Sukanto Ghosh wrote:
On Sat, Aug 9, 2008 at 2:31 PM, Anthony Liguori <[EMAIL PROTECTED]> wrote:
Sukanto Ghosh wrote:
I understand the idea behind ballooning as " it effectively increases
or decreases the amount of physical memory given to the guest, with
the help of the guest's native memory management algorithms". The
benefit is obvious in cases where the hypervisor does a hard
partitioning of the memory between the guests. The guest vm (virtual
memory manager) will indirectly say that these are my least used
pages, or the safest candidates for eviction.
But in case of kvm, the guest memory is itself allocated and managed
by the host linux vm. So, suppose if the guest evicts a lru page
(which might even not actually reside in host physical memory at the
time) and even the host chooses a lru page for eviction, the host
decision is better in a sense that this page is definitely residing in
the physical memory. So, are we gaining from using a balloon driver in
the guest.
What ballooning is useful for in KVM, is reducing the amount of memory a
guest uses by a large amount. It's not terribly useful for shaving a few
dozen MBs from a guest, but it is useful for dropping a 1GB guest down to
512GB.
Since the guest tells the host what portions of it's memory it won't be
using, we can evict it completely from memory (it's not even swapped, it's
just deleted).
But for deleting these pages shouldn't it be ensured that the guest
remains under the illusion that these pages are least recently used
(may be, by modifying the guest pte for those pages, so the reference
bits are always set for them) and the guest's page eviction algorithm
won't touch these pages. Later on when it is possible to give back the
guest those pages, you remove this illusion.
The guest pins the ballooned pages in memory and it won't swap them.
It's simpler than hacking the LRU data.
Another alternative will be, not to have the guest under any sort of
illusion and give it back the pages when it wants to evict them. But
this will defeat the purpose of having the balloon.
Which way is it done in kvm/linux ?
The guest is not under any illusion, it's para virtualization and the
guest kernel is aware about
all these pages.
Host request the balloon to grow->guest allocates pages and does not
touch them -> host frees these pages.
The opposite way for balloon deflate.
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to [EMAIL PROTECTED]
More majordomo info at http://vger.kernel.org/majordomo-info.html
