Re: a 15 GB file on tmpfs

[Antonio Vargas <windenntw@gmail.com>]

On 7/20/05, Erik Mouw <erik@harddisk-recovery.com> wrote:
> On Wed, Jul 20, 2005 at 01:35:07PM +0000, Miquel van Smoorenburg wrote:
> > In article <20050720132006.GI7050@harddisk-recovery.com>,
> > Erik Mouw  <erik@harddisk-recovery.com> wrote:
> > >On Wed, Jul 20, 2005 at 02:16:36PM +0200, Bastiaan Naber wrote:
> > >AFAIK you can't use a 15 GB tmpfs on i386 because large memory support
> > >is basically a hack to support multiple 4GB memory spaces (some VM guru
> > >correct me if I'm wrong).
> >
> > I'm no VM guru but I have a 32 bit machine here with 8 GB of
> > memory and 8 GB of swap:
> >
> > # mount -t tmpfs -o size=$((12*1024*1024*1024)) tmpfs /mnt
> > # df
> > Filesystem           1K-blocks      Used Available Use% Mounted on
> > /dev/sda1             19228276   1200132  17051396   7% /
> > tmpfs                 12582912         0  12582912   0% /mnt
> >
> > There you go, a 12 GB tmpfs. I haven't tried to create a 12 GB
> > file on it, though, since this is a production machine and it
> > needs the memory ..
> 
> I stand corrected.
> 
> > So yes that appears to work just fine.
> 
> The question is if it's a good idea to use a 15GB tmpfs on a 32 bit
> i386 class machine. I guess a real 64 bit machine will be much faster
> in handling suchs amounts of data simply because you don't have to go
> through the hurdles needed to address such memory on i386.
> 
> 
> Erik
> 

On 32bit: you would have to use read() and write() or mmap() munmap()
mremap() to perform your own paging, since you can't fit 15GB on a 4GB
address space.

On 64bit: you would simply mmap() the whole file and you are done.

Most probably the cost of programming and debugging the hand-made
paging on 32bit machines will cost more than the difference for a
64bit machine.

-- 
Greetz, Antonio Vargas aka winden of network

http://wind.codepixel.com/

Las cosas no son lo que parecen, excepto cuando parecen lo que si son.