[Python-Dev] Accepting PEP 3154 for 3.4?

[Nick Coghlan]

On 19 November 2013 09:57, Antoine Pitrou <solipsis at pitrou.net> wrote:
> On Mon, 18 Nov 2013 16:44:59 -0600
> Tim Peters <tim.peters at gmail.com> wrote:
>> [Tim]
>> >> But it has a different kind of advantage:  PREFETCH was optional.  As
>> >> Guido said, it's annoying to bloat the size of small pickles (which
>> >> may, although individually small, occur in great numbers) by 8 bytes
>> >> each.  There's really no point to framing small chunks of data, right?
>>
>> [Antoine]
>> > You can't know how much space the pickle will take until the pickling
>> > ends, though, which makes it difficult to decide whether you want to
>> > emit a PREFETCH opcode or not.
>>
>> Ah, of course.  Presumably the outgoing pickle stream is first stored
>> in some memory buffer, right?  If pickling completes before the buffer
>> is first flushed, then you know exactly how large the entire pickle
>> is.  If "it's small" (say, < 100 bytes), don't write out the PREFETCH
>> part.  Else do.
>
> Yet another possibility: keep framing but use a variable-length
> encoding for the frame size:
>
> - first byte: bits 7-5: N (= frame size bytes length - 1)
> - first byte: bits 4-0: first 5 bits of frame size
> - remaning N bytes: remaining bits of frame size
>
> With this scheme, very small pickles have a one byte overhead; small
> ones a two byte overhead; and the max frame size is 2**61 rather than
> 2**64, which should still be sufficient.
>
> And the frame size is read using either one or two read() calls, which
> is efficient.

And it's only a minimal change from the current patch. Sounds good to me.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia