Re: Speed Traps
Re: Speed Traps
- Subject: Re: Speed Traps
- From: David Remahl <email@hidden>
- Date: Thu, 15 May 2003 00:22:38 +0200
I spent a day rewriting a custom expression parcer for what I hoped
would give me much more speed but the final result is now slower. It
is hard to believe it is slower; could I be missing some Cocoa speed
traps?
First version parced a NSString expression and evaluated as it went.
New version tokenizes the expression on the first past and then all
subsequent passes do no parcing, but just evaluate using the tokens.
The new version evaluation with tokens is slower than the first
version that parced AND evaluated on every pass. It does not make
sense and I am hoping I missed something or am using some Cocoa
feature in new version that is cause speed hang up?
One finding and maybe clue to speed traps:
In evaluating from tokens for expressions with groupings, I recursive
pass sub arrays of tokens to a function. If I allocate the sub arrays
and release them it it is slower than using autoreleased arrays. In
other words
subExpr=[[NSMutableArray alloc] initWithArray:...]
evaluate
[subExpr release];
is noticeably slower than
subExpr=[NSMutableArray arrayWithArray:...]
evaluate
I used the former method because I was concerned this large
calculation would be building up a large pool of autoreleased objects
(several hundred thousand) and slowing things down. I guess that was
wrong.
While I can't comment on your main question, I believe I can give an
explanation why method two is faster than method one.
As you probably know, NSArrays are implemented as class clusters. That
means that objects of different classes may be returned depending on
what sort of data they contain. They way it works, is that
[NSMutableArray alloc] returns one object, which is then replaced by
another one when the initWithArray: method is called. The first
allocation is then useless, and it is released.
NSMutableArray most likely implements arrayWithArray: more efficiently,
since it already at the time of the first allocation knows what data
will be contained in the array. It can thereby avoid a few steps, and
create the correct object straight ahead.
/ Rgds, David
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| References: | |
| | >Speed Traps (From: John Nairn <email@hidden>) |