Strange speeds

Ruby
1

Hello ruby people,
I’ve tried to make some speed test in ruby to find out how fast is
searching in hash (using has_key?)

The results were OK but I discovered curious thing – when i generate
big hash the speed is different when i unserialize (marshal.load) the
same hash.

Unserialized Hash is 1.5x faster in has_key? method. Tried it with
GC.disable as well, results were same.

What am I missing?

Thanx for any ideas

2

kryglik [email protected] wrote:

What am I missing?

Thanx for any ideas

The first reason that comes to mind is different insertion order. But I
hardly believe it could make such a big difference. Care to post your
testing code?

Kind regards

robert
3

Robert K. wrote:

GC.disable as well, results were same.

robert

Well, insertion order will be same, won’t be?

h = {}
h_slovo = []

if not true #HERE YOU SWITCH IF GENERATE/LOAD
for x in 1…40_000
if x % 2000 == 0
puts x
end

     slovo = ""

     for z in 1..32
         slovo += (rand(25)+65).chr
     end

     if rand(10) == 3
         h_slovo << slovo
     end

     h.update slovo => rand(1_000_000)
     slovo = nil
 end

 f = File.new("hash.marshal","w+")
 f.puts(Marshal.dump(h))
 f.close

 f = File.new("hash1.marshal","w+")
 f.puts(Marshal.dump(h_slovo))
 f.close

else

 puts "loading hash"
 h = Marshal.load(File.open("hash.marshal","r"))
 puts "loading hash1"
 h_slovo = Marshal.load(File.open("hash1.marshal","r"))
 puts "done"

end

t1 = Time.now

for slovo1 in h_slovo
h.has_key? h_slovo
end

t2 = Time.now

puts "hash: " + h.size.to_s
puts "hled: " + h_slovo.size.to_s
puts t2-t1

4

kryglik [email protected] wrote:

Unserialized Hash is 1.5x faster in has_key? method. Tried it with
Kind regards

robert

Well, insertion order will be same, won’t be?

Not necessarily. If Marshal.dump uses the current order in the hash for
writing then the insertion order during loading is likely to be
different
from the original insertion order.

end
puts "loading hash"

end

t2 = Time.now

puts "hash: " + h.size.to_s
puts "hled: " + h_slovo.size.to_s
puts t2-t1

Your time measurement is far too unprecise on a modern system. You’ll
have
to repeat lookups to get more accurate results. I get a difference
between 5
and 10 percent with the attached code - regardless of GC enabled or
disabled. Strange though that the copy is always slower. This is an
interesting issue.

Note, if you freeze keys lookups in the original hash are much faster
because then hash keys are not copied on insertion (an optimisation of
unfrozen Strings as hash keys), i.e., during lookup if the key is the
same
it’s also the same object which will be the first test - and of course
this
test is much faster than sequential comparison of characters.

Another note, a more realistic scenario would be if not 10% of all keys
were
used for lookup but if there also were keys that are not present in the
hash.

Kind regards

robert
5

Hello
thanx for nice script!

I find out that i got a little error :o) [see the ‘for’ cycle in
benchmark part:
for slovo1 in h_slovo
h.has_key? h_slovo (SHOULD NOT BE h_slovo BUT slovo1)
end

so when I run your script i got confused about speeds like 1-3s instead
of my 60s. The difference between marshaled and generated was like 35s
and 60s (10000 keys in 100.000 hash).

Still the mysteria continues – why the different speeds.

And also thanx for lecture of ruby optimization - not quite a guru (yet
:-D).

Benchmark is a nice module :slight_smile:

6

“k” == kryglik [email protected] writes:

k> Well, insertion order will be same, won’t be?

This is not the same hash

moulon% cat b.rb
#!/usr/bin/ruby
h = {}
32.times {|i| h[(rand(25)+65).chr] = i}
File.open(“hash.marshal”,“w+”) {|f| f.puts(Marshal.dump(h)) }
h1 = File.open(“hash.marshal”,“r”) {|f| Marshal.load(f) }
puts h.keys.join(’ ‘)
puts h1.keys.join(’ ')
moulon%

moulon% ./b.rb
V K W L A B M X C D O E F Q G R H I U
K V A L W X M B C O D E Q F R G H I U
moulon%

Guy Decoux

7

kryglik [email protected] wrote:

instead of my 60s. The difference between marshaled and generated was
like 35s and 60s (10000 keys in 100.000 hash).

In this case it’s probably due to hash calculation for the hash you
accidentally used as key.

Still the mysteria continues – why the different speeds.

Yeah, I guess we would have to dive into the sources to find out…

And also thanx for lecture of ruby optimization - not quite a guru
(yet :-D).

Take your time. :slight_smile:

Benchmark is a nice module :slight_smile:

It really is!

Kind regards

robert