Re: [gentoo-user] Near freezes during large emerges

[William Kenworthy]

On Mon, 2011-01-17 at 03:08 +0200, Alan McKinnon wrote:
> Apparently, though unproven, at 02:15 on Monday 17 January 2011, Mark Knecht
> did opine thusly:
>
> [snip]
>
> > >> As Volker says, don't turn swap off. Make it small if you must, but
> > >> keep some around. It's just disk space.
> > >
> > > I thought swap was no longer necessary on a machine with sufficient
> > > memory.  I guess I took I some bad advice a while back.
> >
> > I think the idea is never use swap if possible, but in a case where
> > you don't have swap space or run out of swap space I think it's still
> > possible to lose data. I no longer double memory in swap. In the old
> > days I did that. On this server I have 24GB or memory. It seems silly
> > to chew up 50GB of disk space for something that almost never gets
> > touched. If I see this machine swapping I turn something off, but I'm
> > the only user and here to watch what it's doing.
>
> The 2 x RAM rule is an ancient artifact that hasn't been true for, well for
> ages now. It came about because way back when you had to have swap to get
> anything done. The question is how much? The answer sucked out of someone's
> thumb was 2xRAM. This is a pretty useless generic value, but it was less
> useless than any other default.
>

There was in ye old unix days a good technical reason for 2xmemory for
swap - google didnt confirm it for me but I think old solaris used to
coredump memory to swap on a crash.

> Picking swap amounts is like picking a wife - there's no sane default.

love the metaphor!

> A
> modern desktop that swaps is unusable - enormous amounts of data has to be
> pulled back in from the drive. A web server that swaps is already thrashing so
> you always want to avoid that.
>
> Besides, RAM is cheap and a server with 24G is common place. So is 4G on a
> notebook. So your viewpoint is completely correct.
>
> The kernel does need some swap though - it needs wiggle room for when you DO
> run out of RAM, and a little bit of swap gives that. It also staves off that
> bastard demon spawn progeny of satan called the dreaded oom killer....
>

There is one case where ~2xram is still a good idea - when hibernating
to swap using (in my case) tuxonice - 2xram gives a reasonable safety
margin for hibernation plus existing swap contents.

BillK


-- 
William Kenworthy <billk@×××××××××.au>
Home in Perth!