List traffic today asks about changing rsync to use lightweight
threads instead of heavyweight fork.
Before rushing into building a threads version of rsync, please READ
this recent article (its author is also the co-author of the
well-known, and widely used, gc (garbage collecting version of C
malloc and C++ new) library: see
http://www.hpl.hp.com/personal/Hans_Boehm/gc/index.html
):
@String{j-SIGPLAN = "ACM SIG{\-}PLAN Notices"}
@Article{Boehm:2005:TCI,
author = "Hans-J. Boehm",
title = "Threads cannot be implemented as a library",
journal = j-SIGPLAN,
volume = "40",
number = "6",
pages = "261--268",
month = jun,
year = "2005",
CODEN = "SINODQ",
DOI = "http://doi.acm.org/10.1145/1065010.1065042";,
ISSN = "0362-1340",
bibdate = "Tue Jun 21 17:04:05 MDT 2005",
bibsource = "http://portal.acm.org/";,
acknowledgement = ack-nhfb,
abstract = "In many environments, multi-threaded code is written
in a language that was originally designed without
thread support (e.g. C), to which a library of
threading primitives was subsequently added. There
appears to be a general understanding that this is not
the right approach. We provide specific arguments that
a pure library approach, in which the compiler is
designed independently of threading issues, cannot
guarantee correctness of the resulting code. We first
review why the approach almost works, and then examine
some of the surprising behavior it may entail. We
further illustrate that there are very simple cases in
which a pure library-based approach seems incapable of
expressing an efficient parallel algorithm. Our
discussion takes place in the context of C with
Pthreads, since it is commonly used, reasonably well
specified, and does not attempt to ensure type-safety,
which would entail even stronger constraints. The
issues we raise are not specific to that context.",
remark = "This is an important paper: it shows that current
languages cannot be reliable for threaded programming
without language changes that prevent compiler
optimizations from foiling synchronization methods and
memory barriers. The article's author and others are
collaborating on a proposal for changes to the C++
language to remedy this, but that still leaves threads
unreliable in C code, even with POSIX threads.",
}
The gist of the paper is that your threaded program may appear to
work, and even work apparently correctly most of the time. However,
there are going to be nasty race conditions that from time to time
cause unpredictable, and unreproducible, behavior. Most of us would
rather not have to deal with that mess in production software (e.g.,
at my site, rsync is a critical component in nightly updates of about
100 systems running about 20 different flavors of Unix serving
thousands of users).
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- University of Utah FAX: +1 801 581 4148 -
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