Implementing TLS: step 1

[Julian Elischer]

On Thu, 19 Jun 2003, Daniel Eischen wrote:

> On Thu, 19 Jun 2003, Julian Elischer wrote:
> > 
> > BTW Marcel, I think that we can get from where we are now with kse to
> > what we need by just a little massaging of what points to what.
> > it is possible that the register %gs could be pointed to the thread
> > structure directly and we could derive the KSE from that. it will just
> > make the context switches a fraction more expensive if we need to
> > change the segment register..
> > 
> > The big cost is that a processin x86 can only have a limited number of
> > segments set up in teh Local descriptor table (LDT).
> > I forget the actual number but i vaguely remember that it is 16383 or
> > 8191 or something.. A process could theoretically want to have
> > more than that numbe rof threads.. By pointing to the KSE we limit
> > ourselves to having to set up onl a small number of LDT entries
> > which is a big saving.
> 
> Yes, and we can have thousands and thousands of threads.
> 
> > On ia64 we don;t need to use descriptors so there is not that limit so 
> > in effect we could point directly to the thread descriptor and let THAT
> > point to teh VCPU mailbox in question. it's just an extra write or two
> > at context switch time. I think we'll be just fine.
> 
> It is not just that.  It is the cost of an ldt allocation for
> each thread.  In libkse, we only need as many LDTs as there
> are KSEs.  Allocating an ldt per-thread increases thread
> startup and teardown.

that's sort-of-what I was saying..
I think we are ok anyhow.. the spec for i386 is general enough so that
we can use it as it is without problem.

> 
> -- 
> Dan Eischen
> 
>