> Ruksen INANIR ha scritto:
>>>>>> Is there a way to increase the lowmem value for ppc. The 
>>>>>> MAX_LOW_MEM is defined as max 768 MB. But a value around 1.5 G 
>>>>>> works with no problem. But when i try to increase this value to 
>>>>>> 1520 or more, kernel complaints about no space for memory 
>>>>>> allocation when loading kernel modules.
>>>>>> I do not want to use HIGHMEM config. What is the max lowmem value 
>>>>>> for a ppc system? What other setting are needed to use 1520 MB (or 
>>>>>> more) as lowmem .
>>>>>> The ppc card has 2G on board memory. 2.4.22 kernel is used.
>>>>>> Thanks
>>>>>> _______________________________________________
>>>>>> Linuxppc-embedded mailing list
>>>>>> Linuxppc-embedded at
>>>>> With 32-bit arch you may not use more than 1GB to map the memory 
>>>>> (minus some space for some kernel operation). The value of 768MB 
>>>>> was not there by chance.
>>>> Only an additional comment: I meant with the address splitting 
>>>> 3G/1G. To map more than 1GB, you have to change the splitting 2G/2G 
>>>> for example.
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Marco Stornelli
Embedded Software Engineer
CoRiTeL - Consorzio di Ricerca sulle Telecomunicazioni

marco.stornelli at
+39 06 72582838

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