RE: Adding a field to StgTSO

> > There's not too many StgMainThreads around, so why do
> > you think there's a perf win (at the cost of adding another
> > word to every TSO)?
> 
> When implementing my bound threads proposal, I'd have to query the 
> MainThread for every TSO that is scheduled.
> How many is "not too many"? There is one for each call-in to the RTS, 
> couldn't that become a problem? I'm just afraid of doing a 
> linear scan 
> each time.
> 
> But no, I'm not absolutely convinced, and I can just do my 
> linear scans 
> for now. If it's indeed a performance problem, we'll find out soon 
> enough.

The alternative which I considered is to hash the thread id to the
StgMainThread*.  But on balance, adding one word to the TSO structure is
simpler and won't make a noticeable difference to performance: TSOs are
usually a minimum 1k anwyay, and we always copy the whole lot during GC
(we shouldn't - I must get around to optimising that sometime).  TSOs
bigger than about 3k are treated as large objects and not copied.

Cheers,
        Simon