Re: [PATCH 08/12] migration: do not flush_compressed_data at the end of each iteration

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* guangrong.xiao@gmail.com (guangrong.xiao@gmail.com) wrote:
> From: Xiao Guangrong <xiaoguangrong@tencent.com>
> 
> flush_compressed_data() needs to wait all compression threads to
> finish their work, after that all threads are free until the
> migration feed new request to them, reducing its call can improve
> the throughput and use CPU resource more effectively
> 
> We do not need to flush all threads at the end of iteration, the
> data can be kept locally until the memory block is changed
> 
> Signed-off-by: Xiao Guangrong <xiaoguangrong@tencent.com>
> ---
>  migration/ram.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index f9a8646520..0a38c1c61e 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -1994,6 +1994,7 @@ static void ram_save_cleanup(void *opaque)
>      }
>  
>      xbzrle_cleanup();
> +    flush_compressed_data(*rsp);
>      compress_threads_save_cleanup();
>      ram_state_cleanup(rsp);
>  }

I'm not sure why this change corresponds to the other removal.
We should already have sent all remaining data in ram_save_complete()'s
call to flush_compressed_data - so what is this one for?

> @@ -2690,7 +2691,6 @@ static int ram_save_iterate(QEMUFile *f, void *opaque)
>          }
>          i++;
>      }
> -    flush_compressed_data(rs);
>      rcu_read_unlock();

Hmm - are we sure there's no other cases that depend on ordering of all
of the compressed data being sent out between threads?
I think the one I'd most worry about is the case where:

  iteration one:
     thread 1: Save compressed page 'n'

  iteration two:
     thread 2: Save compressed page 'n'

What guarantees that the version of page 'n'
from thread 2 reaches the destination first without
this flush?

Dave

>      /*
> -- 
> 2.14.4
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK