Memory allocation in Linux (fwd)

Memory allocation in Linux (fwd)

[Amit S. Jain]

This was the message I had posted in March expecting some help from the
Masters in Linux-mm...however there was no response...hope someone can
respond to it now.. Pleasse.....and if u do could you please CC the
message to my e-mail address...
 Hello everyone,
               I am confused about the concept of memory allocation in
Linux and hope u all can please clear this.
Obtaining large amount of continuous memory from the kernel is not a
good practice and is also not possible.However,as far as non-contiguous
memory is concerned ...cant those be obtained in huge amounts (I am talkin
in terms of MB).Using get_free_pages or vmalloc cant large amounts of
memory be obtained.I tried doing this but I got continuous message ssayin
PCI bus error 2290...wass this bout???ne idea. 

Also,I will be highly obliged if you could refer a good document which can
gimme a good explaination bout mmap function.I basically want to obtain
zero copy from the user area straigt to the network interface without any
copies in the kernel area. kiobuff can provide one such interface,however
I also want to try using mmap....so please could u refer me some good
document.   

Thanking you
Regards
Amit


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Re: Memory allocation in Linux (fwd)

[Joseph A Knapka]

Amit S. Jain wrote:

> This was the message I had posted in March expecting some help from the
> Masters in Linux-mm...however there was no response...hope someone can
> respond to it now.. Pleasse.....and if u do could you please CC the
> message to my e-mail address...


Well, I may not be telling you anything you don't already
know, but at least here's a reply :-)

>  Hello everyone,
>                I am confused about the concept of memory allocation in
> Linux and hope u all can please clear this.
> Obtaining large amount of continuous memory from the kernel is not a
> good practice and is also not possible.However,as far as non-contiguous
> memory is concerned ...cant those be obtained in huge amounts (I am talkin
> in terms of MB).Using get_free_pages or vmalloc cant large amounts of
> memory be obtained.I tried doing this but I got continuous message ssayin
> PCI bus error 2290...wass this bout???ne idea. 


get_free_pages() allocates physically contiguous RAM of the
requested size (2^order pages). You can get lots of
non-physically-contiguous memory by calling get_free_pages()
many times with order=0.

vmalloc() does just that, and maps the resulting pages
contiguously into kernel virtual memory. However, vmalloc()
can only map pages into kernel addresses that are not
already in use, and in a machine with lots of RAM most of the
kernel's virtual address space is occupied by 1-1 mapping
of physical RAM. So you may be running into that problem,
depending on how much RAM is in the machine. And of
course other users of vmalloc() may fragment the
kernel's virtual space and make allocation of very large
blocks impossible.

I don't know why you would be getting "PCI bus error 2290",
sorry...


> Also,I will be highly obliged if you could refer a good document which can
> gimme a good explaination bout mmap function.I basically want to obtain
> zero copy from the user area straigt to the network interface without any
> copies in the kernel area. kiobuff can provide one such interface,however
> I also want to try using mmap....so please could u refer me some good

> document.   


(I can't comment on this.)

Cheers,

-- Joe
   Using open-source software: free.
   Pissing Bill Gates off: priceless.

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Re: Memory allocation in Linux (fwd)

[Amit S. Jain]

Hi everyone,
            This is a continuation of the mail I had written earlier (see
down)tellin bout my problem that when i use vmalloc()...I get an error stating
 "PCI bus error 2290".I think i have a slight idea what the problem could
be.....Hope u all could comment on it.
The large amount of memory i obtain using vmalloc is then pointed to by
the skb "network" buffers as i copy data into this memory which has to be
transmitted.Since the memory is discontinuous implying data is
discontinuous and the ethernet card I am using is REALTEK8139 which
doesnot support SCATTER/GATHER DMA.... hence the PCI bus cant find the
continuous data which has to be transmitted.

This is my amature fundaa....Hope u can elaborate on this...

Thanks
Amit

<

On Thu, 4 Apr 2002, Joseph A Knapka wrote:

> Amit S. Jain wrote:
> 
> > This was the message I had posted in March expecting some help from the
> > Masters in Linux-mm...however there was no response...hope someone can
> > respond to it now.. Pleasse.....and if u do could you please CC the
> > message to my e-mail address...
> 
> 
> Well, I may not be telling you anything you don't already
> know, but at least here's a reply :-)
> 
> >  Hello everyone,
> >                I am confused about the concept of memory allocation in
> > Linux and hope u all can please clear this.
> > Obtaining large amount of continuous memory from the kernel is not a
> > good practice and is also not possible.However,as far as non-contiguous
> > memory is concerned ...cant those be obtained in huge amounts (I am talkin
> > in terms of MB).Using get_free_pages or vmalloc cant large amounts of
> > memory be obtained.I tried doing this but I got continuous message ssayin
> > PCI bus error 2290...wass this bout???ne idea. 
> 
> 
> get_free_pages() allocates physically contiguous RAM of the
> requested size (2^order pages). You can get lots of
> non-physically-contiguous memory by calling get_free_pages()
> many times with order=0.
> 
> vmalloc() does just that, and maps the resulting pages
> contiguously into kernel virtual memory. However, vmalloc()
> can only map pages into kernel addresses that are not
> already in use, and in a machine with lots of RAM most of the
> kernel's virtual address space is occupied by 1-1 mapping
> of physical RAM. So you may be running into that problem,
> depending on how much RAM is in the machine. And of
> course other users of vmalloc() may fragment the
> kernel's virtual space and make allocation of very large
> blocks impossible.
> 
> I don't know why you would be getting "PCI bus error 2290",
> sorry...
> 
> 
> > Also,I will be highly obliged if you could refer a good document which can
> > gimme a good explaination bout mmap function.I basically want to obtain
> > zero copy from the user area straigt to the network interface without any
> > copies in the kernel area. kiobuff can provide one such interface,however
> > I also want to try using mmap....so please could u refer me some good
> 
> > document.   
> 
> 
> (I can't comment on this.)
> 
> Cheers,
> 
> -- Joe
>    Using open-source software: free.
>    Pissing Bill Gates off: priceless.
> 

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Re: Memory allocation in Linux (fwd)

[Benjamin LaHaise]

On Thu, Apr 11, 2002 at 06:16:56PM +0530, Amit S. Jain wrote:
> Hi everyone,
>             This is a continuation of the mail I had written earlier (see
> down)tellin bout my problem that when i use vmalloc()...I get an error stating
>  "PCI bus error 2290".I think i have a slight idea what the problem could
> be.....Hope u all could comment on it.
> The large amount of memory i obtain using vmalloc is then pointed to by
> the skb "network" buffers as i copy data into this memory which has to be
> transmitted.Since the memory is discontinuous implying data is
> discontinuous and the ethernet card I am using is REALTEK8139 which
> doesnot support SCATTER/GATHER DMA.... hence the PCI bus cant find the
> continuous data which has to be transmitted.

You cannot pass address returned from vmalloc to hardware: vmalloc returns 
a virtual mapping of memory.

		-ben
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Re: Memory allocation in Linux (fwd)

[Amit S. Jain]

You cannot pass address returned from vmalloc to hardware: vmalloc returns
> a virtual mapping of memory.
>
>               -ben


 Thanks Ben,

So wass the solution for this.....??

Thanks
Amit 



On Thu, 11 Apr 2002, Benjamin LaHaise wrote:

> On Thu, Apr 11, 2002 at 06:16:56PM +0530, Amit S. Jain wrote:
> > Hi everyone,
> >             This is a continuation of the mail I had written earlier (see
> > down)tellin bout my problem that when i use vmalloc()...I get an error stating
> >  "PCI bus error 2290".I think i have a slight idea what the problem could
> > be.....Hope u all could comment on it.
> > The large amount of memory i obtain using vmalloc is then pointed to by
> > the skb "network" buffers as i copy data into this memory which has to be
> > transmitted.Since the memory is discontinuous implying data is
> > discontinuous and the ethernet card I am using is REALTEK8139 which
> > doesnot support SCATTER/GATHER DMA.... hence the PCI bus cant find the
> > continuous data which has to be transmitted.
> 
> You cannot pass address returned from vmalloc to hardware: vmalloc returns 
> a virtual mapping of memory.
> 
> 		-ben
> 

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Re: Memory allocation in Linux (fwd)

[Eric W. Biederman]

"Amit S. Jain" <amitjain@tifr.res.in> writes:

> You cannot pass address returned from vmalloc to hardware: vmalloc returns
> > a virtual mapping of memory.
> >
> >               -ben
> 
> 
>  Thanks Ben,
> 
> So wass the solution for this.....??

Call get_free_pages explicitly use the scatter gather support
in the network layer.  You should be able to pass a linked
list of pages.

Eric
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Re: Memory allocation in Linux (fwd)

[Ravi]

> Obtaining large amount of continuous memory from the kernel is not a
> good practice and is also not possible.However,as far as
> non-contiguous memory is concerned ...cant those be obtained in huge 
> amounts (I am talkin in terms of MB).Using get_free_pages or vmalloc 
> cant large amounts of memory be obtained.
 
  __get_free_pages() allocates physically contiguous pages, so it
doesn't help you.  vmalloc() may be used to get more memory than you
would with kmalloc(), but it isn't guaranteed to succeed. vmalloc can
fail for two reasons (AFAIK):
 - vmalloc maps physically discontiguous pages to kernel virtual
addresses in the range of VMALLOC_START and VMALLOC_END. If this
address range is used up, vmalloc will fail.
 - allocations done by vmalloc are always backed by physical pages. So
if enough physical memory is not available to satisfy the request,
vmalloc will fail.

Hope this helps,
Ravi.

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