Re: [Qemu-devel] [RFC 00/12] Migration: Remove copying of guest ram pages

[Paolo Bonzini]

Il 21/03/2013 10:09, 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.

Very nice!  I just disagree on making writev_buffer mandatory; instead,
a QemuFileOps could choose between implementing either put_buffer or
writev_buffer.  This removes the duplicate code you have between
iov_writev and block_writev_buffer.

Thanks,

Paolo

> Orit Wasserman (12):
>   Add iov_writev to use writev to send iovec (also for files)
>   Add QemuFileWritevBuffer QemuFileOps
>   Add socket_writev_buffer function
>   Add stdio_writev_buffer function
>   Add block_writev_buffer function
>   Update bytes_xfer in qemu_put_byte
>   Store the data to send also in iovec
>   Use writev ops instead of put_buffer ops
>   More optimized qemu_put_be64/32/16
>   Add qemu_put_buffer_no_copy
>   Use qemu_put_buffer_no_copy for guest memory pages
>   Bye Bye put_buffer
> 
>  arch_init.c                   |   2 +-
>  include/migration/qemu-file.h |  20 ++++---
>  include/qemu/iov.h            |  12 ++++
>  savevm.c                      | 130 
> +++++++++++++++++++++++++-----------------
>  util/iov.c                    |  36 ++++++++++++
>  5 files changed, 139 insertions(+), 61 deletions(-)
>