Re: [Qemu-devel] [PATCH v3 09/10] migration: fix calculating xbzrle_counters.cache_miss_rate

[Peter Xu]

On Tue, Aug 07, 2018 at 05:12:08PM +0800, address@hidden wrote:
> From: Xiao Guangrong <address@hidden>
> 
> As Peter pointed out:
> | - xbzrle_counters.cache_miss is done in save_xbzrle_page(), so it's
> |   per-guest-page granularity
> |
> | - RAMState.iterations is done for each ram_find_and_save_block(), so
> |   it's per-host-page granularity
> |
> | An example is that when we migrate a 2M huge page in the guest, we
> | will only increase the RAMState.iterations by 1 (since
> | ram_find_and_save_block() will be called once), but we might increase
> | xbzrle_counters.cache_miss for 2M/4K=512 times (we'll call
> | save_xbzrle_page() that many times) if all the pages got cache miss.
> | Then IMHO the cache miss rate will be 512/1=51200% (while it should
> | actually be just 100% cache miss).
> 
> And he also suggested as xbzrle_counters.cache_miss_rate is the only
> user of rs->iterations we can adapt it to count guest page numbers
> 
> After that, rename 'iterations' to 'handle_pages' to better reflect
> its meaning
> 
> Suggested-by: Peter Xu <address@hidden>
> Signed-off-by: Xiao Guangrong <address@hidden>
> ---
>  migration/ram.c | 18 +++++++++---------
>  1 file changed, 9 insertions(+), 9 deletions(-)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index 09be01dca2..bd7c18d1f9 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -300,10 +300,10 @@ struct RAMState {
>      uint64_t num_dirty_pages_period;
>      /* xbzrle misses since the beginning of the period */
>      uint64_t xbzrle_cache_miss_prev;
> -    /* number of iterations at the beginning of period */
> -    uint64_t iterations_prev;
> -    /* Iterations since start */
> -    uint64_t iterations;
> +    /* total handled pages at the beginning of period */
> +    uint64_t handle_pages_prev;
> +    /* total handled pages since start */
> +    uint64_t handle_pages;

The name is not that straightforward to me.  I would think about
"[guest|host]_page_count" or something better, or we just keep the old
naming but with a better comment would be fine too.

>      /* number of dirty bits in the bitmap */
>      uint64_t migration_dirty_pages;
>      /* last dirty_sync_count we have seen */
> @@ -1587,19 +1587,19 @@ uint64_t ram_pagesize_summary(void)
>  
>  static void migration_update_rates(RAMState *rs, int64_t end_time)
>  {
> -    uint64_t iter_count = rs->iterations - rs->iterations_prev;
> +    uint64_t page_count = rs->handle_pages - rs->handle_pages_prev;
>  
>      /* calculate period counters */
>      ram_counters.dirty_pages_rate = rs->num_dirty_pages_period * 1000
>                  / (end_time - rs->time_last_bitmap_sync);
>  
> -    if (!iter_count) {
> +    if (!page_count) {
>          return;
>      }
>  
>      if (migrate_use_xbzrle()) {
>          xbzrle_counters.cache_miss_rate = 
> (double)(xbzrle_counters.cache_miss -
> -            rs->xbzrle_cache_miss_prev) / iter_count;
> +            rs->xbzrle_cache_miss_prev) / page_count;
>          rs->xbzrle_cache_miss_prev = xbzrle_counters.cache_miss;
>      }
>  }
> @@ -1657,7 +1657,7 @@ static void migration_bitmap_sync(RAMState *rs)
>  
>          migration_update_rates(rs, end_time);
>  
> -        rs->iterations_prev = rs->iterations;
> +        rs->handle_pages_prev = rs->handle_pages;
>  
>          /* reset period counters */
>          rs->time_last_bitmap_sync = end_time;
> @@ -3209,7 +3209,7 @@ static int ram_save_iterate(QEMUFile *f, void *opaque)
>              break;
>          }
>  
> -        rs->iterations++;
> +        rs->handle_pages += pages;

So it's still counting host pages, is this your intention to only
change the name in the patch?

>  
>          /* we want to check in the 1st loop, just in case it was the 1st time
>             and we had to sync the dirty bitmap.
> -- 
> 2.14.4
> 

Regards,

-- 
Peter Xu