Re: [PATCH 00 of 28] Transparent Hugepage support #2

[Dave Hansen <dave@linux.vnet.ibm.com>]

On Fri, 2009-12-18 at 12:28 -0600, Christoph Lameter wrote:
> On Fri, 18 Dec 2009, Ingo Molnar wrote:
> > Note that it became more relevant in the past few years due to the arrival of
> > low-latency, lots-of-iops and cheap SSDs. Even on a low end server you can buy
> > a good 160 GB SSD for emergency swap with fantastic latency and for a lot less
> > money than 160 GB of real RAM. (which RAM wont even fit physically on typical
> > mainboards, is much more expensive and uses up more power and is less
> > servicable)
> 
> Swap occurs in page size chunks. SSDs may help but its still a desaster
> area. You can only realistically use swap in a batch environment. It kills
> desktop performance etc etc.

True...  Let's say it takes you down to 20% of native performance.
There are plenty of cases where people are selling Xen or KVM slices
where 20% of native performance is more than *fine*.  It may also let
you have VMs that are 3x more dense than they would be able to be
otherwise.  Yes, it kills performance, but performance isn't everything.

For many people price/performance is much more important, and swapping
really helps the price side of that equation.

We *do* need to work on making swap more useful in a wide range of
workloads, especially since SSDs have changed some of our assumptions
about swap.  I just got a laptop SSD this week, and tuned swappiness so
that I'd get some more swap activity.  Things really bogged down, so I
*know* there's work to do there.

-- Dave

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