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From: Bram Cohen <bram@bittorrent.com>
Date: Fri, 24 Feb 2017 22:23:20 -0800
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To: Peter Todd <pete@petertodd.org>
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Subject: Re: [bitcoin-dev] A Better MMR Definition
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On Fri, Feb 24, 2017 at 8:12 PM, Peter Todd <pete@petertodd.org> wrote:

>
> So to be clear, what you're proposing there is to use the insertion order
> as
> the index - once you go that far you've almost entirely re-invented my
> proposal!
>

I'm not 'proposing' this, I'm saying it could be done simply but I'm
skeptical of the utility. Probably the most compelling argument for it is
that the insertion indexed values are much smaller so they can be compacted
down a lot resulting in using less memory and more locality and fewer
hashes, but your implementation doesn't take advantage of that.


> Your merkle-set implementation is 1500 lines of densely written Python


The reference implementation which is included in those 1500 lines is less
than 300 lines and fairly straightforward. The non-reference implementation
always behaves semantically identically to the reference implementation, it
just does so faster and using less memory.


> with
> almost no comments,


The comments at the top explain both the proof format and the in-memory
data structures very precisely. The whole codebase was reviewed by a
coworker of mine and comments were added explaining the subtleties which
tripped him up.


> and less than a 100 lines of (also uncommented) tests.


Those tests get 98% code coverage and extensively hit not only the lines of
code but the semantic edge cases as well. The lines which aren't hit are
convenience functions and error conditions of the parsing code for when
it's passed bad data.


> By
> comparison, my Python MMR implementation is 300 lines of very readable
> Python
> with lots of comments, a 200 line explanation at the top, and 200 lines of
> (commented) tests. Yet no-one is taking the (still considerable) effort to
> understand and comment on my implementation. :)
>

Given that maaku's Merkle prefix trees were shelved because of performance
problems despite being written in C and operating in basically the same way
as your code and my reference code, it's clear that non-optimized Python
won't be touching the bitcoin codebase any time soon.


>
> Fact is, what you've written is really daunting to review, and given it's
> not
> in the final language anyway, it's unclear what basis to review it on
> anyway.


It should reviewed based on semantic correctness and performance.
Performance can only be accurately and convincingly determined by porting
to C and optimizing it, which mostly involves experimenting with different
values for the two passed in magic numbers.


> I
> suspect you'd get more feedback if the codebase was better commented, in a
> production language, and you have actual real-world benchmarks and
> performance
> figures.
>

Porting to C should be straightforward. Several people have already
expressed interest in doing so, and it's written in intentionally C-ish
Python, resulting in some rather odd idioms which is a bit part of why you
think it looks 'dense'. A lot of that weird offset math should be much more
readable in C because it's all structs and x.y notation can be used instead
of adding offsets.


> In particular, while at the top of merkle_set.py you have a list of
> advantages,
> and a bunch of TODO's, you don't explain *why* the code has any of these
> advantages. To figure that out, I'd have to read and understand 1500 lines
> of
> densely written Python. Without a human-readable pitch, not many people are
> going to do that, myself included.
>

It's all about cache coherence. When doing operations it pulls in a bunch
of things which are near each other in memory instead of jumping all over
the place. The improvements it gets should be much greater than the ones
gained from insertion ordering, although the two could be accretive.

--001a113ed7fef638b9054954e147
Content-Type: text/html; charset=UTF-8
Content-Transfer-Encoding: quoted-printable

<div dir=3D"ltr"><div class=3D"gmail_extra"><div class=3D"gmail_quote">On F=
ri, Feb 24, 2017 at 8:12 PM, Peter Todd <span dir=3D"ltr">&lt;<a href=3D"ma=
ilto:pete@petertodd.org" target=3D"_blank">pete@petertodd.org</a>&gt;</span=
> wrote:<br><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;bo=
rder-left:1px #ccc solid;padding-left:1ex"><span class=3D""><br>
</span>So to be clear, what you&#39;re proposing there is to use the insert=
ion order as<br>
the index - once you go that far you&#39;ve almost entirely re-invented my<=
br>
proposal!<br></blockquote><div><br></div><div>I&#39;m not &#39;proposing&#3=
9; this, I&#39;m saying it could be done simply but I&#39;m skeptical of th=
e utility. Probably the most compelling argument for it is that the inserti=
on indexed values are much smaller so they can be compacted down a lot resu=
lting in using less memory and more locality and fewer hashes, but your imp=
lementation doesn&#39;t take advantage of that.</div><div>=C2=A0</div><bloc=
kquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #cc=
c solid;padding-left:1ex">Your merkle-set implementation is 1500 lines of d=
ensely written Python</blockquote><div><br></div><div>The reference impleme=
ntation which is included in those 1500 lines is less than 300 lines and fa=
irly straightforward. The non-reference implementation always behaves seman=
tically identically to the reference implementation, it just does so faster=
 and using less memory.</div><div>=C2=A0</div><blockquote class=3D"gmail_qu=
ote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex=
"> with<br>
almost no comments,</blockquote><div><br></div><div>The comments at the top=
 explain both the proof format and the in-memory data structures very preci=
sely. The whole codebase was reviewed by a coworker of mine and comments we=
re added explaining the subtleties which tripped him up.</div><div>=C2=A0</=
div><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-lef=
t:1px #ccc solid;padding-left:1ex"> and less than a 100 lines of (also unco=
mmented) tests.</blockquote><div><br></div><div>Those tests get 98% code co=
verage and extensively hit not only the lines of code but the semantic edge=
 cases as well. The lines which aren&#39;t hit are convenience functions an=
d error conditions of the parsing code for when it&#39;s passed bad data.</=
div><div>=C2=A0</div><blockquote class=3D"gmail_quote" style=3D"margin:0 0 =
0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> By<br>
comparison, my Python MMR implementation is 300 lines of very readable Pyth=
on<br>
with lots of comments, a 200 line explanation at the top, and 200 lines of<=
br>
(commented) tests. Yet no-one is taking the (still considerable) effort to<=
br>
understand and comment on my implementation. :)<br></blockquote><div><br></=
div><div>Given that maaku&#39;s Merkle prefix trees were shelved because of=
 performance problems despite being written in C and operating in basically=
 the same way as your code and my reference code, it&#39;s clear that non-o=
ptimized Python won&#39;t be touching the bitcoin codebase any time soon.=
=C2=A0</div><div>=C2=A0</div><blockquote class=3D"gmail_quote" style=3D"mar=
gin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<br>
Fact is, what you&#39;ve written is really daunting to review, and given it=
&#39;s not<br>
in the final language anyway, it&#39;s unclear what basis to review it on a=
nyway.</blockquote><div><br></div><div>It should reviewed based on semantic=
 correctness and performance. Performance can only be accurately and convin=
cingly determined by porting to C and optimizing it, which mostly involves =
experimenting with different values for the two passed in magic numbers.</d=
iv><div>=C2=A0</div><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0=
 .8ex;border-left:1px #ccc solid;padding-left:1ex"> I<br>
suspect you&#39;d get more feedback if the codebase was better commented, i=
n a<br>
production language, and you have actual real-world benchmarks and performa=
nce<br>
figures.<br></blockquote><div><br></div><div>Porting to C should be straigh=
tforward. Several people have already expressed interest in doing so, and i=
t&#39;s written in intentionally C-ish Python, resulting in some rather odd=
 idioms which is a bit part of why you think it looks &#39;dense&#39;. A lo=
t of that weird offset math should be much more readable in C because it&#3=
9;s all structs and x.y notation can be used instead of adding offsets.</di=
v><div>=C2=A0</div><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 =
.8ex;border-left:1px #ccc solid;padding-left:1ex">In particular, while at t=
he top of merkle_set.py you have a list of advantages,<br>
and a bunch of TODO&#39;s, you don&#39;t explain *why* the code has any of =
these<br>
advantages. To figure that out, I&#39;d have to read and understand 1500 li=
nes of<br>
densely written Python. Without a human-readable pitch, not many people are=
<br>
going to do that, myself included.<br></blockquote><div><br></div><div>It&#=
39;s all about cache coherence. When doing operations it pulls in a bunch o=
f things which are near each other in memory instead of jumping all over th=
e place. The improvements it gets should be much greater than the ones gain=
ed from insertion ordering, although the two could be accretive.</div><div>=
<br></div></div></div></div>

--001a113ed7fef638b9054954e147--