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Re: About performance, hash-tables and garbage collection

From: Stefan Monnier <address@hidden>
Subject: Re: About performance, hash-tables and garbage collection
Date: 20 Sep 2002 17:07:38 -0400
User-agent: Gnus/5.09 (Gnus v5.9.0) Emacs/21.3.50

> 1. I have read that hash-tables have a little overhead and that
>    association lists are a little bit faster, when they contain not
>    more than about a dozen of key-value pairs. It is _possible_ that I
>    have to maintain several hundreds or thousand of key/value pairs,
>    but in most cases it will be only a dozen or so. So my question is:
>    how much overhead has a hashtable? Makes it sense to add a
>    condition, like this:
>    (if (> ncolours 15)
>        (gethash colour colour-map)
>      (cdr (assoc (colour colour-map))))

The `if' overhead will probably dwarf the difference, so I'd say,
just use the hash-table.  Most likely any performance problem you'll have
won't come from that anyway.

> 2. I noticed that the garbage collector hits in a little bit too often,

What's too often ?  Why do you care ?
Does it use a significant percentage of the running time ?
If yes, look for things like `append' and things like that to see where
you do most allocation.

>    though I do not know why. I have a lot of `setq'-statements
>    like `(setq pointer (1+ pointer))'. Is it possible that they are
>    the cause?

If the code is interpreted, maybe, tho I don't think so.

>  If so, what could I do?

If you've byte-compiled your code, then an increment (as shown above) will
not allocate any memory and will thus have no influence on GC.


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