Re: [Qemu-devel] [RFC 00/27] Migration thread (WIP)

[Michael Roth <mdroth@linux.vnet.ibm.com>]

On Tue, Jul 24, 2012 at 08:36:25PM +0200, Juan Quintela wrote:
> Hi
> 
> This series are on top of the migration-next-v5 series just posted.
> 
> First of all, this is an RFC/Work in progress.  Just a lot of people
> asked for it, and I would like review of the design.
> 
> It does:
> - get a new bitmap for migration, and that bitmap uses 1 bit by page
> - it unfolds migration_buffered_file.  Only one user existed.
> - it simplifies buffered_file a lot.
> 
> - About the migration thread, special attention was giving to try to
>   get the series review-able (reviewers would tell if I got it).
> 
> Basic design:
> - we create a new thread instead of a timer function
> - we move all the migration work to that thread (but run everything
>   except the waits with the iothread lock.
> - we move all the writting to outside the iothread lock.  i.e.
>   we walk the state with the iothread hold, and copy everything to one buffer.
>   then we write that buffer to the sockets outside the iothread lock.
> - once here, we move to writting synchronously to the sockets.
> - this allows us to simplify quite a lot.
> 
> And basically, that is it.  Notice that we still do the iterate page
> walking with the iothread held.  Light testing show that we got

Is the plan to eventually hold the iothread lock only to re-sync the
dirty bitmap, and then rely on qemu_mutex_lock_ramlist() to walk the
ramlist?

It seems like the non-MRU block list, "local" migration
bitmap copy, and ramlist mutex are all toward this end, but we still
have basically the same locking protocol as before. If not, can you elaborate
on what purpose they serve in this series?

> similar speed and latencies than without the thread (notice that
> almost no optimizations done here yet).
> 
> Appart of the review:
> - Are there any locking issues that I have missed (I guess so)
> - stop all cpus correctly.  vm_stop should be called from the iothread,
>   I use the trick of using a bottom half to get that working correctly.
>   but this _implementation_ is ugly as hell.  Is there an easy way
>   of doing it?
> - Do I really have to export last_ram_offset(), there is no other way
>   of knowing the ammount of RAM?
> 
> Known issues:
> 
> - for some reason, when it has to start a 2nd round of bitmap
>   handling, it decides to dirty all pages.  Haven't found still why
>   this happens.
> 
> If you can test it, and said me where it breaks, it would also help.
> 
> Work is based on Umesh thread work, and work that Paolo Bonzini had
> work on top of that.  All the mirgation thread was done from scratch
> becase I was unable to debug why it was failing, but it "owes" a lot
> to the previous design.
> 
> Thanks in advance, Juan.
> 
> The following changes since commit a21143486b9c6d7a50b7b62877c02b3c686943cb:
> 
>   Merge remote-tracking branch 'stefanha/net' into staging (2012-07-23 13:15:34 -0500)
> 
> are available in the git repository at:
> 
> 
>   http://repo.or.cz/r/qemu/quintela.git migration-thread-v1
> 
> for you to fetch changes up to 27e539b03ba97bc37e107755bcb44511ec4c8100:
> 
>   buffered_file: unfold buffered_append in buffered_put_buffer (2012-07-24 16:46:13 +0200)
> 
> 
> Juan Quintela (23):
>   buffered_file: g_realloc() can't fail
>   savevm: Factorize ram globals reset in its own function
>   ram: introduce migration_bitmap_set_dirty()
>   ram: Introduce migration_bitmap_test_and_reset_dirty()
>   ram: Export last_ram_offset()
>   ram: introduce migration_bitmap_sync()
>   Separate migration bitmap
>   buffered_file: rename opaque to migration_state
>   buffered_file: opaque is MigrationState
>   buffered_file: unfold migrate_fd_put_buffer
>   buffered_file: unfold migrate_fd_put_ready
>   buffered_file: unfold migrate_fd_put_buffer
>   buffered_file: unfold migrate_fd_put_buffer
>   buffered_file: We can access directly to bandwidth_limit
>   buffered_file: Move from using a timer to use a thread
>   migration: make qemu_fopen_ops_buffered() return void
>   migration: stop all cpus correctly
>   migration: make writes blocking
>   migration: remove unfreeze logic
>   migration: take finer locking
>   buffered_file: Unfold the trick to restart generating migration data
>   buffered_file: don't flush on put buffer
>   buffered_file: unfold buffered_append in buffered_put_buffer
> 
> Paolo Bonzini (2):
>   split MRU ram list
>   BufferedFile: append, then flush
> 
> Umesh Deshpande (2):
>   add a version number to ram_list
>   protect the ramlist with a separate mutex
> 
>  arch_init.c      |  108 +++++++++++++++++++++++++-------
>  buffered_file.c  |  179 +++++++++++++++++-------------------------------------
>  buffered_file.h  |   12 +---
>  cpu-all.h        |   17 +++++-
>  exec-obsolete.h  |   10 ---
>  exec.c           |   45 +++++++++++---
>  migration-exec.c |    2 -
>  migration-fd.c   |    6 --
>  migration-tcp.c  |    2 +-
>  migration-unix.c |    2 -
>  migration.c      |  111 ++++++++++++++-------------------
>  migration.h      |    6 ++
>  qemu-file.h      |    5 --
>  savevm.c         |    5 --
>  14 files changed, 249 insertions(+), 261 deletions(-)
> 
> -- 
> 1.7.10.4
> 
>