Re: [Qemu-devel] [PATCH 37/51] ram: Move compression_switch to RAMState

[Peter Xu <peterx@redhat.com>]

On Thu, Mar 23, 2017 at 09:45:30PM +0100, Juan Quintela wrote:
> Rename it to preffer_xbzrle that is a more descriptive name.

s/preffer/prefer/?
       
> 
> Signed-off-by: Juan Quintela <quintela@redhat.com>
> ---
>  migration/ram.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index 6a39704..591cf89 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -217,6 +217,9 @@ struct RAMState {
>      uint64_t dirty_pages_rate;
>      /* Count of requests incoming from destination */
>      uint64_t postcopy_requests;
> +    /* Should we move to xbzrle after the 1st round
> +       of compression */
> +    bool preffer_xbzrle;
>      /* protects modification of the bitmap */
>      QemuMutex bitmap_mutex;
>      /* Ram Bitmap protected by RCU */
> @@ -335,7 +338,6 @@ static QemuCond comp_done_cond;
>  /* The empty QEMUFileOps will be used by file in CompressParam */
>  static const QEMUFileOps empty_ops = { };
>  
> -static bool compression_switch;
>  static DecompressParam *decomp_param;
>  static QemuThread *decompress_threads;
>  static QemuMutex decomp_done_lock;
> @@ -419,7 +421,6 @@ void migrate_compress_threads_create(void)
>      if (!migrate_use_compression()) {
>          return;
>      }
> -    compression_switch = true;
>      thread_count = migrate_compress_threads();
>      compress_threads = g_new0(QemuThread, thread_count);
>      comp_param = g_new0(CompressParam, thread_count);
> @@ -1091,7 +1092,7 @@ static bool find_dirty_block(RAMState *rs, PageSearchStatus *pss,
>                   * point. In theory, xbzrle can do better than compression.
>                   */
>                  flush_compressed_data(rs);
> -                compression_switch = false;
> +                rs->preffer_xbzrle = true;
>              }
>          }
>          /* Didn't find anything this time, but try again on the new block */
> @@ -1323,7 +1324,7 @@ static int ram_save_target_page(RAMState *rs, MigrationState *ms,
>      /* Check the pages is dirty and if it is send it */
>      if (migration_bitmap_clear_dirty(rs, dirty_ram_abs)) {
>          unsigned long *unsentmap;
> -        if (compression_switch && migrate_use_compression()) {
> +        if (!rs->preffer_xbzrle && migrate_use_compression()) {

IIUC this prefer_xbzrle can be dynamically calculated by existing
states:

static inline bool ram_compression_active(RAMState *rs)
{
    /*
     * If xbzrle is on, stop using the data compression after first
     * round of migration even if compression is enabled. In theory,
     * xbzrle can do better than compression.
     */
    return migrate_use_compression() && \
           (rs->ram_bulk_stage || !migrate_use_xbzrle());
}

Then this line can be written as:

    if (ram_compression_active(rs)) {

And if so, we can get rid of prefer_xbzrle, right?

Having prefer_xbzrle should be slightly faster though, since it'll at
least cache the above calculation.

Thanks,

>              res = ram_save_compressed_page(rs, ms, pss, last_stage);
>          } else {
>              res = ram_save_page(rs, ms, pss, last_stage);
> -- 
> 2.9.3
> 
> 

-- peterx