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Re: Today's problem with GUB build

From: David Kastrup
Subject: Re: Today's problem with GUB build
Date: Thu, 16 Jul 2020 10:58:16 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/28.0.50 (gnu/linux)

Han-Wen Nienhuys <> writes:

> On Wed, Jul 15, 2020 at 10:48 PM David Kastrup <> wrote:
>> Well ok.  But only 1000000 random numbers are being used (there is
>> another call using 10000000 instead, the choice appearing random).
>> Let's assume we have 10 processes going through 138 files each.  The
>> processes are going to switch to the next output file asynchronously, so
>> with any change, there is a chance of the old number colliding with the
>> other processes' numbers, and the new number colliding.  The probability
>> that a new number is different from an existing set of 9 is
>> 999991/1000000.  If we do this switch 1380 times, the probability of a
>> collision during one run is 1-(999991/1000000)^1380, about 1 in 80.
> I don't think this analysis is correct. In order for two numbers to
> meaningfully collide, they have to be picked at the same time (since
> we rename the files afterwards.).

And 10 numbers are picked at the same time since 10 processes work on
their temporary files at the same time.

> The birthday attack says you need 1000 parallel instances to get a 50%
> of collision.

For a single occasion of collision.  Not for 1300 occasions in sequence.
How about addressing my actual argument?

>> Now if I remember correctly, there were some changes in how
>> lilypond-book worked that typically resulted in double the number of
>> processes getting spawned than asked for which would give us 19 instead
>> of 9 possibilities for collision.  That would raise the probability of a
>> collision to about 1 in 40 runs.
> I think you misremember. The command
>   make -jN CPU_COUNT=N
> will potentially spawn 2*N -1 processes: 1 x lilypond-book with N
> children and N- 1 other processes.

So back to 1 in 80.  Still not the best odds if a collision is fatal.

David Kastrup

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