RE: Marshaling of custom blocks and threads

["Mattias Waldau" <mattias.waldau@abc.se>]

The easiest way to find threading bugs are to use a multiprocessor machine.

Threading errors occurs a million times more often on a two-processor
machine than on an ordinary.

Why? Modern OS:s actually abort a thread rather seldom, and each thread is
allowed to run thousands of operations until task-switch. The chance that
this task-switch will occur at the wrong location is very small.

/mattias

-----Original Message-----
From: Pierre.Weis@inria.fr [mailto:Pierre.Weis@inria.fr]On Behalf Of
Gerd Stolpmann
Sent: Wednesday, November 29, 2000 2:22 AM
To: Ken Wakita; jserot@lasmea.univ-bpclermont.fr
Cc: caml-list@inria.fr
Subject: Re: Marshaling of custom blocks and threads


On Tue, 28 Nov 2000, Ken Wakita wrote:
>I think the Marshal module is thread unsafe because it uses a shared
>buffer to produce the external image of the ML objects.

In 2.04, this was definitely true
(see http://caml.inria.fr/bin/caml-bugs/fixed?id=24;page=1;user=guest
and http://caml.inria.fr/bin/caml-bugs/fixed?id=25page=1;user=guest).
However, the bug was still subtle; the shared buffers were relatively
well protected by the so-called "master lock". Until now, I thought
that 3.00 fixed the bug finally; i.e. the master lock really locked
the shared buffer. (I checked the code.)

Jocelyn Serot reports that Marshal is not thread-safe for custom blocks.
These did not exist in 2.04, so I suppose that only the new parts of the
marshalling system do not work properly enough. However, multi-threading
programming is error-prone, so it would be helpful to have a small program
that
demonstrates the bug (normally it is sufficient to repeat the errorneous
piece
of code often enough (e.g. 10000 times) to reproduce the incorrect
behaviour).

Gerd
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