uClibc++ associative_base performance

Garrett Kajmowicz gkajmowi at tbaytel.net
Thu Jul 5 23:36:25 UTC 2007


On Wednesday 04 July 2007 03:18, Asier Llano Palacios wrote:
> Quick performance analysis. I think I'm maybe biassed by the tests I've
> been performing, but I'd like to contribute a performance bump to uClibc
> ++, and I'd like to know your point of view.
>
> Current performance situtation
> ==============================
> Timing:
> uClibc++-0.2.1: My test spent 9 seconds.
> uClibc++-0.2.2: My test spent 1 minutes and 30 seconds (aprox).
> Target time: Less than 1 second.

Eek.  I expected that there would be a performance drop under certain 
workloads.  Still, this is worse than I expected.

I changed the underlying infrastructure (as well as representation) of the map 
and set classes so that I might be able to comply with one corner case of the 
spec (upon element deletion, all iterators still need to be valid).  The 
quick and dirty way to do that was to use a list as an underlying 
representation.  It makes me happy to reuse code in such a manner.

> General analysis:
> After some profiling I realized that most of the time is spent in
> std::map operations. In 0.2.1 the time is spent in the std::map::insert
> operation, and in uClibc++-0.2.2 the time is mostly spent in the
> std::map::find operation.
>
> Performance by operation:
> Having a look at the algorithms used in the associative containers in
> uClibc++ I've been looking which operations are not scalable. We will
> have a look at the media of the timing spent for each operation as a
> function of the number of elements in the collection.
> uClibc++-0.2.1:
>   find    = O(log(n))  <--- Binary search (fast)
>   insert  = O(n)       <--- Binary search, but linear storage (slow)
>   iterate = O(1)
> uClibc++-0.2.2:
>   find    = O(n)  <-- Linear search (very slow)
>   insert  = O(n)  <-- Linear search to insert (because of the find)
>   iterate = O(1)

 workload you would be running that would take that amount of time.  On a low 
memory system (which is what uClibc++ is geared towards), it's hard to have 
enough memory to have enough records that this should matter.  If it's a 
matter of inner loop performance, you would be best either taking a reference 
to the node that you are using over and over, or using iterators to go 
through the entire contents.  Both should result in constant-time 
performance.

If you have a very large amount of memory to the point that you are able to 
fill up a very large data set than I would suggest going with libstdc++ - it 
has algorithms that are optimized for speed whereas I went for readability 
(at least in my own mind) and size.

> What I'd like to contribute
> ===========================
> I'd like to contribute a performance improvement in every associative
> container by improving the associative_base. This way the set, mset, map
> and mmap. I'll try to implement it so that it still passes the same
> number of tests with the following timing:
>   find    = O(log(n))  <-- binary search in a tree
>   insert  = O(log(n))  <-- binary search, tree bouncing.
>   iterate = O(1)       <-- walking in a tree (it will be slower than
>                            the previous but will not grow with n).
>
> Of course, I wan't to mantain:
>   - Small code, small binary size and small memory footprint
>   - Iterators point to the same position even after element insertion
>     and deletion.
>
> You'll have news about this implementation as soon as I finish it.

I'm happy to consider any contribution that you would like to make. 

>
> Licensing
> =========
> The only thing I don't like about uClibc++ is its LGPL license, its
> quite restrictive to me. I work for an enterprise where I develop and
> contribute all the general purpose code (the code that is not specific
> for our application). I consider myself a community contributor. I use
> uClibc++ and I have other application specific libraries (that I don't
> see the point of sharing them, because they are too specific of our
> embedded architecture). Even if I contribute the other libraries I
> wouldn't contribute them under LGPL. While mantaining the source code
> like it is now, I would like to build the uClibc++ and my specific
> libraries in one shared library to make the binary size of the code
> smaller (having only one "general purpose for me" shared library). I've
> tested it and I could gain some binary size by joining both libraries
> compilation. I think that I cannot join LGPL and other binary code in a
> single library. Is there any possibility to have a less restrictive
> license (like X11 or BSD)?. I don't want to fork, nor sell uClibc++, I
> only want to have linking flexibility with uClibc++ and some non LGPL
> code.

I personally have objections to using a license similar to BSD or X11.  One 
person has made the argument that I should instead be using the GPL with 
linking exception (same thing that libstdc++ uses), but I haven't bothered 
converting as of yet.


> Thank you,
> Asier
>
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<snark>
It is so privileged that you sent it to a publically accessible mailing list 
with a published public archive?  Unencrypted?  Wow.  And I was just starting 
to think that you knew what you were talking about.  I like the bit about how 
I'm supposed to know to "destroy it without review" the document when the 
super-secret stamp is attached at the bottom.

Finally, the message is scanned for viruses, but you disclaim liability.  Why 
tell me this?  And if somehow the document contained a virus in the main 
message (as opposed to an attachment) would I then be able to sue you because 
your disclaimer doesn't cover it?

*sigh*
</snark>

Thanks for the input.  I look forward to seeing what you come up with.

-     Garrett





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