Re: [Qemu-devel] [PATCH v2 3/8] migration: show the statistics of compression

[Peter Xu <peterx@redhat.com>]

On Mon, Jul 23, 2018 at 03:39:18PM +0800, Xiao Guangrong wrote:
> 
> 
> On 07/23/2018 12:36 PM, Peter Xu wrote:
> > On Thu, Jul 19, 2018 at 08:15:15PM +0800, guangrong.xiao@gmail.com wrote:
> > > @@ -1597,6 +1608,24 @@ static void migration_update_rates(RAMState *rs, int64_t end_time)
> > >               rs->xbzrle_cache_miss_prev) / iter_count;
> > >           rs->xbzrle_cache_miss_prev = xbzrle_counters.cache_miss;
> > >       }
> > > +
> > > +    if (migrate_use_compression()) {
> > > +        uint64_t comp_pages;
> > > +
> > > +        compression_counters.busy_rate = (double)(compression_counters.busy -
> > > +            rs->compress_thread_busy_prev) / iter_count;
> > 
> > Here I'm not sure it's correct...
> > 
> > "iter_count" stands for ramstate.iterations.  It's increased per
> > ram_find_and_save_block(), so IMHO it might contain multiple guest
> 
> ram_find_and_save_block() returns if a page is successfully posted and
> it only posts 1 page out at one time.

ram_find_and_save_block() calls ram_save_host_page(), and we should be
sending multiple guest pages in ram_save_host_page() if the host page
is a huge page?

> 
> > pages.  However compression_counters.busy should be per guest page.
> > 
> 
> Actually, it's derived from xbzrle_counters.cache_miss_rate:
>         xbzrle_counters.cache_miss_rate = (double)(xbzrle_counters.cache_miss -
>             rs->xbzrle_cache_miss_prev) / iter_count;

Then this is suspecious to me too...

> 
> > > +        rs->compress_thread_busy_prev = compression_counters.busy;
> > > +
> > > +        comp_pages = compression_counters.pages - rs->compress_pages_prev;
> > > +        if (comp_pages) {
> > > +            compression_counters.compression_rate =
> > > +                (double)(compression_counters.reduced_size -
> > > +                rs->compress_reduced_size_prev) /
> > > +                (comp_pages * TARGET_PAGE_SIZE);
> > > +            rs->compress_pages_prev = compression_counters.pages;
> > > +            rs->compress_reduced_size_prev = compression_counters.reduced_size;
> > > +        }
> > > +    }
> > >   }
> > >   static void migration_bitmap_sync(RAMState *rs)
> > > @@ -1872,6 +1901,9 @@ static void flush_compressed_data(RAMState *rs)
> > >           qemu_mutex_lock(&comp_param[idx].mutex);
> > >           if (!comp_param[idx].quit) {
> > >               len = qemu_put_qemu_file(rs->f, comp_param[idx].file);
> > > +            /* 8 means a header with RAM_SAVE_FLAG_CONTINUE. */
> > > +            compression_counters.reduced_size += TARGET_PAGE_SIZE - len + 8;
> > 
> > I would agree with Dave here - why we store the "reduced size" instead
> > of the size of the compressed data (which I think should be len - 8)?
> > 
> 
> len-8 is the size of data after compressed rather than the data improved
> by compression that is not straightforward for the user to see how much
> the improvement is by applying compression.
> 
> Hmm... but it is not a big deal to me... :)

Yeah it might be a personal preference indeed. :)

It's just natural to do that this way for me since AFAIU the
compression ratio is defined as:

                           compressed data size
  compression ratio =    ------------------------
                            original data size

> 
> > Meanwhile, would a helper be nicer? Like:
> 
> Yup, that's nicer indeed.

Regards,

-- 
Peter Xu