Thread Local Storage

Daniel Eischen eischen at vigrid.com
Mon Mar 29 13:56:34 PST 2004


On Mon, 29 Mar 2004, Doug Rabson wrote:

> On Monday 29 March 2004 22:26, Daniel Eischen wrote:
> > On Mon, 29 Mar 2004, Doug Rabson wrote:
> > >
> > > Surely the GNU TLS ABI is preferable? It generates much smaller
> > > code and needs many fewer relocations.
> >
> > No, we don't want an LDT for every thread and don't want
> > to force a syscall for a thread switch.
> 
> But the code it generates is at least twice the size for dynamic TLS. It 
> seems that the GNU people have done a better job defining the TLS abi 
> for i386.

About the only thing that uses TLS that I know is nvidia's
openGL.  If you design an API correctly, there's no need
for TLS.  I would hope that it's usage would be limited.

> You don't need a syscall at thread switch if you do something like:
> 
> 	_thread_switch(...)
> 	{
> 		if (tcb doesn't have LDT entry) {
> 			if (!free LDT entries)
> 				steal LDT entry from non-running thread;
> 			allocate LDT entry and point it at TLS goop for tcb.
> 		}
> 		load_gs(tcb's LDT sel);

That's a system call on amd64.

> 		...
> 	}
> 
> I just have this feeling that the GNU ABI is going to get far better 
> testing and support in the future since thats what linux uses. 

-- 
Dan Eischen



More information about the freebsd-threads mailing list