22.10.2013 17:54, David Lang:
Then, there is need for each global variable to be lockable. But
then, it can lead to a deadlocks...
Each global variable needs to eithe be lockable or manipulated with
atomic statements.
And if we talk about global containers which can contain properties, it
becomes even harder.
Rainer is saying that he thinks they may be able to be used in if
statements, you already can't use functions in templates, so I don't
see that as a problem.
Let's see, I would say.
I think that, given current SIMD engine, the problem arises every time
when a global variable is used more than once for one message. But how
to prevent this? Having global(key="$/var") as a function leads to
natural expressions like this:
if global(key="$/var") % 3 == 0 then
Action(1)
else if global(key="$/var") % 3 == 1 then
Action(2)
else
Action(3)
But, obviously, it is not better than having $/ directly.
So, what to do?
We can force user to export global value to userspace, having a way to
access globals, but not using them in expressions. By an action, for
example:
action(type="mmglobal" mode="get" global="$/var" var=".counter")
It is still not prevents a user to access globals twice, but makes it
unnatural. (And it can even be useful to count messages in a batch.)
And one more idea. What if engine caches global value to message space
on first access, then all subsequent reads of the same global are served
locally? It seemes facilitating main issue with batching. Locking and
atomicity are still problematic and need to be resolved, but for
counters it can be easily done with atomic_increment(). But... If batch
size is 10:
set $/var = $/var + 1;
set $!var = $/var;
It is OK so far: each message will get it's number, from 1 to 10, and
global is +10.
But what if we change order:
set $!var = $/var;
set $/var = $/var + 1;
Ooops, it will be strange. Every message will get the same number, and
global will increment by 1. No, this idea is not as good. Just for the
record.
--
Pavel Levshin
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