[Libguestfs] [PATCH nbdkit 1/5] Add new public nbdkit_parse_probability function

[Richard W.M. Jones]

On Tue, May 16, 2023 at 09:22:31PM +0100, Richard W.M. Jones wrote:
> I'm not really sure we reached a conclusion so far, but I did want to
> say that I changed the evil filter impl so that it now treats any
> 0 <= P < 1e-12 as effectively 0.  (P == evil-probability; P < 0 and P > 1
> are still rejected at config time as before).
> 
> With P == 1e-12:
> 
> nbdkit: debug: evil: mode: stuck-bits, P: 1e-12
> nbdkit: debug: evil: block_size: 17592186044416 (2**44)
> nbdkit: debug: evil: expected bits per block: 140.737
> 
> (The large block size isn't an issue here as nothing is allocated.
> However it would be a problem if the block_size calculation overflows,
> and I believe that limiting P to larger values avoids this.)

To clarify what I meant ...

We aim to choose a block size where about 100 bits are corrupted in
the block, which limits the time taken when the code loops over the
block.  Since the block size must also be a power of 2, it ends up
containing slightly more corrupted bits (on average 140 in the case
above).

The block size is calculated as the reciprocal of P.  By limiting P to
be either "effectively 0", or in the range [1e-12 .. 1], we avoid
creating blocks larger than 2**44.  If P < 1e-12 then it is handled by
a separate code path and treated as zero.  If P > 1e-12 then the block
size will be smaller than above.

So my reasoning is that this should not overflow anything.

> Also I'm not sure that handling probabilities as a numerator and
> denominator actually helps here?  Both still need to be stored as
> floats, so it just pushes the same problem to the plugin.

Rich.

-- 
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