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Re: [Bug-apl] segfault when using 'CORE_COUNT_WANTED' configure flag

From: Rowan Cannaday
Subject: Re: [Bug-apl] segfault when using 'CORE_COUNT_WANTED' configure flag
Date: Thu, 17 Oct 2019 11:03:20 -0400


I am new to APL (~9 months). Most of my day-to-day work is sql/shell, however I use APL for a couple things: 1.) as an ad-hoc calculator & 2.) a symbolic notation that greatly simplifies complex mathematical calculations in how I think about, remember, and approach them.

As for things I've built in APL, its been mostly toy projects. I built a simple artificial neural net, however ran into difficulties when attempting to generalize backpropagation
for arbitrary array sizes. I would like to complete this at some point.

Parallelizing APL was more of a curiosity, than an immediate need. I do not plan to invest much energy into pursuing it at this time (especially in light of Jürgen's explanation). To be honest I had watched a talk about cpu caching in C++ and was interested in how that related to APL's handling of arrays.

On Wed, Oct 16, 2019 at 9:53 PM Peter Teeson <address@hidden> wrote:
Hi Rowan:

What classes of problems are you trying to solve that would benefit from parallel processing?


Peter Teeson
On Oct 16, 2019, at 1:27 PM, Dr. Jürgen Sauermann <mail@xn--jrgen-sauermann-zvb.de> wrote:

Hi Rowan,

actually there is no syntax tree in GNU APL. The way in which APL binds names
(*late and ambiguously)  makes it fairly useless to parse it beforehand. What happens
in GNU APL is prefix matching at runtime. The prefix-table is in src/Prefix.def (an
automatically generated hast table that does lookups essentially in time O(1) per prefix.)

This lookup table replaces the AST that you would have in a compiled language,

Best regards,
Jürgen Sauermann

On 10/16/19 6:55 PM, Rowan Cannaday wrote:
Thanks again,

AST = abstract syntax tree. The tree-like structure that is produced by the parser.

Avoiding compilation is a reasonable restriction.

Thanks for the context.

- Rowan

#gdb apl
GNU gdb (Debian 8.3.1-1) 8.3.1
Copyright (C) 2019 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
Find the GDB manual and other documentation resources online at:

For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from apl...
(gdb) run
Starting program: /usr/local/bin/apl
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[Detaching after vfork from child process 23377]
[New Thread 0x7ffff68d7700 (LWP 23381)]
[New Thread 0x7ffff60d6700 (LWP 23382)]
[New Thread 0x7ffff58d5700 (LWP 23383)]

                    ______ _   __ __  __    ___     ____   __
                   / ____// | / // / / /   /   |   / __ \ / /
                  / / __ /  |/ // / / /   / /| |  / /_/ // /
                 / /_/ // /|  // /_/ /   / ___ | / ____// /___
                 \____//_/ |_/ \____/   /_/  |_|/_/    /_____/

                    Welcome to GNU APL version 1.8 / 1191M

                Copyright (C) 2008-2019  Dr. Jürgen Sauermann
                       Banner by FIGlet: www.figlet.org

                This program comes with ABSOLUTELY NO WARRANTY;
                  for details run: /usr/local/bin/apl --gpl.

     This program is free software, and you are welcome to redistribute it
         according to the GNU Public License (GPL) version 3 or later.


Session duration: 5.06809 seconds
Couldn't read debug register: No such process.
(gdb) Cannot find user-level thread for LWP 23382: generic error
(gdb) [Thread 0x7ffff58d5700 (LWP 23383) exited]
[Thread 0x7ffff60d6700 (LWP 23382) exited]
[Thread 0x7ffff68d7700 (LWP 23381) exited]
[Inferior 1 (process 23368) exited normally]

(gdb) bt
No stack.


On Wed, Oct 16, 2019 at 4:18 PM Dr. Jürgen Sauermann <mail@jürgen-sauermann.de> wrote:
Hi Rowan,

a stack-trace for the segfault would be good (command gdb apl then: 'run' and finally 'bt' after
the segfault,

No idea what AST is.
You could try TAB-expansion to get options in various situations and try e.g.

]help ⌹

to get help for APL primitives. Currently system functions and variables are not in )help,
but I suppose extending file src/Help.def could easily add them.

Compiling APL is IMHO a wrong path. Too many problems, too little gain.

Best Regards,
Jürgen Sauermann

On 10/16/19 5:01 PM, Rowan Cannaday wrote:
Thank you for the explanation Jürgen.

That makes intuitive sense. A shared-memory single threaded service is a reasonable abstraction.

Another approach, is to compile a subset of APL to an intermediate representation.

Is there a way to export the AST?
in addition - is there an in-repl method of viewing help and/or arguments for system variables & functions?

By the way, a minor regression: segfaulting, but only after exiting.
thread: 0x7f8747766700
thread_cSegmentation fault

Thanks again,
- Rowan

On Wed, Oct 16, 2019 at 12:06 PM Dr. Jürgen Sauermann <mail@jürgen-sauermann.de> wrote:
Hi Blake,

it is sort of working, but I could well use some help in troubleshooting
the remaining problems. I can help fixing them, but finding their root cause
(and making them reproducible) is a different story.

My current interpretation of various benchmarks that Elias Mårtenson and
myself did some years ago is that the bandwidth of the memory interface
between the CPUs (or cores) and the memory is the limiting factor, and no
matter how efficient the APL interpreter is, this bottleneck will dictate the
speedup that can be achieved.

As an example, from 1985 to 1990, myself and 4 students had built a the
hardware of a parallel APL machine with 32 CPUs and measured a speedup
of close to 32 for sufficiently large vectors.

In contrast, if I remember correctly, then  Elias achieved a speedup of 12 with
80 CPUs using the parallel feature of GNU APL. The only difference that
I can see between our 1990 machine (called Datis-P-256 because the architecture
could be scaled up to 256 processors) was the memory architecture:

Datis-P had one separate memory for each CPU, while current multicore
boxes share their memory module(s) among different cores. That simply
boils down to the fact that the memory bandwidth of Datis-P scaled with the
number of processors, while the number of cores on a typical multi-core box
does not. As long as this is the case, parallel APL remains severely limited
in terms of the speedup that can be achieved.

Best Regards,
Jürgen Sauermann

On 10/16/19 12:58 PM, Blake McBride wrote:

I think getting the parallel processing working is important.  It may be that for various reasons the speedup in general cases is minimal and not worth the effort.  However, I'd imagine that there are particular use-cases utilizing large arrays where the speedup would be substantial.  That is when those types of enhancements would make APL a real benefit.



On Wed, Oct 16, 2019 at 5:27 AM Dr. Jürgen Sauermann <mail@jürgen-sauermann.de> wrote:
Hi Rowan,

fixed in SVN 1191.

You should not be too enthusiastic, though, because the speed-ups that
can be achieved are somewhat disappointing. And due to that, I
haven't put too much effort into fixing faults (sometimes apl hangs
on a semaphore when parallel execution is enabled).

Best Regards,
Jürgen Sauermann

On 10/16/19 5:15 AM, Rowan Cannaday wrote:

intrigued by the ability to parallelize APL, thought I'd try to test it:

`apl --cfg` followed by a line of '=' signs followed by `apl -q`:

configurable options:
    APSERVER_PATH=/tmp/GNU-APL/APserver (default)
    APSERVER_PORT=16366 (default)
    APSERVER_TRANSPORT=0 (default)
    MAX_RANK_WANTED=8 (default)
    SHORT_VALUE_LENGTH_WANTED=12, therefore:
        sizeof(Value)       : 456 bytes
        sizeof(Cell)        :  24 bytes
        sizeof(Value header): 168 bytes

    VF_TRACING_WANTED=no (default)

how ./configure was (probably) called:
    ./configure  'CORE_COUNT_WANTED=2' 'DEVELOP_WANTED=yes' 'VALUE_HISTORY_WANTED=yes' 'VISIBLE_MARKERS_WANTED=yes' '--enable-maintainer-mode'

    Project:        GNU APL
    Version / SVN:  1.8 / 1190M
    Build Date:     2019-10-16 02:45:24 UTC
    Build OS:       Linux 5.2.0-3-amd64 x86_64
    config.status:  'CORE_COUNT_WANTED=2' 'DEVELOP_WANTED=yes' 'VALUE_HISTORY_WANTED=yes' 'VISIBLE_MARKERS_WANTED=yes' '--enable-maintainer-mode'
    Archive SVN:    1161


$ apl -q

thread: 0x7f6078042e00
thread_contexts_count: 2
busy_worker_count:     0
active_core_count:     1
thread # 0:               0 RUN  job:   0 no-name
thread #-1:               0 RUN  job:   0 no-name

-- Stack trace at main.cc:88
0x7F6078FD1BBB __libc_start_main
0x5631406C386D  main
0x5631406CAD8D   init_apl(int, char const**)
0x5631407E881B    Parallel::init(bool)
0x563140832E2D     Thread_context::init_parallel(CoreCount, bool)
0x7F60794E5B18      sem_init

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