On 05/10/2010 12:43 PM, Cam Macdonell wrote:
On Mon, May 10, 2010 at 11:25 AM, Anthony Liguori<anthony@xxxxxxxxxxxxx> wrote:
On 05/10/2010 11:59 AM, Avi Kivity wrote:
On 05/10/2010 06:38 PM, Anthony Liguori wrote:
Otherwise, if the BAR is allocated during initialization, I would have
to use MAP_FIXED to mmap the memory. This is what I did before the
qemu_ram_mmap() function was added.
What would happen to any data written to the BAR before the the
handshake completed? I think it would disappear.
You don't have to do MAP_FIXED. You can allocate a ram area and map that
in when disconnected. When you connect, you create another ram area and
memcpy() the previous ram area to the new one. You then map the second ram
area in.
But it's a shared memory area. Other peers could have connected and
written some data in. The memcpy() would destroy their data.
Why try to attempt to support multi-master shared memory? What's the
use-case?
I don't see it as multi-master, but that the latest guest to join
shouldn't have their contents take precedence. In developing this
patch, my motivation has been to let the guests decide. If the memcpy
is always done, even when no data is written, a guest cannot join
without overwriting everything.
One use case we're looking at is having VMs using a map reduce
framework like Hadoop or Phoenix running in VMs. However, if a
workqueue is stored or data transfer passes through shared memory, a
system can't scale up the number of workers because each new guest
will erase the shared memory (and the workqueue or in progress data
transfer).
(Replying again to list)
What data structure would you use? For a lockless ring queue, you can
only support a single producer and consumer. To achieve bidirectional
communication in virtio, we always use two queues.
If you're adding additional queues to support other levels of
communication, you can always use different areas of shared memory.
I guess this is the point behind the doorbell mechanism?
Regards,
Anthony Liguori
In cases where the latest guest to join wants to clear the memory, it
can do so without the automatic memcpy. The guest can do a memset
once it knows the memory is attached. My opinion is to leave it to
the guests and the application that is using the shared memory to
decide what to do on guest joins.
Cam
Regards,
Anthony Liguori
From the guest's perspective, it's totally transparent. For the backend,
I'd suggest having an explicit "initialized" ack or something so that it
knows that the data is now mapped to the guest.
From the peers' perspective, it's non-transparent :(
Also it doubles the transient memory requirement.
If you're doing just a ring queue in shared memory, it should allow
disconnect/reconnect during live migration asynchronously to the actual qemu
live migration.
Live migration of guests using shared memory is interesting. You'd need
to freeze all peers on one node, disconnect, reconnect, and restart them on
the other node.
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