More on GC Spinlocks

[Fawzi Mohamed]

On 2008-12-06 21:31:34 +0100, dsimcha <dsimcha at yahoo.com> 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