[RFC PATCH 0/4] MultiFD zero-copy improvements

[Leonardo Bras <leobras@redhat.com>]

RFC for an improvement suggested by Juan during the KVM Forum and a few
optimizations I found in the way.

Patch #1 is just moving code to a helper, should have no impact.

Patch #2 is my implementation of Juan's suggestion. I implemented the
simplest way I thought on the array size: a fixed defined value.
I am not sure if this is fine, or if the array size should be either
informed by the user either via QMP or cmdline.
That's an important point I really need feedback on.

Patch #3: Improve the qio_channel_flush() interface to accept flush
waiting for some writes finished instead of all of them. This reduces
the waiting time, since most recent writes/sends will take more time to
finish, while the older ones are probably finished by the first recvmsg()
syscall return.

Patch #4 uses #3 in multifd zero-copy. It flushes the LRU half of the
header array, allowing more writes to happen while the most recent ones
are ongoing, instead of waiting for everything to finish before sending
more.

It all works fine in my tests, but maybe I missed some cornercase.
Please provide any feedback you find fit.

Thank you all!

Best regards,
Leo

Leonardo Bras (4):
  migration/multifd/zero-copy: Create helper function for flushing
  migration/multifd/zero-copy: Merge header & pages send in a single
    write
  QIOChannel: Add max_pending parameter to qio_channel_flush()
  migration/multifd/zero-copy: Flush only the LRU half of the header
    array

 include/io/channel.h |  7 +++-
 migration/multifd.h  |  5 ++-
 io/channel-socket.c  |  5 ++-
 io/channel.c         |  5 ++-
 migration/multifd.c  | 88 ++++++++++++++++++++++++++------------------
 5 files changed, 68 insertions(+), 42 deletions(-)

-- 
2.38.0