Re: 2.4, 2.6, i686/athlon and LDT's

[Arjan van de Ven]

On Mon, 2004-12-27 at 15:55 -0500, Brian Gerst wrote:
> Arjan van de Ven wrote:
> > On Mon, 2004-12-27 at 11:46 -0800, Tymm Twillman wrote:
> > 
> >>Hi all,
> > 
> > 
> >>It appears that use of the LDT is to speed up context switching between 
> >>threads, although I haven't even found especially good references WRT 
> >>that.  I have looked through the info in the IA Developers publications 
> >>and have whacked my head against Google quite a bit.  However, every bit 
> >>of clarity I've found there has been offset by new confuzled bits.
> > 
> > 
> > LDT's are *slow*. That's why glibc will try to avoid using them
> > nowadays, and with 2.6 it won't; as for 2.4.. it depends if you use a
> > vendor 2.4 it might be able to avoid using LDT's as well.
> 
> Using the LDT isn't inherently slower, since the cpu caches the segment 
> descriptor regardless of if it came from the GDT or LDT. 

it does however increase context switch costs significantly since the
LDT needs to be reloaded and reverified...


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