Re: [Qemu-devel] [PATCH v14 03/13] Add XBZRLE documentation

[Orit Wasserman]

On 07/03/2012 10:45 PM, Eric Blake wrote:
> On 07/03/2012 07:52 AM, Orit Wasserman wrote:
>> Signed-off-by: Orit Wasserman <address@hidden>
>> ---
>>  docs/xbzrle.txt |  133 
>> +++++++++++++++++++++++++++++++++++++++++++++++++++++++
>>  1 files changed, 133 insertions(+), 0 deletions(-)
>>  create mode 100644 docs/xbzrle.txt
> 
>> +
>> +Example
>> +old buffer:
>> +1000 zeros
>> +05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 00 00 6b 00 6d
>> +3072 zeros
> 
> That's only 4092 bytes, but a page is typically 4096.  I think you meant
> 3076 trailing zeros.
> 
>> +
>> +new buffer:
>> +1000 zeros
>> +01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 00 00 67 00 69
>> +3702 zeros
> 
> That's 4722 bytes; looks like a transposition error, and given the above
> comment, I still think you want 3076 trailing zeros.
> 
>> +
>> +encoded buffer:
>> +
>> +encoded length 29
>> +e8 07 18 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 00 00 67 00 69 00 
>> 00
>> +00 80 18
> 
> Looks better, but still not quite accurate.  Per the spec, you always
> end on a nzrun, which means you should not have transmitted the last two
> bytes (80 18 => 3072, which is the length of your trailing zrun).  I
> also find it weak that your example never shows an unchanged byte on
> anything other than 00; having at least one non-zero unchanged byte may
> make it more obvious in the encoded example that it is the xor
> difference that determines a zrun vs. nzrun, and not the old or new
> buffer contents.
> 
> Also, reading that encoding says you have 1000 zrun, then 24 bytes of
> nzrun starting with a leading 00, but based on your old and new buffer,
> the only way to have a leading 00 in your nzrun is if you count a zrun
> of 999 bytes.  Are you sure you didn't test with a buffer of 1001
> leading zeros, then 20 bytes of interest, then 3073 trailing zeros?
yes you are right 1001 zeros ...
I fix it .
> 
> Given your old and new buffer as-is, and my assumption that you are
> compressing a page of exactly 4096 bytes (3076 trailing zeros after your
> 20-byte window of interesting data), I see at least the following four
> valid encodings, listed in increasing difficulty of computation:
> 
> 8-byte long at a time; all chunks that differ in at least one byte are
> treated as at least 8 bytes of nzrun, transmit 27 bytes
> e8 07 18 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 00 00 67 00 69 00
> 00 00 00
> 
> 4 bytes at a time; all chunks that differ in at least one byte are
> treated as at least 4 bytes of nzrun, transmit 23 bytes
> e8 07 14 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 00 00 67 00 69
> 
> list every single zrun (except trailing), even if only one byte long,
> transmit 24 bytes
> e8 07 0f 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 02 01 67 01 01 69
> 
> avoid a 1-byte zrun, transmit 23 bytes
> e8 07 0f 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 02 03 67 00 69
>