[TUHS] Moto and MMU issues -- was Unix & Memory Management Units (MMU)

Joerg Schilling schily at schily.net
Fri Dec 9 20:25:32 AEST 2016


Chet Ramey <chet.ramey at case.edu> wrote:

> On 12/8/16 11:38 AM, Ron Natalie wrote:
> > /
> > 
> > The original Bourne Shell did not use malloc, but rather had a SIGSEGV handler
> > that used to extend the "string stack" called "stak" via sbrk() whenever the
> > code tried to access data beyond the end ot the heap./
> > 
> > / /
> > 
> > The V6 (Mashey) shell did that.   I thought they???d gotten rid of that by
> > the time the Bourne Shell rolled around.

Do you like to say that the Mashey shell also used the string stack and a 
SIGSEV handler?

Well, from a talk from Stephen Bourne I know that both the Bourne Shell and the 
Mashey shell did start as a modified Thompson shell. I should have a look at the
Thompson shell sources...

> That was one of the changes Bourne made during the deliberations over
> which shell would be the Unix "standard" for Bell Labs.  It was always
> an efficiency hack, meant to address Mashey shell user concerns about
> performance.

I am not sure whether this kind of SIGSEGV based string handling helps to have 
a good performance. Having a string stack (regardless of what the low level 
part of the implementation looks like) definitely helps. I am using a similar
technique in my "smake" program and it turns out that the concept of a string
stack is helpful when you write software that needs to create many 
intermediate string copies of unpredictable length.

What I definitely know is that replacing the low level support code from 
Stephen Bourne (stak.c) with an implementation based on the ideas from Geoff 
Collyer did not affect the performance of the Bourne Shell.

The Korn shell did introduce something similar (maybe even also from Geoff 
Collyer with ksh88) and ksh93 is the fastest known shell implementation if it 
is compiled the way, it is in the OpenSolaris integration of the Solaris "ON"
code base.

For today, numbers definitely look different. 30% of the total amount of CPU 
time spend by the Bourne Shell is spend by the conversion from multy-byte 
strings to wide strings and vice versa. BTW: "dash" is faster than bash just 
because it does not imlement multy-byte support. If it did, it would become 
slower than bash.

Jörg

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