Re: [bitcoin-dev] Some real-world results about the current Segwit Discount

From: James Hilliard <james.hilliard1@gmail•com>
To: Sergio Demian Lerner <sergio.d.lerner@gmail•com>
Cc: bitcoin-dev <bitcoin-dev@lists•linuxfoundation.org>
Subject: Re: [bitcoin-dev] Some real-world results about the current Segwit Discount
Date: Tue, 9 May 2017 09:33:34 -0500	[thread overview]
Message-ID: <CADvTj4pz9HauV7LD_uSnOJcrQ60W_2E76e16Ym4n6==NVffQvA@mail.gmail.com> (raw)
In-Reply-To: <CAKzdR-on-w9EF+2hLjchdyHB1gj7fi4QnybA=J4Cz7yyN3KKNA@mail.gmail.com>

The discount is designed to reduce UTXO bloat primarily, witness spam
data would not make it into the UTXO set. The discount brings the fee
of a transaction more in line with the actual costs to the network for
the transaction. A miner spamming the network with 4MB witness blocks
would have very little impact on the UTXO size compared with 1MB
non-witness blocks. UTXO size is a bigger issue than blockchain size
since full nodes can't prune the UTXO set.

The discount of 75% for the SegWit softfork doesn't really have any
effect on future hard forks as it can always be adjusted as needed
later on as part of a HF.

On Tue, May 9, 2017 at 8:49 AM, Sergio Demian Lerner via bitcoin-dev
<bitcoin-dev@lists•linuxfoundation.org> wrote:
> This [1] article says the current discount prevents witness spam. Witness
> spam is free space in the witness part of the block that can be filled by
> miners to create bigger blocks with almost no cost for the benefit a cluster
> of miners with low latency, increasing centralization.
>
> The 75% discount does not prevent it, but on the contrary leaves a lot of
> extra witness space for spam.
>
> If the maximum block weight is set to 2.7M, each byte of non-witness block
> costs 1.7, and each byte of witness costs 1, then a normal filled block
> would be 2.7M bytes (1.7+1), and there will be no need to create ever a 4
> Mbyte block. The worst case would be the average case, and the transaction
> rate would be the maximum possible.
>
> The current 75% discount can only achieve more transactions per second if
> the type of transactions change. Therefore the current 75% discount only
> makes the block size worst case worse (4 Mbytes when it should be 2.7
> Mbytes).
>
> 80% of all inputs/outputs are P2PKH. The only way to make use of the extra
> witness
> space If most P2PKH transactions are replaced by multisigs (typically for
> LN).
>
> So it seems the 75% discount has been chosen with the idea that in the
> future the current transaction pattern will shift towards multisigs. This is
> not a bad idea, as it's the only direction Bitcoin can scale without a HF.
> But it's a bad idea if we end up doing, for example, a 2X blocksize increase
> HF in the future. In that case it's much better to use a 50% witness
> discount, and do not make scaling risky by making the worse case block size
> 8 Mbytes, when it could have been 2*2.7=5.4 Mbytes.
>
> I've uploaded the code here:
> https://github.com/SergioDemianLerner/SegwitStats
>
>  [1]
> https://segwit.org/why-a-discount-factor-of-4-why-not-2-or-8-bbcebe91721e.
>
>
> On Mon, May 8, 2017 at 8:47 PM, Alphonse Pace via bitcoin-dev
> <bitcoin-dev@lists•linuxfoundation.org> wrote:
>>
>> Sergio,
>>
>> I'm not sure what the data you present has to do with the discount.  A 75%
>> discount prevents witness spam precisely because it is 75%, nothing more.
>> The current usage simply gives a guideline on how much capacity is gained
>> through a particular discount.  With the data you show, it would imply that
>> those blocks, with SegWit used where possible, would result in blocks of
>> ~1.8MB.
>>
>>
>>
>> On Mon, May 8, 2017 at 5:42 PM, Sergio Demian Lerner via bitcoin-dev
>> <bitcoin-dev@lists•linuxfoundation.org> wrote:
>>>
>>> I have processed 1000 blocks starting from Block #461653.
>>>
>>> I computed several metrics, including the supposed size of witness data
>>> and non-witness data (onchain), assuming all P2SH inputs/outputs are
>>> converted to P2PWSH and all P2PKH inputs/outputs are converted to P2WPKH.
>>>
>>> This takes into account that other types of transactions will not be
>>> modified by Segwit (e.g. OP_RETURN outputs, or P2PK). This analysis doesn't
>>> take into account that LN transactions may affect the current state,
>>> increasing the segwit/nosegwit ratio.
>>>
>>> Among a lot of information, I've got the following real world results...
>>>
>>> acMainChainSpace =352608924
>>> acSegwitSpace =599400403
>>> Ratio segwit/nosegwit=1.6999
>>>
>>> This implies that the 75% that discount is not the best option to prevent
>>> witness spam in a block of 4 MB, as stated in
>>> https://segwit.org/why-a-discount-factor-of-4-why-not-2-or-8-bbcebe91721e.
>>>
>>> The non-witness data weight factor should not be 4 but 2.35. The closest
>>> integer value is 2, which leads to a 50% witness discount.
>>>
>>> The Bitcoinj source code is available for anyone to review. I encourage
>>> anyone to re-compute this with another utility to cross-check. Maybe Antoine
>>> Le Calvez (p2sh.info) would like to double-check.
>>>
>>>
>>>
>>>
>>>
>>>
>>> _______________________________________________
>>> bitcoin-dev mailing list
>>> bitcoin-dev@lists•linuxfoundation.org
>>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>>
>>
>>
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