[TUHS] mh/hm, mmh (was: fmt(1): history, POSIX, -t, -c)

Larry McVoy lm at mcvoy.com
Mon Jun 1 02:50:50 AEST 2020

On Sun, May 31, 2020 at 12:25:47PM -0400, Richard Salz wrote:
> Originally ihnp4!mirror!rs at seismo.arpa, then ihnp4!mirror!rs at seismo.css.gov
> and then rs%mirror.uucp at seismo.css.gov and then rs at mirror.tmc.com  Thanks
> to Mary Ann and the "UUCP Mapping Project" for making much of that possible
> and Peter Honeyman for Pathlias

My first real research paper was with Udi Manber and it was all about 
compressing the pathalias maps.  I had them on a 40MB disk on a Masscomp
that had 20 users.  The format was

seismo.css.gov	geowhiz!geophiz!uwisc!ihnp4!mirror

in other words

destination	path!to!get!there

I went to Udi and explained that all of our stuff started with
so that string was replicated over and over and it took too much

Once Udi understood the problem he went "You have time best case,
space worst case, you can reverse them like so"

seismo	-> mirror
mirror	-> ihnp4
ihnp4	-> uwisc
uswisc	-> geophiz
geophiz	-> geowhiz

and you'll have time worst case and space best case.  I was like "oh,
yeah, you are right, cool, thanks" and got up to leave.  Not so fast,
says Udi, that's not "interesting".  What would be interesting is if
you could approximate time best case and space best case.

With that, I started down the only dynamic programming problem I have
ever solved.  Udi's idea was to break the graph at what he called
pivot points, where the pivot points as much as possible got rid of
the replicated strings.  He also wanted to limit lookups to just

So I had to read in the maps and build a graph from them on my
creaky microvax with 4MB of ram.  Doing so lead me to the first
place I taught Udi about systems.  I was fgets() each line and 
mallocing a copy of it.  Then I sorted with qsort() and it took
overnight to get the sorted array.  Udi asked what sort?  Qsort.
Oh, that's garbage, use a radix sort, that will be better.  WTF
is a radix sort, look that up, implemented it, it was worse.

So I start poking around and somehow realized we were thrashing
VM, we didn't fit, and I have no idea how I figured out that
malloc was fragmenting memory but I did.  Probably figured out
that most of my program was backed by swap, not files, and that's
usually malloc or brk.

So I wrote my own malloc that grabbed memory in 300KB chunks (why
that size, I dunno, seemed right).  Then I allocated memory out
of those chunks.  Got rid of the fragmentation and we fit in
4MB.  qsort worked fine, took about 20 minutes (we didn't really
fit but we were way closer).

Udi was watching as I was doing all this, I think I was working
in his office, it was his microvax.  The fragmentation thing 
taught him that when Unix says it has virtual memory and you
can just use it, it only works well when you fit.

So the programming problem turned out that you calculated the 
space it work take for every possible pivot point and then at
the end, you took the set that gave you the minimum space.  We
wrote, heh, who am I kidding, Udi wrote, a paper about it.  I
was still green on writing papers, they were hard for me.  I told
Udi that and he stared at me blankly and said "Writing papers is
easy if you know the content.  You just make a good outline and
fill it in."  He's right, that's how you do it, and it is easy
if you know and can make the outline.

I went on to do a lot of work with/for him.  I wrote a user level
thread library that made me write swtch() in assembler for the VAX
(I believe I did a 68K version too but the source I have is VAX
only).  I "cheated" and did 99% of the work in C and popped down
the rabbit hole to assembler, did the nasty, and popped back out
as a different thread.  112 lines:


Funny Udi story, I was young and wild and drove too fast and got 
tickets.  So I got a radar detector that beeped when it saw the
radar gun.  Udi upgraded the microvax to some newer release of
X10 or X11 and the new xclock beeped on the hour.  Every time
it beeped, I jumped because it sounded just like the radar 
detector.  Udi asked about it, I explained, he laughed and said
"Larry, you're hacking too fast" each time I jumped :)

Good times.  I almost did a PhD under him but I didn't care for
the CS department at Tucson where he went and I liked industry
too much.

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