Re: kernel: ldt allocation failed

[Andi Kleen <ak@suse.de>]

On Wed, Feb 06, 2002 at 02:13:45PM +0000, Alan Cox wrote:
> > glibc 2.3 seems to plan to use segment register based thread local data for 
> > even non threaded programs, so it would be a good idea to optimize LDT 
> > allocation a bit (= not allocate 64K of vmalloc space every time 
> > sys_modify_ldt is called - there is only 8MB of it) 
> 
> I think it would be a good idea to modify the glibc authors in that case.
> The ldt costs real performance on task switches. It would be very dumb of
> glibc to use it except when justified in the bigger picture - ie threaded
> apps

Are you sure it does? LGDT with non zero argument shouldn't be that costly. 
The %fs switching adds some locked cycles for reloading the segment cache, 
but because Windows uses that I would it expect to be reasonably optimized 
on CPUs. 

I actually tried to complain because on x86-64 it is more costly, but to
no avail. 

-Andi