Re: [RFC] VM: I have a dream...

[Jamie Lokier <jamie@shareable.org>]

linux-os (Dick Johnson) wrote:
> On Mon, 23 Jan 2006, Diego Calleja wrote:
> > However, I doubt the approach is really useful. If you need that much
> > swap space, you're going well beyond the capabilities of the machine.
> > In fact, I bet that most of the cases of machines needing too much
> > memory will be because of bugs in the programs and OOM'ing would be
> > a better solution.
> 
> You have roughly 2 GB of dynamic address-space avaliable to each
> task (stuff that's not the kernel and not the runtime libraries).
> You can easily have 500 tasks, even RedHat out-of-the-box creates
> about 60 tasks. That's 1,000 GB of potential swap-space required
> to support this.

And how many machines is it useful to use that much swap-space on?

> This is not beyond the capabilites of a 32-bit
> machine with a fast front-side bus and fast I/O (like wide SCSI).

Anything but the most expensively RAM-equipped machine would be stuck
in a useless swap-storm, if it's got 1000GB of GB of active swap space
and only a relatively tiny amount of physical RAM (e.g. 16GB).  The
same is true if only, say, 10% of the swap space is in active use.

Wide SCSI isn't fast enough to make that useful.

I think that was the point Diego was making: you can use that much
swap space, but by the time you do, whatever task you hoped to
accomplish won't get anywhere due to the swap-storm.

> Some persons tend to forget that 32-bit address space is available
> to every user, some is shared, some is not. A reasonable rule-of-
> thumb is to provide enough swap-space to duplicate the address-
> space of every potential task.

I think that's a ridiculous rule of thumb.  Not least because (a) even
the biggest drive available (e.g. 1TB) doesn't provide that much
swap-space, and (b) if you're actively using only a tiny fraction of
that, your machine has already become uselessly slow - even root
logins and command prompts don't work under those conditions.

-- Jamie
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