Re: Memory allocation in Linux (fwd)

["Amit S. Jain" <amitjain@tifr.res.in>]

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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