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

[Juan Quintela <quintela@redhat.com>]

Orit Wasserman <owasserm@redhat.com> wrote:
> 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

it is still sendv_v2 or sendv_v3 O:-)

> 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
>
> Future work: Make number of iovec changeable
>
> Orit Wasserman (9):
>   Add QemuFileWritevBuffer QemuFileOps
>   Add socket_writev_buffer function
>   Update bytes_xfer in qemu_put_byte
>   Store the data to send also in iovec
>   Use writev ops if available
>   More optimized qemu_put_be64/32/16
>   Add qemu_put_buffer_no_copy
>   Use qemu_put_buffer_no_copy for guest memory pages
>   coalesce adjacent iovecs
>
>  arch_init.c                   |   2 +-
>  include/migration/qemu-file.h |  12 ++++
>  savevm.c                      | 127 ++++++++++++++++++++++++++++++++++--------
>  3 files changed, 116 insertions(+), 25 deletions(-)

Really nice series.  Just a couple of small nits.

Later, Juan.