Subject: Re: Swap overcommit (was Re: Replacement for grep(1) (part 2))
To: None <freebsd-hackers@FreeBSD.ORG>
From: Noriyuki Soda <>
List: tech-userlevel
Date: 07/16/1999 02:38:22
> On Thu, 15 Jul 1999, Daniel C. Sobral wrote:
>> Uh... like any modern unix, Solaris overcommits.

>>>>> On Thu, 15 Jul 1999 08:46:36 -0700 (PDT),
	"Eduardo E. Horvath" <> said:

> Where do you guys get this misinformation?  
> Note the `19464k reserved'; that space has been reserved but not yet
> allocated.

Both Dillon and Sobral mistakenly claimed that "Solaris overcommits",
this fact seems to be somewhat suggestive.

And also, the followings are allocated memory and reserved memory 
in my environment. (This table also includes Eduardo's example)

	SunOS	allocated reserved    total total/allocated
	-----	--------- -------- -------- ------------
	4.1.4       4268k    1248k    5516k 1.2924  
	4.1.2       7732k    1492k    9224k 1.193   
	4.1.4       8848k    3080k   11928k 1.3481  
	4.1.4      13532k    6772k   20304k 1.5004  
	5.5.1      15312k    5092k   20404k 1.3325  
	4.1.3      16112k    6512k   22624k 1.4042  
	4.1.2      26356k    1620k   27976k 1.0615  
	4.1.4      26560k    3756k   30316k 1.1414  
	5.5        26076k   11348k   37424k 1.4352  
	4.1.4      32984k    5556k   38540k 1.1684  
	5.6        32448k    7072k   39520k 1.2179  
	4.1.4      38056k    3692k   41748k 1.097   
	4.1.4      49064k    7672k   56736k 1.1564  
	4.1.4      67012k    7800k   74812k 1.1164  
	4.1.4      99348k   16956k  116304k 1.1707  
	4.1.4     118288k   11780k  130068k 1.0996  
	5.6       231968k   18880k  250848k 1.0814  
	5.7       307240k   19464k  326704k 1.0634  

	(sorted by total amount of used swap)

In those examples, non-overcommiting system requires 1.06x ... 1.50x
more swap space than overcommiting system.  This table also indicates
that in proportion as total used swap increase the ratio will
decrease. And extra swap space required on non-overcommiting system is
approximately several tens mega bytes. i.e. The extra cost of
non-overcommiting system is less than ten dollers in my environment.

Matt Dillon claimed that non-overcommiting system requires 8x or more
swap space than overcommiting system. That's just wrong as above.
(There might be cases which requires 8x swap, but it is not typical
 like Dillon said.)

If you don't want non-overcommiting system, because you don't want to
pay it's cost. That's OK, but please don't force us to accept your
limited view.