Re: [uml-devel] memory

[Doug Dumitru <doug@easyco.com>]

Vaibhav Sharma, Noida wrote:
> Hi,
> I have to build a simulator, to check the scalability limits in memory 
> management of linux. For this, the simulator  will provide a large 
> amount of RAM (in GBs), even if the amout of physical memory is only 512MB.
> 
> Can UML be used for this purpose? I had seen the "mem" option in the 
> kernel switches option of UML. Can UML simulate such a case? If it does, 
> how UML handles this? Specifically, can the UML kernel allocate all this 
> amount of memory (leaving aside the page faults which will be generated 
> in case of read/write requests to the allocated pages, for it has to use 
> the actual RAM)? Say, for example if I allocate 4 GB, will the UML 
> kernel allocate all the amount in the simulated RAM. (maybe it does it 
> in the some swap, or the /tmp, as the documentation says for the "mem" 
> usage <_http://user-mode-linux.sourceforge.net/switches.html_> . 
> 
> If I have wrong interpretation of the "mem" option in UML, please do 
> correct me.
> 
> TIA,
> Vaibhav

Mr. Sharma,

What you are trying to do will work, but not for large amounts of 
memory.  UML runs the client using a single user mode memory block as 
the entire client's core.  Thus the clients core size is limited to what 
a single task can allocate as plain memory.  With x86 architechure cpus, 
this is 2G or 3G depending on how you build a kernel (but usually 2G). 
There are no paging mechanism here so there is no way to make it "gigabytes"

Others can correct me if I have the exact numbers wrong.



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Doug Dumitru     800-470-2756     (610-237-2000)
EasyCo LLC       doug@easyco.com  http://easyco.com
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