On Sun August 16 2009, Dor Laor wrote: > On 08/16/2009 05:18 PM, Thomas Fjellstrom wrote: > > On Sun August 16 2009, Avi Kivity wrote: > >> On 08/16/2009 12:55 PM, Thomas Fjellstrom wrote: > >>> I'm wondering if kvm supports automatic memory ballooning. I've had a > >>> kvm guest running for a couple days, and the balloon driver was loaded, > >>> and I could manually change the amount of ram it had allocated in the > >>> console, but it never seemed to change automatically. > >>> > >>> Is there any support for that? > >> > >> That would be part of a management application. qemu only knows about > >> the guest it controls, while ballooning needs a global view of the > >> system. > > > > All a single guest needs to do is only use as much ram as it needs at any > > given time (up to the max allocated). So if the guest hasn't used much > > ram in a given time frame, "free" the free ram from the host, and only > > reallocate when needed. It doesn't _need_ a management application, just > > happens to be the way people do it. > > This is far from being an accurate description of the reality ( ;) ) > You cannot just expect the guest to do so. The guest has page cache that > uses memory, it might run many processes that consume lots of memory, etc. > Even if you could have done it, the translation between the guest-host > is not 1-1 and the host needs to be aware of the guest memory usage. > > This is what ballooning does. A target is determined by host management > daemon. As a response, the guest balloon driver try to allocate memory > and pass it as Guest Physical Addresses to the host. Now the host can > use madvise in order to mark these pages as not needed (and free the mmu > of pinning them). > > The complexity is for the management to dynamically shift memory between > the host and the guest to reach maximum performance. > > Regards, > Dor > -- > 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 One thing I found odd about kvm's ballooning is that it actually seems to change how much ram the guest has. I really didn't expect "free -m" to report that the guest only had 64M ram after I manually ballooned the ram. I was however expecting it just to free ram it wasn't using in the host. To me, it just doesn't seem to be the same thing. now it'll start swapping at 64M ram instead of just reallocating the ram it used to have. -- Thomas Fjellstrom tfjellstrom@xxxxxxx -- 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
