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Re: [Qemu-devel] [RFC 00/12] Migration: Remove copying of guest ram page
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [RFC 00/12] Migration: Remove copying of guest ram pages |
Date: |
Thu, 21 Mar 2013 13:09:10 +0200 |
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
- [Qemu-devel] [RFC 08/12] Use writev ops instead of put_buffer ops, (continued)
- [Qemu-devel] [RFC 08/12] Use writev ops instead of put_buffer ops, Orit Wasserman, 2013/03/21
- [Qemu-devel] [RFC 09/12] More optimized qemu_put_be64/32/16, Orit Wasserman, 2013/03/21
- [Qemu-devel] [RFC 10/12] Add qemu_put_buffer_no_copy, Orit Wasserman, 2013/03/21
- [Qemu-devel] [RFC 11/12] Use qemu_put_buffer_no_copy for guest memory pages, Orit Wasserman, 2013/03/21
- [Qemu-devel] [RFC 12/12] Bye Bye put_buffer, Orit Wasserman, 2013/03/21
- Re: [Qemu-devel] [RFC 00/12] Migration: Remove copying of guest ram pages, Paolo Bonzini, 2013/03/21
- Re: [Qemu-devel] [RFC 00/12] Migration: Remove copying of guest ram pages, Michael S. Tsirkin, 2013/03/21