[erlang-questions] Erlang basic doubts about String, message passing and context switching overhead

[Bob Ippolito]

On Tue, Jan 10, 2017 at 10:58 AM, Bhag Chandra <bhag.chandra.erl@REDACTED>
wrote:

> Hello,
>
> I have been coding in Erlang for 2 years.  A wonderful language but not
> very big community, so I cant discuss my questions with programmers around
> me (Java, Python guys). I found out about this list today.
>
> I have some fundamental doubts about the Erlang. It would be great if
> someone can help me clarify them.
>
>
> 1) "Strings in Erlang are internally treated as a list of integers of each
> character's ASCII values, this representation of string makes operations
> faster. For example, string concatenation is constant time operation in
> Erlang."  Can someone explain why?
>
>
The description of a string's internal representation is correct, but
concatenation is O(n). Most people end up using binaries of utf8 encoded
text instead of strings (lower constant factors), and iolists to
concatenate data for output (which is constant time, since you're just a
rose tree of data to be traversed later).


> 2) "It makes sense to use Erlang only where system's availability is very
> high".  Is it not a very general requirement of most of the systems?
> Whatsapp to Google to FB to Amazon to Paypal to Barclays etc they all are
> high availability systems, so we can use Erlang in all of them?
>

You can use Erlang any time, but high availability in distributed systems
is where Erlang is a better fit than many other options. Whatsapp in
particular does use Erlang. The reasons not to use Erlang in these
situations are generally related to familiarity and momentum.


> 3) "Every message which is sent to a process, goes to the mailbox of that
> process. When process is free, it consumes that message from mailbox". So
> how exactly does process ask from the mailbox for that message? Is there a
> mechanism in a process' memory which keeps polling its mailbox. I basically
> want to understand how message is sent from mailbox to my code in process.
>
> 4) We say that a message is passed from process A to process B by simply
> using a bang (!) character, but what happens behind the scenes to pass this
> message? Do both processes establish a tcp connection first and then pass
> message or what?
>

There's plenty written on these topics available in this list archive and
various places online. I'd start by searching for "erlang scheduler" and
then scan for the bits that talk about messages and mailboxes. TCP
connections are only involved when sending messages between separate notes
in distributed erlang, local messages are sent and received by manipulating
local data structures.


> 5) At 30:25 in this video ( https://youtu.be/YaUPdgtUYko?t=1825 ) Mr.
> Armstrong is talking about the difference between the context switching
> overhead between OS threads and Erlang processes. He says, thread context
> switching is of order 700 words but Erlang process context switching is ...
> ?
> I cant understand what he said, if someone could tell.
>

Words is a measure of memory, an Erlang process (by default) has an
overhead of 309 words according to the efficiency guide.
http://erlang.org/doc/efficiency_guide/processes.html

-bob
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