[Beowulf] [OOM killer/scheduler] disabling swap on cluster nodes?

Mark Hahn hahn at mcmaster.ca
Tue Feb 10 21:25:58 PST 2015

>> is net-swap really ever a good idea?  it always seems like asking for
>> trouble, though in principle there's no reason why net IO should be
>> "worse" than disk IO...
> ... except for the need to allocate memory to build packets to send the swap 
> data.

I thought the implication was clear, that doing disk IO may also 
require memory allocations.

> There are still a few places that look at you funny if you suggest running 
> w/o swap.  The 6 orders of magnitude performance difference for random page 
> touching performance suggests you should stare them back down.

absolutely: if you have reason to believe all your pages are uniformly hot,
more power to you!

> Seriously, if you can avoid under-spec'ing/provisioning ram, you should.

in other words: buy extra ram to hold your cold pages!  after all, dram
is only O($10/GB), and disk is O($0.05/GB).  oh, wait...

> wish for the wild west of OOM shooting random things in comparison to random 
> 4k page touches.  Yes, I've seen the latter.

thrashing is bad.  it's not the same as *using* swap.  that's why swap 
still makes sense.

interesting thought: SSD is about $0.5/GB, so would make a great swap dev - 
has anyone tried tuning the swap cluster size to match the SSD flash block?

regards, mark.

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