Kenni Lund wrote: > Hi > > I was wondering; Does it make any sense to use swap inside guests? > Wouldn't it give better performance to just skip swap entirely in the > guest, assign it more memory and then increase the swap size on the > host? The Gold Rule: do not swap guests. If a guest does not know it is swapped, and an interrupt comes which requires memory which is swapped on host but guest thinks it's right here, all bad things happens. Guest is not ready to face a situation when access to system RAM takes several _seconds_, usually it's in range of nanoseconds. For another thing, guest knows better what to swap. Generally, swap serves as a safety measure in case we run into a situation when the amount of RAM isn't sufficient. Amount of memory should be enough for "typical" workload of a system, and swap serves for situation where it requires a bit more during peaks etc. For that, let the particular guest handle the (light) swap load, instead of slowing down _all_ guests by swapping in host. (And a side note to stop possible flame war here: there are different views as of what swap is and is not. Another "version" is that it is used to free system memory from tasks which are not used right now, and so on. The main rule stands still: there should be very few swapping. And for light swapping it does not really matter if we do swapping a bit faster in host than a bit slower in guest (due to virtual i/o overhead): the difference isn't that high and for _small_ amount of swapping it does not really matter speed- wise. But swapping in guest makes things more predictable and smooth due to all the above). /mjt -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
