Re: [Qemu-devel] [PATCH 2/5] migration: rename max_size to threshold_size

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

* Peter Xu (peterx@redhat.com) wrote:
> In migration codes (especially in migration_thread()), max_size is used
> in many place for the threshold value that we will start to do the final
> flush and jump to the next stage to dump the whole rest things to
> destination. However its name is confusing to first readers. Let's
> rename it to "threshold_size" when proper and add a comment for it. No
> functional change is made.
> 
> CC: Juan Quintela <quintela@redhat.com>
> CC: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  include/migration/vmstate.h |  3 ++-
>  migration/migration.c       | 17 +++++++++--------
>  migration/savevm.c          |  4 ++--
>  3 files changed, 13 insertions(+), 11 deletions(-)
> 
> diff --git a/include/migration/vmstate.h b/include/migration/vmstate.h
> index f2dbf84..dad3984 100644
> --- a/include/migration/vmstate.h
> +++ b/include/migration/vmstate.h
> @@ -56,7 +56,8 @@ typedef struct SaveVMHandlers {
>  
>      /* This runs outside the iothread lock!  */
>      int (*save_live_setup)(QEMUFile *f, void *opaque);
> -    void (*save_live_pending)(QEMUFile *f, void *opaque, uint64_t max_size,
> +    void (*save_live_pending)(QEMUFile *f, void *opaque,
> +                              uint64_t threshold_size,
>                                uint64_t *non_postcopiable_pending,
>                                uint64_t *postcopiable_pending);
>      LoadStateHandler *load_state;
> diff --git a/migration/migration.c b/migration/migration.c
> index f9f4d98..b065fe4 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -1907,7 +1907,8 @@ static void *migration_thread(void *opaque)
>      int64_t initial_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
>      int64_t setup_start = qemu_clock_get_ms(QEMU_CLOCK_HOST);
>      int64_t initial_bytes = 0;
> -    int64_t max_size = 0;
> +    /* We'll do the final flush when reachs threshold_size */

I think that's 'reaches' - however perhaps we should make a more
explicit comment:
   'The final stage happens when the remaining data is smaller than
    this threshold; it's calculated from the requested downtime
    and measured bandwidth'

other than that:
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

Dave

> +    int64_t threshold_size = 0;
>      int64_t start_time = initial_time;
>      int64_t end_time;
>      bool old_vm_running = false;
> @@ -1951,17 +1952,17 @@ static void *migration_thread(void *opaque)
>          if (!qemu_file_rate_limit(s->to_dst_file)) {
>              uint64_t pend_post, pend_nonpost;
>  
> -            qemu_savevm_state_pending(s->to_dst_file, max_size, &pend_nonpost,
> -                                      &pend_post);
> +            qemu_savevm_state_pending(s->to_dst_file, threshold_size,
> +                                      &pend_nonpost, &pend_post);
>              pending_size = pend_nonpost + pend_post;
> -            trace_migrate_pending(pending_size, max_size,
> +            trace_migrate_pending(pending_size, threshold_size,
>                                    pend_post, pend_nonpost);
> -            if (pending_size && pending_size >= max_size) {
> +            if (pending_size && pending_size >= threshold_size) {
>                  /* Still a significant amount to transfer */
>  
>                  if (migrate_postcopy_ram() &&
>                      s->state != MIGRATION_STATUS_POSTCOPY_ACTIVE &&
> -                    pend_nonpost <= max_size &&
> +                    pend_nonpost <= threshold_size &&
>                      atomic_read(&s->start_postcopy)) {
>  
>                      if (!postcopy_start(s, &old_vm_running)) {
> @@ -1993,13 +1994,13 @@ static void *migration_thread(void *opaque)
>                                           initial_bytes;
>              uint64_t time_spent = current_time - initial_time;
>              double bandwidth = (double)transferred_bytes / time_spent;
> -            max_size = bandwidth * s->parameters.downtime_limit;
> +            threshold_size = bandwidth * s->parameters.downtime_limit;
>  
>              s->mbps = (((double) transferred_bytes * 8.0) /
>                      ((double) time_spent / 1000.0)) / 1000.0 / 1000.0;
>  
>              trace_migrate_transferred(transferred_bytes, time_spent,
> -                                      bandwidth, max_size);
> +                                      bandwidth, threshold_size);
>              /* if we haven't sent anything, we don't want to recalculate
>                 10000 is a small enough number for our purposes */
>              if (s->dirty_bytes_rate && transferred_bytes > 10000) {
> diff --git a/migration/savevm.c b/migration/savevm.c
> index 3b19a4a..59c04eb 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -1197,7 +1197,7 @@ void qemu_savevm_state_complete_precopy(QEMUFile *f, bool iterable_only)
>   * the result is split into the amount for units that can and
>   * for units that can't do postcopy.
>   */
> -void qemu_savevm_state_pending(QEMUFile *f, uint64_t max_size,
> +void qemu_savevm_state_pending(QEMUFile *f, uint64_t threshold_size,
>                                 uint64_t *res_non_postcopiable,
>                                 uint64_t *res_postcopiable)
>  {
> @@ -1216,7 +1216,7 @@ void qemu_savevm_state_pending(QEMUFile *f, uint64_t max_size,
>                  continue;
>              }
>          }
> -        se->ops->save_live_pending(f, se->opaque, max_size,
> +        se->ops->save_live_pending(f, se->opaque, threshold_size,
>                                     res_non_postcopiable, res_postcopiable);
>      }
>  }
> -- 
> 2.7.4
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK