RE: The history of the term "to block on a channel"

I suspect the term has been lifted from the telecoms industry where telephone exchange equipment was considered to be non-blocking when a caller was guaranteed always to be able to get an immediate connection to another non-busy user on a fully functioning, non-blocking exchange. In the UK, local exchanges were non-blocking while trunk connections were not.

I would have thought the usage referring to the possibility of sending a communication was still pertinent.

Chris

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From: occam-com-request@xxxxxxxxxx [mailto:occam-com-request@xxxxxxxxxx] On Behalf Of Larry Dickson
Sent: 27 September 2014 18:32
To: Matt.Pedersen@xxxxxxxx
Cc: Roger Shepherd; Teig, Oyvind BIS; occam-com@xxxxxxxxxx; "Øyvind Teig (oyvind.teig@xxxxxxxxxxx)"; David May
Subject: Re: The history of the term "to block on a channel"

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It seems to me that “blocking” applies to an alt as well as a one-to-one communication (or to a timer). It’s not something that CSP people need to be embarrassed about. On the contrary. Any model that does NOT account for blocking is a false model. Think on the instruction level. Each sequential logic primitive (like an add or multiply) is a wrapper around some combinatorial logic that needs time to settle. The system clock gives it that time — so every instruction blocks! And if it did not block, it would give incorrect results.

Before an instruction completes, the conditions necessary for its completion have to hold. If this is a read, write, alt, or timeout, the required blocking can be “macroscopic,” and efficient chips yield control of the instruction stream to other tasks at that point.

Larry

On Sep 26, 2014, at 2:22 AM, matt.pedersen@xxxxxxxx wrote:

Blocking is probably used because a read or write will do exactly that in a one to one straight up communication. Though used in an alt blocking might not aptly describe the state as only communications that are ready (i.e., will _not_ block) will be considered.

I dislike yield because yield gives the idea that you can back off from a blocking read or write call which you cannot.

Matt

On Sep 26, 2014, at 2:19, "Roger Shepherd" <rog@xxxxxxxx> wrote:

Oyvind,

I don’t know the answer to you question.

I can’t say that I like “block” - but it usage is certainly old and is common for multitask systems where the ability to create an extra task/thread/whatever to do communication is considered to be advantageous - hence “non-blocking communication”.

I don’t like “yield” either as this has a meaning along the lines of "to give place or precedence (usually followed by to): to yield to another; Will the senator from New York yield?” (from dictionary.com) and it is not necessarily the case the case that there is anything else to yield to.

I prefer “wait”.

On the subject of language, I think the term “synchronous” is plain wrong when used to describe (occam) channel communication. The processes are “synchronised” by the communication; the communication is “asynchronous” - there is no clock which causes the communication to happen.

Roger

On 26 Sep 2014, at 07:27, Teig, Oyvind BIS <Oyvind.Teig@xxxxxxxxxxxxxxxx> wrote:

Sirs

When was the phrase “blocking on a channel” introduced and by whom? Hoare does not use it in his 1985 book. Roscoe almost does not use it, or I would say, he does not use it at all in this context in his version of the book that’s PDFed. If this group does not know this, none will.

I am suggesting using “to yield on a channel” rather than “to block on a channel”. I have a blog note about it [1] and there is a discussion thread on golang-nuts [2]. I include the figure here (and the intro text in golang-nuts):

<image001.jpg>

Readers of golang-nuts know that “blocking is fine in Go”. But often, when I talk with other programmers I would hear that “blocking is evil”. I suggest that we go over to saying that “yielding on a channel is fine”. I’ll explain:

The literate meaning of blocking is about something we don’t want. It means I want to go somewhere but am stopped or delayed so I arrive too late. Or a door is blocked, in which case we must unblock it, hopefully without a bulldozer. Since this semantics outnumbers the people who understand CSP-based channel semantics we have an explanation problem.

With an explanation problem follows a mission problem.

Tell a basically single-thread programmer in C++ that blocking is good and you ask for much. His attention to try to understand something rather new, even if he’s used to linux select. Because often the code that does this linux select also does other rather independent matters. And it’s in his spec that these other matters also need to complete. And when you block on one matter it’s easy to see blocking as something evil. Because he or she is right in their own mind.

So which “blocking” do you mean?

1.     “Blocking on a channel” or some shared resource controlled by a non-blocking algorithm. I believe these may be in the same group, ref. the Wikipedia page about Non-blocking algorithm

2.     “Blocking” away other required functionality

3.     “Blocking” as in deadlock, where the system cannot proceed, where there is a cycle in the dependency tree

We already have good words for 2. = blocking as is, and 3 = deadlock. But we reuse blocking for 1.) which is not optimal. As said, I suggest 1. = yielding. This is an implicit yield that the application doesn’t have control of. Not the explicit yield that some operating systems would supply in the API. The channel semantics as implemented in the scheduler does it for us.

What do you think of this? If we start to write “to yield on a channel” or “yielding on a channel” it could slowly creep into our language. And the C/C++ (and even Scala or Erlang) people would give us an ear. Especially if we agree with them that blocking is evil.

(I alse dare take comments on the idea.. Here, there or there)

[1] - http://www.teigfam.net/oyvind/home/technology/092-not-so-blocking-after-all/

[2] - https://groups.google.com/forum/#!topic/golang-nuts/OSBU8wjlES8

Med vennlig hilsen / Best regards

Øyvind Teig

Senior utviklingsingeniør, siv.ing. / Senior Development Engineer, M.Sc.
Autronica Fire and Security AS
Research and Development

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Phone: +47 735 824 68
E-mail:  oyvind.teig@xxxxxxxxxxxxxxxx
www.autronicafire.no
www.teigfam.net/oyvind/home/ (Also work-related)

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