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

[Xiao Guangrong <guangrong.xiao@gmail.com>]

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.

> 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;

>> +        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... :)

> Meanwhile, would a helper be nicer? Like:

Yup, that's nicer indeed.