Re: [Qemu-devel] [PATCH 6/8] migration: move calling save_zero_page to the common place

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

* guangrong.xiao@gmail.com (guangrong.xiao@gmail.com) wrote:
> From: Xiao Guangrong <xiaoguangrong@tencent.com>
> 
> save_zero_page() is always our first approach to try, move it to
> the common place before calling ram_save_compressed_page
> and ram_save_page
> 
> Signed-off-by: Xiao Guangrong <xiaoguangrong@tencent.com>
> ---
>  migration/ram.c | 106 ++++++++++++++++++++++++++++++++------------------------
>  1 file changed, 60 insertions(+), 46 deletions(-)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index 839665d866..9627ce18e9 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -1021,15 +1021,8 @@ static int ram_save_page(RAMState *rs, PageSearchStatus *pss, bool last_stage)
>      trace_ram_save_page(block->idstr, (uint64_t)offset, p);
>  
>      XBZRLE_cache_lock();
> -    pages = save_zero_page(rs, block, offset);
> -    if (pages > 0) {
> -        /* Must let xbzrle know, otherwise a previous (now 0'd) cached
> -         * page would be stale
> -         */
> -        xbzrle_cache_zero_page(rs, current_addr);
> -        ram_release_pages(block->idstr, offset, pages);
> -    } else if (!rs->ram_bulk_stage &&
> -               !migration_in_postcopy() && migrate_use_xbzrle()) {
> +    if (!rs->ram_bulk_stage && !migration_in_postcopy() &&
> +           migrate_use_xbzrle()) {
>          pages = save_xbzrle_page(rs, &p, current_addr, block,
>                                   offset, last_stage);
>          if (!last_stage) {
> @@ -1172,40 +1165,23 @@ static int ram_save_compressed_page(RAMState *rs, PageSearchStatus *pss,
>  
>      p = block->host + offset;
>  
> -    /* When starting the process of a new block, the first page of
> -     * the block should be sent out before other pages in the same
> -     * block, and all the pages in last block should have been sent
> -     * out, keeping this order is important, because the 'cont' flag
> -     * is used to avoid resending the block name.
> -     */
>      if (block != rs->last_sent_block) {
> -        flush_compressed_data(rs);
> -        pages = save_zero_page(rs, block, offset);
> -        if (pages > 0) {
> -            ram_release_pages(block->idstr, offset, pages);
> -        } else {
> -            /*
> -             * Make sure the first page is sent out before other pages.
> -             *
> -             * we post it as normal page as compression will take much
> -             * CPU resource.
> -             */
> -            ram_counters.transferred += save_page_header(rs, rs->f, block,
> -                                            offset | RAM_SAVE_FLAG_PAGE);
> -            qemu_put_buffer_async(rs->f, p, TARGET_PAGE_SIZE,
> -                                  migrate_release_ram() &
> -                                  migration_in_postcopy());
> -            ram_counters.transferred += TARGET_PAGE_SIZE;
> -            ram_counters.normal++;
> -            pages = 1;
> -        }
> +        /*
> +         * Make sure the first page is sent out before other pages.
> +         *
> +         * we post it as normal page as compression will take much
> +         * CPU resource.
> +         */
> +        ram_counters.transferred += save_page_header(rs, rs->f, block,
> +                                        offset | RAM_SAVE_FLAG_PAGE);
> +        qemu_put_buffer_async(rs->f, p, TARGET_PAGE_SIZE,
> +                              migrate_release_ram() &
> +                              migration_in_postcopy());
> +        ram_counters.transferred += TARGET_PAGE_SIZE;
> +        ram_counters.normal++;
> +        pages = 1;
>      } else {
> -        pages = save_zero_page(rs, block, offset);
> -        if (pages == -1) {
> -            pages = compress_page_with_multi_thread(rs, block, offset);
> -        } else {
> -            ram_release_pages(block->idstr, offset, pages);
> -        }
> +        pages = compress_page_with_multi_thread(rs, block, offset);
>      }
>  
>      return pages;
> @@ -1447,6 +1423,25 @@ err:
>      return -1;
>  }
>  
> +static bool save_page_use_compression(RAMState *rs)
> +{
> +    if (!migrate_use_compression()) {
> +        return false;
> +    }
> +
> +    /*
> +     * 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.
> +     */
> +    if (rs->ram_bulk_stage || !migrate_use_xbzrle()) {
> +        return true;
> +    }
> +
> +    return false;
> +
> +}
> +
>  /**
>   * ram_save_target_page: save one target page
>   *
> @@ -1472,12 +1467,31 @@ static int ram_save_target_page(RAMState *rs, PageSearchStatus *pss,
>          }
>  
>          /*
> -         * 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.
> +         * When starting the process of a new block, the first page of
> +         * the block should be sent out before other pages in the same
> +         * block, and all the pages in last block should have been sent
> +         * out, keeping this order is important, because the 'cont' flag
> +         * is used to avoid resending the block name.
>           */
> -        if (migrate_use_compression() &&
> -            (rs->ram_bulk_stage || !migrate_use_xbzrle())) {
> +        if (block != rs->last_sent_block && save_page_use_compression(rs)) {
> +            flush_compressed_data(rs);
> +        }
> +
> +        res = save_zero_page(rs, block, offset);
> +        if (res > 0) {
> +            /* Must let xbzrle know, otherwise a previous (now 0'd) cached
> +             * page would be stale
> +             */
> +            if (!save_page_use_compression(rs)) {

This test is quite interesting; I think the
reason it's different in the compression case is that since we don't put
any none-0 data in the xbzrle cache, we don't need to knock any old
none-0 pages out of the cache.

> +                XBZRLE_cache_lock();
> +                xbzrle_cache_zero_page(rs, block->offset + offset);
> +                XBZRLE_cache_unlock();
> +            }
> +            ram_release_pages(block->idstr, offset, res);
> +            goto page_saved;
> +        }
> +
> +        if (save_page_use_compression(rs)) {
>              res = ram_save_compressed_page(rs, pss, last_stage);
>          } else {
>              res = ram_save_page(rs, pss, last_stage);


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

> -- 
> 2.14.3
> 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK