Re: [PATCH v2 8/8] migration: do not flush_compressed_data at the end of each iteration

[Peter Xu <peterx@redhat.com>]

On Thu, Jul 19, 2018 at 08:15:20PM +0800, 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 feeds 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 or
> memory migration starts over in that case we will meet a dirtied
> page which may still exists in compression threads's ring
> 
> Signed-off-by: Xiao Guangrong <xiaoguangrong@tencent.com>
> ---
>  migration/ram.c | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index 89305c7af5..fdab13821d 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -315,6 +315,8 @@ struct RAMState {
>      uint64_t iterations;
>      /* number of dirty bits in the bitmap */
>      uint64_t migration_dirty_pages;
> +    /* last dirty_sync_count we have seen */
> +    uint64_t dirty_sync_count;

Better suffix it with "_prev" as well?  So that we can quickly
identify that it's only a cache and it can be different from the one
in the ram_counters.

>      /* protects modification of the bitmap */
>      QemuMutex bitmap_mutex;
>      /* The RAMBlock used in the last src_page_requests */
> @@ -2532,6 +2534,7 @@ static void ram_save_cleanup(void *opaque)
>      }
>  
>      xbzrle_cleanup();
> +    flush_compressed_data(*rsp);

Could I ask why do we need this considering that we have
compress_threads_save_cleanup() right down there?

>      compress_threads_save_cleanup();
>      ram_state_cleanup(rsp);
>  }
> @@ -3203,6 +3206,17 @@ static int ram_save_iterate(QEMUFile *f, void *opaque)
>  
>      ram_control_before_iterate(f, RAM_CONTROL_ROUND);
>  
> +    /*
> +     * if memory migration starts over, we will meet a dirtied page which
> +     * may still exists in compression threads's ring, so we should flush
> +     * the compressed data to make sure the new page is not overwritten by
> +     * the old one in the destination.
> +     */
> +    if (ram_counters.dirty_sync_count != rs->dirty_sync_count) {
> +        rs->dirty_sync_count = ram_counters.dirty_sync_count;
> +        flush_compressed_data(rs);
> +    }
> +
>      t0 = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
>      i = 0;
>      while ((ret = qemu_file_rate_limit(f)) == 0 ||
> @@ -3235,7 +3249,6 @@ static int ram_save_iterate(QEMUFile *f, void *opaque)
>          }
>          i++;
>      }
> -    flush_compressed_data(rs);

This looks sane to me, but I'd like to see how other people would
think about it too...

>      rcu_read_unlock();
>  
>      /*
> -- 
> 2.14.4
> 

Regards,

-- 
Peter Xu