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Re: [libvirt] [Qemu-devel] [PATCH v4] XBZRLE delta for live migration of large memory apps

On 08/08/2011 05:15 PM, Anthony Liguori wrote:

I think workload aware migration compression is possible for a lot of
different types of workloads. That makes me a bit wary of QEMU growing
quite a lot of compression mechanisms.

It makes me think that this logic may really belong at a higher level
where more information is known about the workload. For instance, I
can imagine XBZRLE living in something like libvirt.

A better model would be plugin based.

exec helpers are plugins. They just live in a different address space and a channel to exchange data (pipe).

libvirt isn't an exec helper.

If we did .so plugins, which I'm really not opposed to, I'd want the interface to be something like:

typedef struct MigrationTransportClass
   ssize_t (*writev)(MigrationTransport *obj,
                     struct iovec *iov,
                     int iovcnt);
} MigrationTransportClass;

I think it's useful to use an interface like this because it makes it easy to put the transport in a dedicated thread that didn't hold qemu_mutex (which is sort of equivalent to using a fork'd helper but is zero-copy at the expense of less isolation).

If we have a shared object helper, the thread should be maintained by qemu proper, not the plugin.

I wouldn't call it "migration transport", but instead a compression/decompression plugin.

I don't think it merits a plugin at all though. There's limited scope for compression and it best sits in qemu proper. If anything, it needs to be more integrated (for example turning itself off if it doesn't match enough).

error compiling committee.c: too many arguments to function

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