I do not understand what is happening here.
oc := OrderedCollection new: 10000.
oc size. "0"
a := Array new: 10000.
a size. "10000"
So it seems that the #new: isn't creating an OrderedCollection of the
size we pass in.
I discovered this when I attempted to do the identical test but with an
Array instead of an OrderedCollection. Since the Array doesn't #add: I
had to change the code to #at:put:.
When I noticed all of the OC code did #add: even if supposedly setting
its size to 10000(0). I tried to use #at:put: in the OrderedCollection
but it failed because their was no index 1, because the size was still 0.
I naively would have thought that OrderedCollection new: 10000 would
return an OrderedCollection of 10000 empty/nil elements and that I could
#at:put: into any of those 10000 slots and #add if I needed to go beyond
those 10000.
Tests done on a laptop with Windows 7, Pharo 4 latest image, latest
stable vm, 3rd gen i7 processor, 12gb ram.
I did all of the tests in Pharo 4 and added my Array test.
0:00:00:04.37
0:00:00:03.183
0:00:00:03.674
0:00:00:03.753
0:00:00:01.647 and with an Array instead of OrderedCollection
So we can see if we know the max size and create the collection with
that size it should improve performance.
I also do not understand why Pharo is so slow on this?
With the preallocated Array above it took 1.647 seconds.
Python 3.4.2, 0.003 seconds with 10,000 ints, and 0.17 with 1,000,000
ints.
Julia 3.5, 0.005 seconds with 1,000,000 Int64.
Now I don't expect Pharo to keep up with Julia. But we aren't even in
the neighborhood with Python.
I really would like to use Pharo. It really is my favorite language and
environment. But these performance numbers give me pause.
Jimmie
On 1/14/2015 3:53 AM, Max Leske wrote:
Hi Alex
I thought the following little experiment might interest you.
I was trying to find out (as per your suggestion) how big the difference would
be between:
- setting the array in OrderedCollection to nil and initializing it to size 10
on first access
- initializing the array in OrderedCollection to size 0.
Expectation: #grow will take time, ergo, using an empty array should be slower
than using
a preallocated array.
Experiment (Pharo 1.3 on SqueakVM):
[ 10000 timesRepeat: [ | c | c := OrderedCollection new: 100000.
1 to: 10000 do: [ :i | c add: i ] ] ] timeToRun 17566
[ 10000 timesRepeat: [ | c | c := OrderedCollection new: 10000.
1 to: 10000 do: [ :i | c add: i ] ] ] timeToRun 19717
[ 10000 timesRepeat: [ | c | c := OrderedCollection new: 0.
1 to: 10000 do: [ :i | c add: i ] ] ] timeToRun 17598
[ 10000 timesRepeat: [ | c | c := OrderedCollection new: 1000.
1 to: 10000 do: [ :i | c add: i ] ] ] timeToRun 18084
Result: growing a collection is always faster (or at least nearly equally fast)!
Explanation: What makes preinitialized collections so slow is the fact the the
firstIndex variable will be set to
the first third of the array (3333 in this case). When the last
position in the array is reached, all elements
are copied to the beginning of the array (2n operations). This copy
operation is really slow (which can be seen
from the first example with an oversized collection that does not
require copying because the last position
of the array is never reached).
Experiment (Pharo 4 on PharoVM):
(I’m using bigger numbers here to compensate for the faster VM :) )
[ 10000 timesRepeat: [ | c | c := OrderedCollection new: 1000000.
1 to: 100000 do: [ :i | c add: i ] ] ] timeToRun "0:00:01:09.225"
[ 10000 timesRepeat: [ | c | c := OrderedCollection new: 100000.
1 to: 100000 do: [ :i | c add: i ] ] ] timeToRun "0:00:00:37.653"
[ 10000 timesRepeat: [ | c | c := OrderedCollection new: 0.
1 to: 100000 do: [ :i | c add: i ] ] ] timeToRun "0:00:00:40.152"
[ 10000 timesRepeat: [ | c | c := OrderedCollection new: 10000.
1 to: 100000 do: [ :i | c add: i ] ] ] timeToRun "0:00:00:42.121”
Result: growing a collection is easily fast enough, although minute performance
gains are possible with preinitialized collections.
Explanation: #makeRoomAtLast has been improved. Also: firstIndex is now set to
the beginning of the array, preventing the copy
operation that made the former measurements so slow.
The run with the oversized collection is only so slow because it
apparently takes a lot of time to assign the large array (?).
Consequence of my little experiment: My initial implementation of the idea in
you paper set the array in OrderedCollection to nil
under the assumption that grow operations are very costly. That choice
leads to many changes in methods of OrderedCollection
where it is necessary to check if the array has been initialized yet.
This introduces overhead and makes the code more error prone.
I see now that it makes much more sense to use an empty array, the
performance lost by growing ist neglectable.
It is also interesting to note that there almost never seems to be a
good reason to initialize an OrderedCollection with a large array.
Cheers,
Max
