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

[Orit Wasserman <owasserm@redhat.com>]

On 03/21/2013 11:29 AM, Paolo Bonzini wrote:
> 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.
Sure, I can change the code that if put_buffer exists to revert to the previous
implementation.

Orit
> 
> 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(-)
>>
>