On May 29, 2013, at 3:52 PM, jb <[email protected]> wrote:
> Yes, there is some confusion about the diff, if any, between paging and
> swapping.
>
> Paging - copying or moving pages between physical memory (RAM) and secondary
> storage (e.g. hard disk), in both directions.
> Swapping - nowdays is synonymous with "paging".
> You say that FB supports both, Linux supports paging only.
> Well, Linux utilizes swap space as part of virtual memory.
> So, can you elaborate more on that - what is the essence of the diff, why
> should I avoid the term "swapping" when referring to Linux, assuming VMM
> systems on both ?
When I started working professionally with Unix systems in 1995, I was
taught that paging was the process of copying least used "pages" of RAM onto
disk so that the RAM could be freed if the system needed more RAM. Swapping was
the process of moving an entire program from RAM to disk in order to free up
RAM.
In other words, a process can be "swapped out" and placed on disk until
it comes up to run again, at which point it can be "swapped in" and executed.
I think that much of the confusion comes from the use of the SWAP
device by the PAGING system. When the concept of paging came about, it just
used the already existing SWAP space to store it's "paged out" pages of memory.
On the systems I worked on at the time (SunOS / Solaris), paging was a
sign of pressure on the physical memory (RAM) of a system, swapping was a sign
of _severe_ physical memory pressure. This was a time when we configured 2 to 4
times the amount of physical RAM as SWAP space. RAM was very expensive and hard
drives just expensive :-) It was common on a "normally" operating system to see
the page scanner* running up to 100 times per second. A scan rate of over 100
was considered a sign of pressure on RAM that needed to be addressed, any
SWAPing was considered a sign that the system needed more physical RAM.
Today RAM is so cheap that _any_ paging is often considered bad and an
indication that more Ram should be added.
*Solaris Page Scanner: This is a kernel level process that wakes up, examines
the amount of free RAM, and takes action based on that value. The thresholds
are all dynamic and based on the amount of RAM in the system. Above a high
water mark the scanner does nothing. As the amount of free RAM drops, various
pages of RAM are copied to SWAP space and the RAM freed. Eventually, if the
amount of free Ram falls low enough, even parts of the kernel will be paged
out. This is very bad and can lead to a system "thrashing" where it spends the
vast majority of it's time just paging in and out and not actually getting
anything done.
--
Paul Kraus
Deputy Technical Director, LoneStarCon 3
Sound Coordinator, Schenectady Light Opera Company
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