29 Nov
2006
29 Nov
'06
9:44 a.m.
This definitely sounds impressive, but, FMI:
On 11/28/06, Andrei Pelinescu-Onciul <andrei(a)iptel.org> wrote:
Why would you want to change the hash size from the
config? Do you
really know somebody who wanted/needed to do this? If you use a variable
for the hash size the compiler will not be able to optimize the modulo
operation (x % hash_size) and it will have to implement it using slow
DIVs. Using a 2^k constant the whole modulo operation will be optimized
to & (2^k - 1). A DIV takes minimum 56 cycles on a P4 and 16 on an
AMD64 (and this if the operands are in _registers_ which will probably
not happen for the variable containing the hash size). An "and" takes
only 1 cycle (and the operand is an immediate constant).
Look also at the rest of the hash function: it uses only XORs, shifts
and additions, operations that all execute in 1 cycle => for normal small
uris your DIV operation will take a very significant time from the
total ammount spent hashing.
Unfortunately Daniel didn't reply anymore (maybe he wants to cover trade
secrets ;-) ), but OpenSER uses now the much faster masking operation
instead of the always costly modulo (which I expect to execute faster even
with the cost of extra memory fetch for the hash size). I don't know whether
hashing is also cheap or the result has a good distribution, but this should
be something you should look at: I expect he did some research too, before
changing, maybe this is a fix worth porting.
WL.