Concerns about performance w/Python, Pysco on Pentiums

John Machin sjmachin at lexicon.net
Fri Mar 7 16:57:11 CET 2003


Pedro Rodriguez <pedro_rodriguez at club-internet.fr> wrote in message news:<b49iot$r78$1 at s1.read.news.oleane.net>...
> On Thu, 06 Mar 2003 20:35:14 +0100, Peter Hansen wrote:
> 
> >     def step(self):
> >         opcodeByte = self.readByte(self.PC)
> >         try:
> >             opcode = self.opcodes[opcodeByte]
> >         except KeyError:
> >             raise UnimplementedOpcode('$%02X %s' % (opcodeByte,

> if you are using a dictionnary for opcodes, wouldn't you gain
> time by not putting a try/except clause but by going straight
> with self.opcodes.get(...) and checking the returned value 
> against None.

Rule 1: optimize for frequent events, not for infrequent events. In
emulating a CPU, which is more frequent, valid opcode or invalid
opcode? In your get-versus-try example, you do "valid" 256 times and
"invalid" (1000000 - 256) times.

Below is a slightly more realistic test, which seems to indicate that
the try-except caper is about 50% faster than dict.get() if you never
hit the "except" clause.

Cheers,
John

8<------

import time

def getter(q):
   l = {}
   for i in range(256):
       l[i] = i
   t0 = time.time()
   npass = nfail = 0
   lget = l.get
   for i in xrange(1000000):
       x = lget(q)
       if x is None:
           nfail += 1
       else:
           npass += 1
   t1 = time.time()
   print "get", q, t1 - t0, npass, nfail

def tryer(q):
   l = {}
   for i in range(256):
       l[i] = i
   t0 = time.time()
   npass = nfail = 0
   for i in xrange(1000000):
       try:
          x = l[q]
       except KeyError:
           nfail += 1
       else:
           npass += 1
   t1 = time.time()
   print "try", q, t1 - t0, npass, nfail

for q in (0, 1, 255, -1, 256, 257):
   getter(q)
   tryer(q)

8<-------
=== output (Python 2.2 Windows 32 bit version, on a 1.4Ghz Athlon)

get 0 1.30199992657 1000000 0
try 0 0.871000051498 1000000 0
get 1 1.27199995518 1000000 0
try 1 0.881000041962 1000000 0
get 255 1.36199998856 1000000 0
try 255 0.950999975204 1000000 0
get -1 1.31200003624 0 1000000
try -1 7.77199995518 0 1000000
get 256 1.32100009918 0 1000000
try 256 7.77199995518 0 1000000
get 257 1.30200004578 0 1000000
try 257 7.79100000858 0 1000000




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