Re: [Qemu-devel] [PATCH v3 0/9] Migration: Remove copying of guest ram pages

[Paolo Bonzini]

Il 21/03/2013 17:05, Orit Wasserman ha scritto:
> In migration all data is copied to a static buffer in QEMUFile,
> this hurts our network bandwidth and CPU usage especially with large guests.
> We switched to iovec for storing different buffers to send (even a byte field 
> is
> considered as a buffer) and use writev to send the iovec.
> writev was chosen (as apposed to sendmsg) because it supprts non socket fds.
>   
> Guest memory pages are not copied by calling a new function 
> qemu_put_buffer_no_copy.
> The page header data and device state data are still copied into the static
> buffer. This data consists of a lot of bytes and integer fields and the static
> buffer is used to store it during batching.
> Another improvement is changing qemu_putbe64/32/16 to create a single
> buffer instead of several byte sized buffer.
> 
> git repository: git://github.com/oritwas/qemu.git sendv_v2
> 
> Change from v2:
> Always send data for the iovec even if writev_buffer is not implemented.
> Coalesce adjacent iovecs to create one big buffer from small adjacent buffer.
> 
> Changes from v1:
> Use iov_send for socket.
> Make writev_buffer optional and if it is not implemented use put_buffer

I didn't review it too closely, so I don't feel like giving my
Reviewed-by.  Still, I must say it is a very nice and simple solution.
Kudos!

Just one question: patch 6 is not strictly needed anymore now that you
have coalescing, no?  Should Juan leave it out?

Paolo