[OpenIndiana-discuss] Swap during install
richard.elling at richardelling.com
Tue Sep 25 20:00:47 UTC 2012
On Sep 25, 2012, at 11:41 AM, Peter Tribble <peter.tribble at gmail.com> wrote:
> On Tue, Sep 25, 2012 at 1:50 PM, Richard Elling
> <richard.elling at richardelling.com> wrote:
>> Use what you need. Most people don't need or want to use swap. Why? Because...
>> if you have to swap, performance will suck. Period. Case closed. Game, set, match.
>> HDDs are 5 orders of magnitude slower than RAM and all the king's horses and all
>> the king's men can't fix that.
>> The old rule of "2x" RAM has not been true since around the time you could put 1GB of
>> RAM into a machine. Interestingly, the place we normally see vehement arguments for
>> 2x RAM is from Oracle DBAs who believe everything ever written in an Oracle manual :-)
>> hint: run "swap -l" and see if free == blocks. If so, then you've never used
>> swap since the system was booted.
> That's not true. Anonymous reservations go against swap, if it's available.
Technically, you are correct...
> Not having adequate swap can kill performance, because you end up forcing
> reservations to be made against real memory rather than swap. And I would
> much rather have idle garbage sat out in swap rather than have it block
> valuable RAM. In both cases inadequate swap depletes the availability of
> real memory, and you want as much of that free as possible.
... in which case swap will have been used. So the test still stands as empirically
For the curious, pmap -x will show the summary of anonymous memory
mapping for a process. For the whole system, as root:
echo ::memstat | mdb -k
Measure it, and use what you need.
> I run general-purpose workloads, and find 2x RAM to be a good starting
> point. I've got systems with 32G RAM with 32G swap in use (half really in
> use, half just reserved). They aren't swapping, at all, performance isn't
> impacted, whereas if I didn't have the swap some applications wouldn't
> even run.
> So it varies; with disk relatively cheap I would rather be generous than
32GB is 2 DIMMs today. It is quite easy to build a machine with 256GB for
some more dollars, 1TB for a modest pile of dollars, at which point 2x RAM for
swap becomes silly once again.
illumos Day & ZFS Day, Oct 1-2, 2012 San Fransisco
Richard.Elling at RichardElling.com
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