[gentoo-amd64] Re: What should the swap size be for 4G ram?

[Duncan]

Richard Freeman <rich@××××××××××××××.net> posted
46F53191.8040204@××××××××××××××.net, excerpted below, on  Sat, 22 Sep 2007
11:15:29 -0400:

> That's Ok, I got a chuckle out of it.  You have Duncan who doesn't use
> swap at all (I think),

I ran entirely without for awhile back when I had a gig of memory, but I 
turned it back on to allow suspend to disk (aka hibernate), then got the 
4-way RAID before I upped memory, and set it up for 4x4-gig of striped 
swap.  The kernel will stripe swap automatically when you set swap 
partitions to the same priority, as I've done here.  I didn't setup the 
swap on top of RAID-0.

I do use swap occasionally; the last time was when I recompiled qt-3 I 
think -- with unlimited jobs and $PORTAGE_TMPDIR on tmpfs!  IIRC it ran 
up 200+ load average (1 minute), used up all 8 gigs memory, and ran 7 
gigs into swap, before the system got slow enough it wasn't updating 
ksysguard any more and I couldn't tell how much worse it got. =8^)

I'm glad I was running striped swap or it probably would have been there 
quite some time, but it ran a nearly frozen GUI for several minutes, then 
started coming back to life, as I could see the load and memory usage 
climbing back down.

FWIW I'm running the kernel 2.6.23-rcs, which of course have the new CFS 
CPU scheduler.  I wasn't too happy with it originally, as it'd zero out X 
responsiveness much faster than the old scheduler did, but it has gotten 
somewhat better as the rcs have progressed.  I don't think it's quite 
where the old one was yet, but if one truly wants "fair", one should 
expect to play with priorities/niceness a bit more to keep smooth X 
operation when running 200+ load average and heavy swapping.  It's 
certainly reasonable now; something I couldn't have said back around rc-3 
or 4, when I was pretty unhappy with it.  I guess we see how good they 
did when it releases and we see if there's any outcry on people having 
trouble with X or whatever.

Back to swap, striped swap isn't so bad.  Running more than 4 gig of 
single-spindle swap can be a killer tho, as the effects of a multi-gig 
swap-storm on a slow single-spindle disk setup aren't pretty! =8^(

That said, the main reason I run swap now is for suspend-to-disk aka 
hibernate.  It's nice to be able to have the system shut off when I'm at 
work or sleeping, and turn it back on to have it restore the session as I 
left it.  Unfortunately, it can only use a single swap device for the 
suspend image, so since I have the 16 gigs spread across 4 disks equally, 
the max suspend image I can create is 4 gigs, only half of my physical 
memory size!  I'm thinking about layering swap on top of RAID-0 
(assembled based on the kernel command line, so before the restore 
starts) to see if I can avoid that and get a full 8 gig suspend image to 
match my memory size, plus faster read-in as well, but haven't tried it 
yet and am not positive it's supported.  IOW, the automatic combination 
of swap partitions into striped swap works for swap, but not for the 
suspend image, unfortunately, so (if it works) there's still reason to 
layer swap on RAID-0 after all.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman

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