Date: Wed, 29 May 2013 16:13:39 -0400 From: Paul Kraus <paul@kraus-haus.org> To: jb <jb.1234abcd@gmail.com> Cc: freebsd-questions@freebsd.org Subject: Re: "swap" partition leads to instability? Message-ID: <7695CBFB-60A6-4E64-BB8B-FD0413271A9D@kraus-haus.org> In-Reply-To: <loom.20130529T213928-77@post.gmane.org> References: <1369558712.96152.YahooMailNeo@web165006.mail.bf1.yahoo.com> <loom.20130526T143506-872@post.gmane.org> <20130529133516.295084a6@gumby.homeunix.com> <loom.20130529T213928-77@post.gmane.org>
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On May 29, 2013, at 3:52 PM, jb <jb.1234abcd@gmail.com> wrote: > Yes, there is some confusion about the diff, if any, between paging = and=20 > swapping. >=20 > 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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