[Cython] Hash-based vtables

[Robert Bradshaw]

On Tue, Jun 12, 2012 at 10:21 AM, Dag Sverre Seljebotn
<d.s.seljebotn at astro.uio.no> wrote:
> On 06/12/2012 01:01 PM, Dag Sverre Seljebotn wrote:
>>
>> On 06/10/2012 11:53 AM, Robert Bradshaw wrote:
>>>
>>> On Sun, Jun 10, 2012 at 1:43 AM, Dag Sverre Seljebotn
>>>>
>>>> About signatures, a problem I see with following the C typing is that
>>>> the
>>>> signature "ill" wouldn't hash the same as "iii" on 32-bit Windows and
>>>> "iqq"
>>>> on 32-bit Linux, and so on. I think that would be really bad.
>>>
>>>
>>> This is why I suggested promotion for scalars (divide ints into
>>> <=sizeof(long) and sizeof(long)< x<= sizeof(long long)), checked at
>>> C compile time, though I guess you consider that evil. I don't
>>> consider not matching really bad, just kind of bad.
>>
>>
>> Right. At least a convention for promotion of scalars would be good
>> anyway.
>>
>> Even MSVC supports stdint.h these days; basing ourselves on the random
>> behaviour of "long" seems a bit outdated to me. "ssize_t" would be
>> better motivated I feel.
>>
>> Many linear algebra libraries use 32-bit matrix indices by default, but
>> can be swapped to 64-bit indices (this holds for many LAPACK
>> implementations and most sparse linear algebra). So often there will at
>> least be one typedef that is either 32 bits or 64 bits without the
>> Cython compiler knowing.
>>
>> Promoting to a single type "[u]int64" is the only one that removes
>> possible combinatorial explosion if you have multiple external typedefs
>> that you don't know the size of (although I guess that's rather
>> theoretical).
>>
>> Anyway, runtime table generation is quite fast, see below.
>>
>>>
>>>> "l" must be banished -- but then one might as well do "i4i8i8".
>>>>
>>>> Designing a signature hash where you select between these at
>>>> compile-time is
>>>> perhaps doable but does generate a lot of code and makes everything
>>>> complicated.
>>>
>>>
>>> It especially gets messy when you're trying to pre-compute tables.
>>>
>>>> I think we should just start off with hashing at module load
>>>> time when sizes are known, and then work with heuristics and/or build
>>>> system
>>>> integration to improve on that afterwards.
>>>
>>>
>>> Finding 10,000 optimal tables at runtime better be really cheap than
>>> for Sage's sake :).
>>
>>
>> The code is highly unpolished as I write this, but it works so here's
>> some preliminary benchmarks.
>>
>> Assuming the 64-bit pre-hashes are already computed, hashing a 64-slot
>> table varies between 5 and 10 us (microseconds) depending on the set of
>> hashes.
>>
>> Computing md5's with C code from ulib (not hashlib/OpenSSL) takes ~400ns
>> per hash, so 26 us for the 64-slot table => it dominates!
>>
>> The crapwow64 hash takes ~10-20 ns, for ~1 us per 64-slot table.
>> Admittedly, that's with hand-written non-portable assembly for the
>> crapwow64.
>>
>> Assuming 10 000 64-slot tables we're looking at something like 0.3-0.4
>> seconds for loading Sage using md5, or 0.1 seconds using crapwow64.
>>
>>
>> https://github.com/dagss/pyextensibletype/blob/master/include/perfecthash.h
>>
>> http://www.team5150.com/~andrew/noncryptohashzoo/CrapWow64.html
>
>
> Look: A big advantage of the hash-vtables is that subclasses stay
> ABI-compatible with superclasses, and don't need recompilation when
> superclasses adds or removes methods.
>
> => Finding the hash table must happen at run-time in a lot of cases anyway,
> so I feel Robert's chase for a compile-time table building is moot.
>
> I feel this would also need to trigger automatically heap-allocated tables
> if the statically allocated. Which is good to have in the very few cases
> where a perfect table can't be found too.

Finding the hash table at runtime should be supported, but the *vast*
majority of methods sets is known at compile time. 0.4 seconds is a
huge overhead to just add to Sage (yes, it's an exception, but an
important one), and though crapwow64 helps I'd rather rely on a known,
good standard hash. I need to actually look at Sage to see what the
impact would be. Also, most tables would probably have 2 entries in
them (e.g. a typed one and an all-object one).

long int will continue to be an important type as long as it's the
default for int literals and Python's "fast" ints (whether in type or
implementation), so we can't just move to stdint. I also don't like
that the form of the table (and whether certain signatures match)
being platform-dependent: the less variance we have from one platform
to the next is better.

On an orthogonal note, sizeof(long)-sensitive tables need not be
entirely at odds with compile-time table compilation, as most
functions will probably have 0 or 1 parameters that are of unknown
size, so we could spit out 1 or 2 statically compiled tables and do
generate the rest on the fly. I still would rather have fixed
Cython-compile time tables though.

> One thing is that, which makes me feel uneasy about the relatively
> unexplored crapwow64 is that we really don't want collisions in the 64-bit
> prehashes within a single table (which would raise an exception -- which I
> think is OK from a security perspective, you can always have a MemoryError
> at any point too, so programmers should not expose class creation to
> attackers without being able to deal with it failing).
>
> For the record, I found another md5 implementation that's a bit faster;
> first one is "sphlib" and second is "ulib":
>
> In [2]: %timeit extensibletype.extensibletype.md5bench2(10**3)
> 1000 loops, best of 3: 237 us per loop
>
> In [3]: %timeit extensibletype.extensibletype.md5bench(10**3)
> 1000 loops, best of 3: 374 us per loop
>
> http://www.saphir2.com/sphlib/
>
> It's really only for extremely large projects like Sage where this can be
> noticed in any way.
>
>
> Dag
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