Date: Fri, 18 Jun 1999 09:02:37 +0930 From: Greg Lehey <grog@lemis.com> To: Mitch Collinsworth <mkc@Graphics.Cornell.EDU> Cc: Thomas David Rivers <rivers@dignus.com>, rfg@monkeys.com, questions@FreeBSD.ORG Subject: Swap usage (was: Desperate to shrink a partition) Message-ID: <19990618090237.M9893@freebie.lemis.com> In-Reply-To: <199906172028.AA210841314@broccoli.graphics.cornell.edu>; from Mitch Collinsworth on Thu, Jun 17, 1999 at 04:28:33PM -0400 References: <199906171950.PAA77238@lakes.dignus.com> <199906172028.AA210841314@broccoli.graphics.cornell.edu>
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On Thursday, 17 June 1999 at 16:28:33 -0400, Mitch Collinsworth wrote: > >> And - it may be a requirement that swap be as large as physical memory, >> if not twice as large... > > On p. 62 of the Lehey book (2nd ed.) there is a lengthy discussion > entitled "How much swap?" > > The last point says: > > "Even with lightly memory loads, the virtual memory system slowly pages > out data in preparation for a possible sudden demand for memory. This > means that it can be mor responsive to such requests. As a result, you > should have at least as much swap as memory." Right. But that's a "should", not a "must". Here's the complete text: It's very difficult to predict how much swap space you need. The automatic option gave us 42 MB. In the manual example, we chose 32 MB. Maybe you can get by with 16 MB. Maybe you'll need 512 MB. How do you decide? It's almost impossible to know in advance what your system will require. Here are some considerations: o Swap space is needed for all pages of virtual memory which contain data that is not locked in memory and which can't be recreated automatically. This is the majority of virtual memory in the system. o Some people use rules of thumb like ``2.5 times the size of physical memory, or 64 MB, whichever is bigger''. These rules work only by making assumptions about your workload. If you're using more than 2.5 times as much swap space as physical memory, performance will suffer. o Known memory hogs are X11 and the GNU C compiler (gcc). If you use these, you will probably need more swap space. o You can add additional swap partitions on other disks. This has the additional advantage of balancing the disk load if your machine swaps a lot. o About the only ways to change the size of a swap partition are to add another partition or to reinstall the system, so if you're not sure, a little bit more won't do any harm, but too little can really be a problem. o If your system panics, and memory dumping is enabled, it will write the contents of memory to the swap partition. This will obviously not work if your swap partition is smaller than main memory. Under these circumstances, the system refuses to dump, but it's not impossible that a bug might cause the dump to write beyond the bounds of the swap partition, probably causing irreperable damage to your /usr partition. o Even with light memory loads, the virtual memory system slowly pages out data in preparation for a possible sudden demand for memory. This means that it can be more responsive to such requests. As a result, you should have at least as much swap as memory. A couple of examples might make this clearer: 1. I run X, StarOffice, Netscape and a whole lot of other memory-hungry applications on my old 486 with 16 MB. Sure, it's really slow, especially when changing from one application to another, but it works. Since there's not much memory, it uses a lot of swap. To view the current swap usage, use pstat. Here's a typical view of this machine's swap space: $ pstat -s Device 1024-blocks Used Avail Capacity Type /dev/sd0s1b 122880 65148 57668 53% Interleaved 2. I run much more stuff on my Pentium with 96 MB of memory. I've got lots of swap space, but what I see is: $ pstat -s Device 1024-blocks Used Avail Capacity Type /dev/wd0s1b 51200 14416 36720 28% Interleaved /dev/sd0b 66036 14332 51640 22% Interleaved /dev/sd2b 204800 14384 190352 7% Interleaved Total 321844 43132 278712 13% It's not so important that the Pentium is using less swap: it's using 0.67 of its memory in swap, whereas the 486 is using 4 times its memory. Look at it from a different point, and it makes more sense: swap makes up for the lack of real memory, so the 486 is using a total of 80 MB of memory, and the Pentium is using 140 MB. In other words, there is a tendency to be able to say ``the more main memory you have, the less swap you need''. If, however, you look at it from the point of view of acceptable performance, you will hear things like ``you need at least one-third of your virtual memory in real memory''. That makes sense from a performance point of view, assuming all processes are relatively active. And, of course, it's another way of saying ``take twice as much swap as real memory''. In summary: be generous in allocating swap space. In this example, we have a tiny disk, so we're forced to use a small swap partition. If you have the choice, use more. If you really can't make up your mind, take 256 MB of swap space. > [Buy the book, it's loaded with useful stuff, even if you've been > a sysadmin for decades.] Thanks. Greg -- When replying to this message, please copy the original recipients. For more information, see http://www.lemis.com/questions.html See complete headers for address, home page and phone numbers finger grog@lemis.com for PGP public key To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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