The rule of thumb comes from UNIX days (BSD and even before that with
AT&T UNIX). In order to be completely sure you would be able to swap
out a program when memory became full, UNIX allocated a page of swap
for every page of virtual memory a program occupied. So if vi
required 256K to run, there was 256K of swap space allocated to it.
The 2 to 1 ratio came from the observation that a busy UNIX time-
sharing system with lots of users ran most of it's time with half the
users doing something that required CPU/memory resources and the
other half thinking, so you could afford to overcommit memory by a
factor of two.
These days, Linux uses a less straightforward, but in some ways more
effective, algorithm for deciding when and where to swap. That
combined with the availability of large cheap memory and lots of
bloated programs to fill it up, and users who expect instant
response, has made the old rule of thumb obsolete. But old habits
die hard, and you often hear the old rules of thumb quoted without
thinking where they came from.
On my own systems, I make swap huge (10 GB or more for 1 GB RAM --
Disk is cheap!) so I can mount /tmp on a tmpfs filesystem.
Rick
On Jan 22, 2008, at 8:08 PM, David Brodbeck wrote:
On Jan 21, 2008, at 5:45 PM, Ron Johnson wrote:
The old "you need 2x RAM for swap" rule is hard to forget.
I never really understood the rationale for that rule. It seems
like a system with more RAM would need less swap, not more. In
particular, it always seemed to me like it'd be a bit silly to use
8 gigs of disk for swap on a system with 4 gigs of RAM. Can
someone explain the reasons for the 2x rule? Is there a
performance boost?
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