On Tue, 12 Oct 2010, Igor Stasenko wrote:
On 12 October 2010 16:51, Levente Uzonyi <[email protected]> wrote:
On Tue, 12 Oct 2010, Igor Stasenko wrote:
Hello, i just thought, that it would be cool to have a special bytecode,
which guarantees atomicity for swapping values between two variables.
To swap two values, you usually do:
| var1 var2 temp |
temp := var1.
var1 := var2.
var2 := temp.
But since its non-atomic, a process can be interrupted and such operation
is not thread-safe.
In order to make it thread safe, you must add even more boilerplate:
| var1 var2 temp |
semaphore critical: [
temp := var1.
var1 := var2.
var2 := temp.
]
An alternative solution:
| a b |
a := 1.
b := 2.
[
| tmp |
tmp := a.
a := b.
b := tmp ] valueUnpreemptively
Yeah, another boilerplate under the hood, also highly dependent from
scheduling nuances :)
I don't get the "dependency from scheduling nuances part", but here's my
idea:
Add the compiler changes to support :=: as atomic swap. We don't really
need a bytecode for now, since a sequence of assignments is currently
atomic. So the compiler could compile :=: as three assignments using a
hidden temporary variable. On other systems, :=: can be compiled
differently.
Levente
valueUnpreemptively
"Evaluate the receiver (block), without the possibility of preemption
by higher priority processes. Use this facility VERY sparingly!"
"Think about using Block>>valueUninterruptably first, and think about
using Semaphore>>critical: before that, and think about redesigning
your application even before that!
After you've done all that thinking, go right ahead and use it..."
| activeProcess oldPriority result |
activeProcess := Processor activeProcess.
oldPriority := activeProcess priority.
activeProcess priority: Processor highestPriority.
result := self ensure: [activeProcess priority: oldPriority].
"Yield after restoring priority to give the preempted processes a
chance to run"
Processor yield.
^result
Levente
--
Best regards,
Igor Stasenko AKA sig.
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