More on swap.

[Aaron Konstam]

Yesterday in answer to the question as to how much swap is needed it was
suggested to read the article at:
http://www.redhat.com/magaszine/001nov04/features/vm/

Well this is an interesting article but it tells one nothing about swap
fits into the virtual memory structure for the 2.4 kernel in this case.

Unfortunately , it is sort of complicated and as other people have said
how much swap you need depends a good deal on what you are doing. Swap
is an concept whose name was incorrectly borrowed from the OLD 370 and
360 IBM systems and to understand how it fits in to the VM system of the
Linux kernel you need to read books like, "Linux Kernel Development,
Second Edition", by Robert Love.

In many cases you can run a Linux system without swap.
--
=======================================================================
You have a tendency to feel you are superior to most computers.
=======================================================================
Aaron Konstam telephone: [phone] e-mail: akonstam(a)sbcglobal.net

[Laurent Vaills]

Here is the correct link :
http://www.redhat.com/magazine/001nov04/features/vm/

Laurent


2007/2/28, Aaron Konstam <akonstam(a)sbcglobal.net&gt;:
>  Yesterday in answer to the question as to how much swap is needed it was
>  suggested to read the article at:
>  http://www.redhat.com/magaszine/001nov04/features/vm/
>
>  Well this is an interesting article but it tells one nothing about swap
>  fits into the virtual memory structure for the 2.4 kernel in this case.
>
>  Unfortunately , it is sort of complicated and as other people have said
>  how much swap you need depends a good deal on what you are doing. Swap
>  is an concept whose name was incorrectly borrowed from the OLD 370 and
>  360 IBM systems and to understand how it fits in to the VM system of the
>  Linux kernel you need to read books like, "Linux Kernel Development,
>  Second Edition", by Robert Love.
>
>  In many cases you can run a Linux system without swap.
>  --
>  =======================================================================
>  You have a tendency to feel you are superior to most computers.
>  =======================================================================
>  Aaron Konstam telephone: [phone] e-mail: akonstam(a)sbcglobal.net
>
>  --
>  fedora-list mailing list
>  fedora-list(a)redhat.com
>  To unsubscribe: https://www.redhat.com/mailman/listinfo/fedora-list
>

[Aaron Konstam]

On Wed, 2007-02-28 at 16:30 +0100, Laurent Vaills wrote:
>  Here is the correct link : 
>  http://www.redhat.com/magazine/001nov04/features/vm/
>  
>  Laurent 
Huh!. Is that not the link I mentined below? Have I spelled something
wrong?
>  
>  
>  
>  
>  
>  2007/2/28, Aaron Konstam < akonstam(a)sbcglobal.net&gt;:
>          Yesterday in answer to the question as to how much swap is
>          needed it was 
>          suggested to read the article at:
>          http://www.redhat.com/magaszine/001nov04/features/vm/
>          
>          Well this is an interesting article but it tells one nothing
>          about swap 
>          fits into the virtual memory structure for the 2.4 kernel in
>          this case.
>          
>          Unfortunately , it is sort of complicated and as other people
>          have said
>          how much swap you need depends a good deal on what you are
>          doing. Swap 
>          is an concept whose name was incorrectly borrowed from the OLD
>          370 and
>          360 IBM systems and to understand how it fits in to the VM
>          system of the
>          Linux kernel you need to read books like, "Linux Kernel
>          Development, 
>          Second Edition", by Robert Love.
>          
>          In many cases you can run a Linux system without swap.
>          --
>          =======================================================================
>          You have a tendency to feel you are superior to most
>          computers. 
>          =======================================================================
>          Aaron Konstam telephone: [phone] e-mail:
>          akonstam(a)sbcglobal.net
>          
>          --
>          fedora-list mailing list 
>          fedora-list(a)redhat.com
>          To unsubscribe:
>          https://www.redhat.com/mailman/listinfo/fedora-list
>  
>  -- 
>  fedora-list mailing list
>  fedora-list(a)redhat.com
>  To unsubscribe: https://www.redhat.com/mailman/listinfo/fedora-list 
--
=======================================================================
It has just been discovered that research causes cancer in rats.
=======================================================================
Aaron Konstam telephone: [phone] e-mail: akonstam(a)sbcglobal.net

[Todd Zullinger]

Aaron Konstam wrote:
>  Have I spelled something wrong? 
Yes.  An 's' crept in to the word magazine.  Perhaps you were thinking
like you were from England or Oz? ;-)

 http://www.redhat.com/magazine/001nov04/features/vm/ !=
 http://www.redhat.com/magaszine/001nov04/features/vm/

-- 
Todd        OpenPGP -> KeyID: 0xBEAF0CE3 | URL: www.pobox.com/~tmz/pgp
======================================================================
There, I've gone and soiled myself, are you happy now?!
    -- Stewie Griffin

More on swap. clarification

[Aaron Konstam]

In a previous post I made the statement below.
>  
>  In many cases you can run a Linux system without swap.
>  -- 
To avoid possible rants, let me say I do not recommend running Linux
without swap.
--
=======================================================================
Life in the state of nature is solitary, poor, nasty, brutish, and
short. - Thomas Hobbes, Leviathan
=======================================================================
Aaron Konstam telephone: [phone] e-mail: akonstam(a)sbcglobal.net

More on swap. clarification

[Laurent Vaills]

2007/2/28, Aaron Konstam <akonstam(a)sbcglobal.net&gt;:
>  In a previous post I made the statement below.
>  >
>  > In many cases you can run a Linux system without swap.
>  > --
>  To avoid possible rants, let me say I do not recommend running Linux
>  without swap. 


I'm currently experimenting that , and I have some freezes. I think I'll use
a swap file as someone advised me earlier on this list.

Laurent


--
> =======================================================================
>  Life in the state of nature is solitary, poor, nasty, brutish, and
>  short. - Thomas Hobbes, Leviathan
>  =======================================================================
>  Aaron Konstam telephone: [phone] e-mail: akonstam(a)sbcglobal.net
>
>  --
>  fedora-list mailing list
>  fedora-list(a)redhat.com
>  To unsubscribe: https://www.redhat.com/mailman/listinfo/fedora-list
>

[Gordon Messmer]

Aaron Konstam wrote:
>  
>  Well this is an interesting article but it tells one nothing about swap
>  fits into the virtual memory structure for the 2.4 kernel in this case. 
No, but it does cover the "overcommit" feature of the Linux VM. 
Understanding overcommit and the requirements of fork() in a posix 
environment are critical to deciding how much swap you need.

Generally, the overcommit feature may allow some things to work better, 
but introduces some serious problems when the assumptions behind 
overcommit don't apply.  Hence, it's off by default.  If you're not 
using overcommit, then you should have considerably more swap than you 
intend to actually use.  Swapping is slow, and many people prefer to 
have very little because they believe that if they allow memory use to 
grow that large, the system will be very slow to respond.  In some cases 
they are right.  However, if you have insufficient swap, the login 
processes may not be able to fork, and you may not be able to log in and 
correct the problem.  Larger applications may be unable to fork, and use 
external helpers, even when there appears to be sufficient memory for 
the helpers.

I'm of the opinion that it's best to have plenty of swap, as previously 
discussed.

[Aaron Konstam]

On Wed, 2007-02-28 at 12:36 -0800, Gordon Messmer wrote:
>  Aaron Konstam wrote:
>  > 
>  > Well this is an interesting article but it tells one nothing about swap
>  > fits into the virtual memory structure for the 2.4 kernel in this case.
>  
>  No, but it does cover the "overcommit" feature of the Linux VM. 
>  Understanding overcommit and the requirements of fork() in a posix 
>  environment are critical to deciding how much swap you need. 
The only point I am
trying to make is that strictly speaking in a VM
system when the memory gets full segments (or pages) from memory are
sent back to the disk (in the place on the disk where they come from).
The swap area has a system of indexing the pages it contains that allows
the system to retrieve segments (or pages) from the swap area faster
than they can be retrieved from the regular disk partition. What Unix
(and Linux) introduced is a algorithm to decide under what circumstances
the strict requirements of a VM system would be violated and segments
(or pages) would be temporarily placed in the swap space rather than
back into their original position on the disk. Unless you know under
which circumstances this occurs you can't really evaluate your need for
swap space.
>  
>  Generally, the overcommit feature may allow some things to work better, 
>  but introduces some serious problems when the assumptions behind 
>  overcommit don't apply.  Hence, it's off by default.  If you're not 
>  using overcommit, then you should have considerably more swap than you 
>  intend to actually use.  Swapping is slow, and many people prefer to 
>  have very little because they believe that if they allow memory use to 
>  grow that large, the system will be very slow to respond.  In some cases 
>  they are right.  However, if you have insufficient swap, the login 
>  processes may not be able to fork, and you may not be able to log in and 
>  correct the problem.  Larger applications may be unable to fork, and use 
>  external helpers, even when there appears to be sufficient memory for 
>  the helpers.
>  
>  I'm of the opinion that it's best to have plenty of swap, as previously 
>  discussed.
>   
--
=======================================================================
A penny saved kills your career in government.
=======================================================================
Aaron Konstam telephone: [phone] e-mail: akonstam(a)sbcglobal.net

[James Wilkinson]

Aaron Konstam wrote:
>  The only point I am trying to make is that strictly speaking in a VM
>  system when the memory gets full segments (or pages) from memory are
>  sent back to the disk (in the place on the disk where they come from). 
Anonymous pages (those that weren't on disk in the first place, such as
temporary data structures, or where the data originally came from disk,
the data in memory has been modified, but the data on disk is not
supposed to be modified) obviously can't be sent back to the place on
disk where they came from! They have to go to swap space.

>  The swap area has a system of indexing the pages it contains that
> allows
>  the system to retrieve segments (or pages) from the swap area faster
>  than they can be retrieved from the regular disk partition. What Unix
>  (and Linux) introduced is a algorithm to decide under what circumstances
>  the strict requirements of a VM system would be violated and segments
>  (or pages) would be temporarily placed in the swap space rather than
>  back into their original position on the disk. Unless you know under
>  which circumstances this occurs you can't really evaluate your need for
>  swap space. 
This sounds implausible. By far the greatest cost of swapping is simply
the time it takes for the disk drive to read or write the data -- any
overhead in fancy algorithms really is lost in the noise.

James.

-- 
E-mail:     james@ | In a serial interface, the data bits move down a single
aprilcottage.co.uk | channel one after the other, like railway trains. This is
                   | different from the parallel interface in which groups of
                   | bits arrive together, like London buses.
                   |     -- 'The Computer Dictionary', Jon Wedge