Re: Why is java considered a language for "web" or "internet" programming?
Tom Forsmo wrote:
Mishagam wrote:
I just used my little bit C / Asm / Knuth education, and my very very
limited experience writing interpreters / compilers. I don't remember
reading exact things about Perl interpreter, for example. I assume
that for operation i = i+2 on Java after JIT (or on C / C++ after
compiler) you have do something like (asm instructions for native CPU,
r - register):
load r, i
add r, #2
store r, i
For perl, assuming what should be done, and taking into account what
you wrote, and using higher level operation here, you have to do:
load and parse bytecodes.
load i ptr
load i type bits
check i type
go to correct procedure for integers
load i integer value
add 2
store i value
return to start of next bytecode.
This is where you are mistaken, because many of the steps you mention
here are automaticaly performed by the processor and some dont need to
be any more elaborate than it is for c or java. The only thing that
takes more times is performing the bit test. Loading pointer and type
bits are predone, so is loading integer value along with i+2.
I don't understand what "predone" means here. I think, integer in Perl
or Python or other scripting languages is some structure, with at the
least field for type and value, and pointer to this structure you hold
in 'i' variable. So you have to load i, and then load i type and value.
(and also may be / probably reference counter and so on).
The actual
decision on whether to use integer procedure or not is branch-predicted,
so are the instructions for each branch. In addition to all this, much
of the bytecode is hotspotted or cached so the conversion to binary does
not need to happen again.
I again don't understand what "hotspotted" means here. I now think, that
you can convert bytecode to sequence of branch instructions, through it
appears that you have to have rather large branch table (to jump
depending from i type) so you cannot put these tables inline for each
command.
In my sequence I already assumed that conversion of '2' to binary and
checking of '2' type is done in (Perl) compiler.
So I assume shortened sequence can be:
jump to summing procedure
load i ptr
load i type
switch to procedure/case for integer addition
load i value
add 2
store i value
break // switch
return to next command
Which looks only about 3 times longer than Java version, takes more
space and more jumps.
As you can see below, this is definitely not how things are done in Python.
So the only real difference is the type
checking of the bits which is done in a couple of cycles.
So the end result is pretty much the same as teh above example, pluss a
couple of cycles as penalty, which does not amount to much.
Finally I decided to run real benchmarks. I wrote very short, but not
trivial integer calculations (so compiler will not optimize it away):
On VC++:
const int NRUNS = 1000;
const int NNUM = 1000000;
const int A = 23491;
const int B = 789175;
.....
clock_t t0 = clock();
for (int i=0; i<NRUNS; i++) {
int k = 5;
for (int j = 0; j<NNUM; j++) {
k = k*A + B;
kk = kk + k + j*j;
}
}
clock_t t1 = clock();
int rep = (NRUNS*NNUM);
double time = ((t1-t0)*1.0)/CLOCKS_PER_SEC;
printf("Speed test ended, kk = %d rep = %d, time = %9.3f\n", k, rep,
time);
On Java:
final static int NRUNS = 1000;
final static int NNUM = 1000000;
final static int A = 23491;
final static int B = 789175;
.....
long t0 = System.currentTimeMillis();
for (int i=0; i<NRUNS; i++) {
int k = 5;
for (int j = 0; j<NNUM; j++) {
k = k*A + B;
kk = kk + k + j*j;
}
}
long t1 = System.currentTimeMillis();
int rep = (NRUNS*NNUM);
double time = (t1 - t0)/1000.0;
System.out.println("Speed test ended, kk = " + kk +
" rep = " + rep + ", time = " + time);
On Python:
NRUNS = 1000
NNUM = 1000
A = 23491
B = 789175
MAXI = (1 << 32)
import time
print "Speed Test Started"
t0 = time.time()
i = 0
kk = 0
while i < NRUNS:
k = 5
j = 0
while j < NNUM:
k = (k*A + B) % MAXI;
kk = (kk + k + j*j) % MAXI;
j=j+1
i=i+1
t1 = time.time()
rep = (NRUNS*NNUM)
tm = (t1 - t0)/1.0
print "Speed test ended, kk = " + str(kk) + " rep = " + str(rep) + ",
time = " + str(tm)
Results :
VC++/Release - 2 sec
VC++/Debug - 4 sec
Java - 4 sec
Python - 2.5 sec - but you can see, that C / Java make 10 ^ 9
repetitions, and python makes 10^6 repetitions, so Python is 1000 times
slower - just what I expected !!
I agree, that we compare different things here, first in Python I
used while cycle, because 'for' has different semantics, (I don't think
this is very important here) and second - I had to make % MAXI for
Python, because C and Java ignored integer overflows, but Python tried
to make exact computations using unlimited precision.
Again, apparently Python doesn't make optimizations based on fact
that k is integer - but I think this is design choice if you use
typeless scripting language.
I didn't test Perl, because I hate Perl and it's syntax and I never
seen uglier language, so I choose Python as modern scripting language
that I like and assumed it is representative.
Do you really think that Perl would work substantially better in
this case?
Of course, such a micro operation, is going to take a little more time
because the language is dynamically typed and interpreted, but that is
the point of knuths assertion, dont: "dont pre-optimise".
I generally like you comments, but I usually really hate when
something makes advices against micro-optimization,
I like to be referred to as someone, not something, unless you think my
brain is such a super computer :)
Please excuse me for my typing error. I am sorry.