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Re: [Qemu-devel] [RFC 00/13] Multiple fd migration support

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [RFC 00/13] Multiple fd migration support
Date: Wed, 20 Apr 2016 18:46:31 +0300

On Wed, Apr 20, 2016 at 04:44:28PM +0200, Juan Quintela wrote:
> Hi
> This patch series is "an" initial implementation of multiple fd migration.
> This is to get something out for others to comment, it is not finished at all.
> So far:
> - we create threads for each new fd
> - only for tcp of course, rest of transports are out of luck
>   I need to integrate this with daniel channel changes
> - I *think* the locking is right, at least I don't get more random
>   lookups (and yes, it was not trivial).  And yes, I think that the
>   compression code locking is not completely correct.  I think it
>   would be much, much better to do the compression code on top of this
>   (will avoid a lot of copies), but I need to finish this first.
> - Last patch, I add a BIG hack to try to know what the real bandwidth
>   is.
> Preleminar testing so far:
> - quite good, the latency is much better, but was change so far, I
>   think I found the problem for the random high latencies, but more
>   testing is needed.
> - under load, I think our bandwidth calculations are *not* completely
>   correct (This is the way to spell it to be allowed for a family audience).
> ToDo list:
> - bandwidth calculation: I am going to send another mail
>   with my ToDo list for migration, see there.
> - stats: We need better stats, by thread, etc
> - sincronize less times with the worker threads.
>   right now we syncronize for each page, there are two obvious optimizations
>   * send a list of pages each time we wakeup an fd
>   * if we have to sent a HUGE page, dont' do a single split, just sent the 
> whole page
>     in one send() and read things with a single recv() on destination.
>     My understanding is that this would make Transparent Huge pages trivial.
> - measure things under bigger loads
> Comments, please?

Nice to see this take shape.
There's something that looks suspicious from quick look at
the patches:
- imagine that the same page gets transmitted on two sockets
  on first, then on second one
- it's possible that the second update is received and handled on
  destination before the first one

Note: you do make sure a single thread sends data for
a page at a time, but that does not seem to affect the order
in which it's received.

In that case, I suspect the first one will overwrite the
page with stale data.

A simple fix would be to change

        static int multifd_send_page(uint8_t *address)

to calculate the fd based on address. E.g.
(long)address/PAGE_SIZE % thread_count.

Or split memory between threads in some other way.


> Later, Juan.
> Juan Quintela (13):
>   migration: create Migration Incoming State at init time
>   migration: Pass TCP args in an struct
>   migration: [HACK] Don't create decompression threads if not enabled
>   migration: Add multifd capability
>   migration: Create x-multifd-threads parameter
>   migration: create multifd migration threads
>   migration: Start of multiple fd work
>   migration: create ram_multifd_page
>   migration: Create thread infrastructure for multifd send side
>   migration: Send the fd number which we are going to use for this page
>   migration: Create thread infrastructure for multifd recv side
>   migration: Test new fd infrastructure
>   migration: [HACK]Transfer pages over new channels
>  hmp.c                         |  10 ++
>  include/migration/migration.h |  13 ++
>  migration/migration.c         | 100 ++++++++----
>  migration/ram.c               | 350 
> +++++++++++++++++++++++++++++++++++++++++-
>  migration/savevm.c            |   3 +-
>  migration/tcp.c               |  76 ++++++++-
>  qapi-schema.json              |  29 +++-
>  7 files changed, 540 insertions(+), 41 deletions(-)
> -- 
> 2.5.5

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