Re: [Qemu-devel] [PATCH v2 1/8] migration: do not wait for free thread

[Peter Xu <peterx@redhat.com>]

On Thu, Jul 19, 2018 at 08:15:13PM +0800, guangrong.xiao@gmail.com wrote:
> @@ -3113,6 +3132,8 @@ static Property migration_properties[] = {
>      DEFINE_PROP_UINT8("x-compress-threads", MigrationState,
>                        parameters.compress_threads,
>                        DEFAULT_MIGRATE_COMPRESS_THREAD_COUNT),
> +    DEFINE_PROP_BOOL("x-compress-wait-thread", MigrationState,
> +                      parameters.compress_wait_thread, false),

This performance feature bit makes sense to me, but I would still
think it should be true by default to keep the old behavior:

- it might change the behavior drastically: we might be in a state
  between "normal" migration and "compressed" migration since we'll
  contain both of the pages.  Old compression users might not expect
  that.

- it might still even perform worse - an extreme case is that when
  network bandwidth is very very limited but instead we have plenty of
  CPU resources. [1]

So it's really a good tunable for me when people really needs to
understand what's it before turning it on.

>      DEFINE_PROP_UINT8("x-decompress-threads", MigrationState,
>                        parameters.decompress_threads,
>                        DEFAULT_MIGRATE_DECOMPRESS_THREAD_COUNT),
> diff --git a/migration/migration.h b/migration/migration.h
> index 64a7b33735..a46b9e6c8d 100644
> --- a/migration/migration.h
> +++ b/migration/migration.h
> @@ -271,6 +271,7 @@ bool migrate_use_return_path(void);
>  bool migrate_use_compression(void);
>  int migrate_compress_level(void);
>  int migrate_compress_threads(void);
> +int migrate_compress_wait_thread(void);
>  int migrate_decompress_threads(void);
>  bool migrate_use_events(void);
>  bool migrate_postcopy_blocktime(void);
> diff --git a/migration/ram.c b/migration/ram.c
> index 52dd678092..0ad234c692 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -1889,30 +1889,34 @@ static int compress_page_with_multi_thread(RAMState *rs, RAMBlock *block,
>                                             ram_addr_t offset)
>  {
>      int idx, thread_count, bytes_xmit = -1, pages = -1;
> +    bool wait = migrate_compress_wait_thread();
>  
>      thread_count = migrate_compress_threads();
>      qemu_mutex_lock(&comp_done_lock);
> -    while (true) {
> -        for (idx = 0; idx < thread_count; idx++) {
> -            if (comp_param[idx].done) {
> -                comp_param[idx].done = false;
> -                bytes_xmit = qemu_put_qemu_file(rs->f, comp_param[idx].file);
> -                qemu_mutex_lock(&comp_param[idx].mutex);
> -                set_compress_params(&comp_param[idx], block, offset);
> -                qemu_cond_signal(&comp_param[idx].cond);
> -                qemu_mutex_unlock(&comp_param[idx].mutex);
> -                pages = 1;
> -                ram_counters.normal++;
> -                ram_counters.transferred += bytes_xmit;
> -                break;
> -            }
> -        }
> -        if (pages > 0) {
> +retry:
> +    for (idx = 0; idx < thread_count; idx++) {
> +        if (comp_param[idx].done) {
> +            comp_param[idx].done = false;
> +            bytes_xmit = qemu_put_qemu_file(rs->f, comp_param[idx].file);
> +            qemu_mutex_lock(&comp_param[idx].mutex);
> +            set_compress_params(&comp_param[idx], block, offset);
> +            qemu_cond_signal(&comp_param[idx].cond);
> +            qemu_mutex_unlock(&comp_param[idx].mutex);
> +            pages = 1;
> +            ram_counters.normal++;
> +            ram_counters.transferred += bytes_xmit;
>              break;
> -        } else {
> -            qemu_cond_wait(&comp_done_cond, &comp_done_lock);
>          }
>      }
> +
> +    /*
> +     * if there is no thread is free to compress the data and the user
> +     * really expects the slowdown, wait it.

Considering [1] above, IMHO it might not really be a slow down but it
depends.  Maybe only mentioning about the fact that we're sending a
normal page instead of the compressed page if wait is not specified.

> +     */
> +    if (pages < 0 && wait) {
> +        qemu_cond_wait(&comp_done_cond, &comp_done_lock);
> +        goto retry;
> +    }
>      qemu_mutex_unlock(&comp_done_lock);
>  
>      return pages;
> @@ -2226,7 +2230,10 @@ static int ram_save_target_page(RAMState *rs, PageSearchStatus *pss,
>       * CPU resource.
>       */
>      if (block == rs->last_sent_block && save_page_use_compression(rs)) {
> -        return compress_page_with_multi_thread(rs, block, offset);
> +        res = compress_page_with_multi_thread(rs, block, offset);
> +        if (res > 0) {
> +            return res;
> +        }
>      } else if (migrate_use_multifd()) {
>          return ram_save_multifd_page(rs, block, offset);
>      }
> diff --git a/qapi/migration.json b/qapi/migration.json
> index 186e8a7303..b4f394844b 100644
> --- a/qapi/migration.json
> +++ b/qapi/migration.json
> @@ -462,6 +462,11 @@
>  # @compress-threads: Set compression thread count to be used in live migration,
>  #          the compression thread count is an integer between 1 and 255.
>  #
> +# @compress-wait-thread: Wait if no thread is free to compress the memory page
> +#          if it's enabled, otherwise, the page will be posted out immediately
> +#          in the main thread without compression. It's off on default.
> +#          (Since: 3.0)
> +#

Should "Since 3.1" in all the places.

We'll need to touch up the "by default" part depending on whether we'd
need to change it according to above comment.

Otherwise it looks good to me.

Thanks,

-- 
Peter Xu