Re: RAM shows less

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On Tue, 11 May 2010 09:17:38 -0500
Aaron Konstam <akonstam@xxxxxxxxxxxxx> wrote:

> On Tue, 2010-05-11 at 06:33 -0400, John Aldrich wrote: 
> > Just a question here... are you running a 32-bit version of Fedora or a 64-
> > bit version? 32-bit versions will only show about 3 gigs of memory, no 
> > matter how much if you have 20 Gigs, you'll only see 3.
> 
> I would expect 32 bit machinews to be able to handle 4G of memory. 
> 2^32=4G

It isn't quite that simple in either direction. Most x86 processors have
a 32bit virtual but a 36 or even 40bit physical address space. So you can
"handle" very large amounts of memory. In fact if you have 4GB of RAM the
kernel is probably seeing 4.25GB or so - because of the graphics card
memory, PCI devices etc.

> And the putput of free tells me he has roughly 4G of memmory reported.

The x86 has a 4GB virtual address space and in some cases a 36bit
physical one. So you can have up to above 64GB of memory in theory but
only 4GB of anything can be mapped at a time.

To get performance you need the user address space (3GB), the kernel,
some virtual mappings and some of memory mapped so we have to do a lot
more work when there is over about 1GB of physical RAM.

The practical impact tends to be that above 1GB of memory (actually about
900Mb due to kernel mappings) the 64bit kernel performs better, and as
you head above 4GB it rapidly increases its advantage as the 32bit kernel
has to spend all its time changing what it has mapped (think of it like
reading a book versus seeing the entire thing as a poster, the 32bit
kernel spends all of its time adjusting the pages it can see)

-- 
users mailing list
users@xxxxxxxxxxxxxxxxxxxxxxx
To unsubscribe or change subscription options:
https://admin.fedoraproject.org/mailman/listinfo/users
Guidelines: http://fedoraproject.org/wiki/Mailing_list_guidelines
[Index of Archives]     [Older Fedora Users]     [Fedora Announce]     [Fedora Package Announce]     [EPEL Announce]     [EPEL Devel]     [Fedora Magazine]     [Fedora Summer Coding]     [Fedora Laptop]     [Fedora Cloud]     [Fedora Advisory Board]     [Fedora Education]     [Fedora Security]     [Fedora Scitech]     [Fedora Robotics]     [Fedora Infrastructure]     [Fedora Websites]     [Anaconda Devel]     [Fedora Devel Java]     [Fedora Desktop]     [Fedora Fonts]     [Fedora Marketing]     [Fedora Management Tools]     [Fedora Mentors]     [Fedora Package Review]     [Fedora R Devel]     [Fedora PHP Devel]     [Kickstart]     [Fedora Music]     [Fedora Packaging]     [Fedora SELinux]     [Fedora Legal]     [Fedora Kernel]     [Fedora OCaml]     [Coolkey]     [Virtualization Tools]     [ET Management Tools]     [Yum Users]     [Yosemite News]     [Gnome Users]     [KDE Users]     [Fedora Art]     [Fedora Docs]     [Fedora Sparc]     [Libvirt Users]     [Fedora ARM]

  Powered by Linux