2012-07-02

asynchronous communication and promises

6.15: adda/co/asynchronous communication and promises:

. the idea of protected types vs task types
 is about whether the interaction is using an entry queue .
[6.17:
. this reminded me of what promises are:
task types are taking orders on paper,
ie, instead of being called directly,
the callers put their call details in a queue
just like a consumer-waiter-cook system .
. the ada task is designed to make the consumer wait
(ie, do nothing until their order is done);
but this doesn't have to be the case;
because, instead of blocking the caller's entire process
we could put a block on some of its variables instead;
eg, say y`= f(x) is a task call,
so the caller must wait because
the rest of its program depends on y's value;
and, y's value depends on that task call, f, finishing .
. however, we could block access to y,
such that the caller can do other things
up until trying to access y .
]
. notice in ada's style
both protected and tasking types are blocking;
ie, synchronously communicating .
. how do we ask instead for
asynchronous communication ?

we don't need new syntax because
functions logically require blocking
whereas message-sends do not .
. by logically I mean
the function code is saying:
"( I am waiting for this return value), ...
[6.17: not true:
. asynchronous behaviour should be encoded in syntax
because it does make a difference at a high level:
. the only reason to feature asynchronous calling
is when the job or its shipping could take a long time;
if the job is quick
then there's no use bothering with multitasking,
and conversely, if the job is slow,
then multitasking is practically mandatory;
and, multitasking can be changing the caller's algorithm .
]
. there are 2 ways to ask for asynch:
#  pass the address of a promise-kept flag
that the caller can check to unblock a promised var;
# caller passes an email address
for sending a promise-kept notification:
the caller selects an event code
to represent a call being finished,
and that event code gets emailed to caller
when job is done .

. the usual protocol is to
have the called task email back to the caller
a pointer to the var that has been updated,
so if there are multiple targets,
the async'ly called might send an email for each completion,
or they could agree that when all is done
to email the name of the async sub that has finished;
but if caller wants to know which job was done;
then only the event code idea would be indicating
a per-call completion event .

6.26:  news.adda/co/perfbook:
Is Parallel Programming Hard, And, If So, What Can You Do About It?
Paul E. McKenney, December 16, 2011
Linux Technology Center, IBM Beaverton
paulmck@linux.vnet.ibm.com
2011 version:
src:
--
seen from here: cpu-and-gpu-trends-over-time
from: JeanBaptiste Poullet @jpoullet
Bioinformatician/statistician
, next generation sequencing (NGS),
NMR, Linux, Python, Perl, C/C++