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

[Xiao Guangrong]

On 07/23/2018 04:05 PM, Peter Xu wrote:
> 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, address@hidden 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?

You're right, thank you for pointing it out.

So, how about introduce a filed, posted_pages, into RAMState that is used
to track total pages posted out.

Then will use this filed to replace 'iter_count' for compression and use
'RAMState.posted_pages - ram_counters.duplicate' to calculate
xbzrle_cache_miss as the zero page is not handled by xbzrle.

Or introduce a new function, total_posted_pages, which returns the
sum of all page counts:

   static total_posted_pages(void)
   {
       return ram_counters.normal + ram_counters.duplicate + 
compression_counters.pages
              +  xbzrle_counters.pages;
   }

that would be a bit more complexity...

> > > 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

Er, we do it as following:
            compression_counters.compression_rate =
                (double)(compression_counters.reduced_size -
                rs->compress_reduced_size_prev) /
                (comp_pages * TARGET_PAGE_SIZE);

We use reduced_size, i.e,:

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

for example, for 100 bytes raw data, if we posted 99 bytes out, then
the compression ration should be 1%.

So if i understand correctly, the reduced_size is really you want? :)