On 2008-12-06 21:31:34 +0100, dsimcha <[EMAIL PROTECTED]> said:
A few days ago, I commented that I thought that maybe the GC should be using
spinlocks, given how little time a typical allocation takes compared to
context switches, etc. I've created a version of the D 2.21 druntime GC with
spinlocks instead of synchronized, and created the following simple benchmark
to just generate a ton of contention for the GC:
import core.thread, core.memory, std.perf, std.stdio, std.c.time, std.c.stdio;
void main() {
readln; //Allow for affinity to be changed.
GC.disable;
auto T = new Thread(&foo);
T.start;
scope auto pc = new PerformanceCounter;
pc.start;
foo();
T.join;
pc.stop;
writeln(pc.milliseconds);
}
void foo() {
foreach(i; 0..10_000_000) {
auto foo = GC.malloc(8);
GC.free(foo);
}
}
Here are the times:
Using both of my CPU cores, meaning serious contention, in milliseconds:
Spinlock: 10006
Synchronized: 28563
The synchronized version uses ~25-30% CPU, because of OS rescheduling, while
the spinlock version uses 100%.
Setting the affinity to only one CPU to simulate a single-CPU environment:
Spinlock: 4356
Synchronized: 4758
Replacing one thread's foo() by a dummy function so that the lock is never
even contested:
Spinlock: 1876
Synchronized: 2589
I will acknowledge that this is an extremely simple benchmark, but I think
it's reasonably representative of a severely contested memory allocation lock.
The spinlock I used was the simplest possible atomic CAS lock, nothing fancy.
Nice, I think indeed for the allocation I think that spinlock are
definitely preferable, but I think that they should be integrated in
the runtime, so that the runtime can be built also on platforms that
do not support them (maybe a spinlock module that can be implemented on
the top of normal locks if the need arises, it should not add too much
overhead I hope.
Fawzi
