Re: 2.4, 2.6, i686/athlon and LDT's
From: Brian Gerst (bgerst_at_didntduck.org)
Date: 12/27/04
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Date: Mon, 27 Dec 2004 15:55:53 -0500 To: Arjan van de Ven <arjan@infradead.org>
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. Using an LDT
however consumes kernel memory, which can slow down the system from
memory pressure if you have many processes/threads using them.
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