On 04/09/2011 03:20 PM, Alexander Graf wrote:
On 09.04.2011, at 14:14, Sasha Levin wrote:
> Attempt to use mmap first for working with a disk image, if the attempt is
failed (for example, large image on a 32bit system) fallback to using read/write.
That reminds me of an idea I had quite a while back.
What if we mmap'ed a raw disk image directly into the guest's address space?
This could for example be done through a virtio feature addition, keeping the
disk accessible through normal virtio plus the mmap'ed part. At least in
writeback mode, this should perform pretty well, as we'd save all the userspace
exits. It'd basically be almost like vhost-blk :).
Have you thought about trying out to implement such a feature?
A creative idea, but I don't think it will work. On EPT hosts we don't
have accessed/dirty bits so you have to incur at least write faults to
track dirty data and perhaps read faults to gather recency information.
On non-EPT you have to scan page tables to find out what you have to
write out, and flush TLBs. Cache misses, which you'd expect there to be
quite a few, would stall the vcpu (unless you use asynchronous page
faults) and contribute less information to the host than virtio-blk
(location of access but not size). Write misses are converted to
read-modify-write operations.
Even in userspace I think mmap() is usually not a performance win. It's
advantages are that it's simple to use and has low set-up time, which is
useful for short lived processes, especially with read-only backing
files. For virtual machines it's much worse.
--
error compiling committee.c: too many arguments to function
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