Re: [PATCH v3] migration/xbzrle: add encoding rate

[Wei Wang <wei.w.wang@intel.com>]

On 06/04/2020 11:22 AM, Richard Henderson wrote:
> On 6/3/20 7:58 PM, Wei Wang wrote:
>> It is possible that encoded_size==0, but unencoded_size !=0. For example,
>> a page is written with the same data that it already has.
> That really contains 0 bytes?
> Not even the ones that say "same data"?

Yes. It's a just delta operation, the diff (encoded_size) is 0 in that case.

>
> You certainly have a magical compression algorithm there.
> Or bad accounting.
>
>> The encoding_rate is expected to reflect if the page is xbzrle encoding friendly.
>> The larger, the more friendly, so 0 might not be a good representation here.
>>
>> Maybe, we could change UINT64_MAX above to "~0ULL" to avoid the issue?
> ~0ull is no different than UINT64_MAX -- indeed, they are *exactly* the same
> value -- and is not an exactly representible floating-point value.
>
> If unencoded_size != 0, and (somehow) encoded_size == 0, then
>
>    unencoded_size / encoded_size = Inf
>
> which is indeed the limit of x -> 0, n / x.
>
> Which is *also* printable by %0.2f.
>
> I still contend that the middle if should be removed, and you should print out
> whatever's left.  Either NaN or Inf is instructive.  Certainly nothing in the
> middle cares about the actual value.
>

OK, leave it as "inf" looks good to me. Will send a patch to remove the 
middle. Thanks!

Best,
Wei