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

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Mar 21, 2013 at 10:53:55AM +0100, Paolo Bonzini wrote:
> Il 21/03/2013 10:48, Michael S. Tsirkin ha scritto:
> > A recent discussion about overcommitted memory made me think:
> > this will still need to read pages into memory even
> > if all we will do is send it out on the wire immediately,
> > dirtying cache etc.
> > 
> > Now, one property of RAM writes is that if guest changes
> > the memory that we are migrating, we really
> > don't care that remote will get a new copy and not
> > the old copy.
> 
> Orit's patches already do this (patch 11).  The only copy that remains
> is from memory to socket buffers.
> 
> 
> > For this to work, however, we need to have a way
> > for QEMUFile to know whether a specific iovec
> > references RAM (so it's ok to use vmsplice)
> > or a malloced buffer for device state (so it must use write
> > to ensure kernel copies data).
> > 
> > What do you think?
> 
> Was SPLICE_F_GIFT/SPLICE_F_MOVE ever implemented in the end?  If not,
> patch 11 of this series does the same thing you are suggesting, without
> the indirection of an additional pipe file descriptor that is required
> for splice/vmsplice.
> 
> Paolo

GIFT seems to be implemented so no problem there.
MOVE seems not to be needed for TCP - I think it just uses sendpage
internally which will do zero copy without special hints.


> >> 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(-)
> >>
> >> -- 
> >> 1.7.11.7