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behaviour of "long" seems a bit outdated to me.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.
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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