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Date: Wed, 6 Feb 2002 15:09:49 +0100
From: Andi Kleen <ak@suse.de>
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Andi Kleen <ak@suse.de>,
        Denis Vlasenko <vda@port.imtp.ilyichevsk.odessa.ua>,
        linux-kernel@vger.kernel.org
Subject: Re: kernel: ldt allocation failed
Message-ID: <20020206150949.A10871@wotan.suse.de>
In-Reply-To: <p73ofj2lpdg.fsf@oldwotan.suse.de> <E16YSpV-0005Es-00@the-village.bc.nu>
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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
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