Re: The myth of fingerprints, er, no, garbage collectors

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Rewrite it in Rust =F0=9F=99=84

On Thu, Jun 25, 2026 at 6:19=E2=80=AFPM Tim Bradshaw (as tfb at tfeb dot or=
g) <
[email protected]> wrote:

> Given this silly function:
>
> (defun smash (n m)
>   (declare (type fixnum n m))
>   (let ((c nil))
>     (dotimes (i n c)
>       (setf c (cons 1 (if (zerop (mod i m)) nil c))))))
>
> You can time it.  (smash (expt 10 9) 10)) allocates about 16GB, and takes
> 25 seconds.  GC time is about 0.3% of that time.
>
> You can also watch GC time grow as it has to copy more: (smash (expt 10 9=
)
> 1000) makes lists which can grow to 1000 elts: GC now is well over 0.5% o=
f
> the time. Keeping lists of a million elements results in the GC taking 10=
%
> of the runtime.
>
> The reason I have been playing with all these things is I keep finding
> myself writing functions which carefully allow you to, for instance pass =
in
> some object which they will then reuse instead of allocating one.  And
> after trying to measure if this was worthwhile, I realised that it almost
> never is: it's better to allocate the thing on the fly and throw it away,
> because the GC will then never see it at all, whereas if you hold onto it=
,
> it will.
>
> As someone who remembers when a GC meant 'you can go and make a cup of te=
a
> now', modern GCs are wonderful things: we forget how spoiled we are.
>
> --tim
>
> _______________________________________________
> Lisp Hug - the mailing list for LispWorks users
> [email protected]
> http://www.lispworks.com/support/lisp-hug.html
>


--=20
Marco Antoniotti, Professor, Director         tel. +39 - 02 64 48 79 01
DISCo, University of Milan-Bicocca U14 2043   http://dcb.disco.unimib.it
Viale Sarca 336
I-20126 Milan (MI) ITALY

REGAINS: https://regains.disco.unimib.it/

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<div dir=3D"ltr">Rewrite it in Rust=C2=A0=F0=9F=99=84</div><br><div class=
=3D"gmail_quote gmail_quote_container"><div dir=3D"ltr" class=3D"gmail_attr=
">On Thu, Jun 25, 2026 at 6:19=E2=80=AFPM Tim Bradshaw (as tfb at tfeb dot =
org) &lt;<a href=3D"mailto:[email protected]">[email protected]</=
a>&gt; wrote:<br></div><blockquote class=3D"gmail_quote" style=3D"margin:0p=
x 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Gi=
ven this silly function:<br>
<br>
(defun smash (n m)<br>
=C2=A0 (declare (type fixnum n m))<br>
=C2=A0 (let ((c nil))<br>
=C2=A0 =C2=A0 (dotimes (i n c)<br>
=C2=A0 =C2=A0 =C2=A0 (setf c (cons 1 (if (zerop (mod i m)) nil c))))))<br>
<br>
You can time it.=C2=A0 (smash (expt 10 9) 10)) allocates about 16GB, and ta=
kes 25 seconds.=C2=A0 GC time is about 0.3% of that time.<br>
<br>
You can also watch GC time grow as it has to copy more: (smash (expt 10 9) =
1000) makes lists which can grow to 1000 elts: GC now is well over 0.5% of =
the time. Keeping lists of a million elements results in the GC taking 10% =
of the runtime.<br>
<br>
The reason I have been playing with all these things is I keep finding myse=
lf writing functions which carefully allow you to, for instance pass in som=
e object which they will then reuse instead of allocating one.=C2=A0 And af=
ter trying to measure if this was worthwhile, I realised that it almost nev=
er is: it&#39;s better to allocate the thing on the fly and throw it away, =
because the GC will then never see it at all, whereas if you hold onto it, =
it will.<br>
<br>
As someone who remembers when a GC meant &#39;you can go and make a cup of =
tea now&#39;, modern GCs are wonderful things: we forget how spoiled we are=
.<br>
<br>
--tim<br>
<br>
_______________________________________________<br>
Lisp Hug - the mailing list for LispWorks users<br>
<a href=3D"mailto:[email protected]" target=3D"_blank">lisp-hug@lispwo=
rks.com</a><br>
<a href=3D"http://www.lispworks.com/support/lisp-hug.html" rel=3D"noreferre=
r" target=3D"_blank">http://www.lispworks.com/support/lisp-hug.html</a><br>
</blockquote></div><div><br clear=3D"all"></div><br><span class=3D"gmail_si=
gnature_prefix">-- </span><br><div dir=3D"ltr" class=3D"gmail_signature"><d=
iv dir=3D"ltr"><div><span style=3D"font-family:monospace">Marco Antoniotti,=
 Professor, Director =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 tel. +39 - =
02 64 48 79 01<br>DISCo, University of Milan-Bicocca U14 2043=C2=A0=C2=A0 <=
a href=3D"http://dcb.disco.unimib.it" target=3D"_blank">http://dcb.disco.un=
imib.it</a><br>Viale Sarca 336<br>I-20126 Milan (MI) ITALY<br><br></span></=
div><span style=3D"font-family:monospace">REGAINS: <a href=3D"https://regai=
ns.disco.unimib.it/" target=3D"_blank">https://regains.disco.unimib.it/</a>=
<br></span></div></div>

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_______________________________________________
Lisp Hug - the mailing list for LispWorks users
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http://www.lispworks.com/support/lisp-hug.html