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Date: Mon, 12 Aug 2002 11:21:55 -0400
From: Jakub Jelinek <jakub@redhat.com>
To: Ingo Molnar <mingo@elte.hu>
Cc: Linus Torvalds <torvalds@transmeta.com>, linux-kernel@vger.kernel.org,
       Alexandre Julliard <julliard@winehq.com>,
       Luca Barbieri <ldb@ldb.ods.org>
Subject: Re: [patch] tls-2.5.31-D5
Message-ID: <20020812112155.S1596@devserv.devel.redhat.com>
Reply-To: Jakub Jelinek <jakub@redhat.com>
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On Mon, Aug 12, 2002 at 07:06:50PM +0200, Ingo Molnar wrote:
> 
> okay, the attached patch does some more things:
> 
>  - moves the first two TLS entries and the user CS/DS entries on the same
>    cacheline.
> 
>  - excludes CS/DS from the TLS space - Luca is right in that it only slows
>    things down unnecesserily, and there is nothing that cannot be done by
>    changing the %ds %cs selectors - and every cycle counts in the 
>    context-switch path.
> 
> the only open issues are the number of TLSs supported. I'd vote for making
> them 4 and then we can inline the copy and make it unconditional, it will
> be 12 cycles to copy them all which alone is better than a branch miss. In
> this patch it's 2, thus the copying cost is 6 cycles.
> 
> with 4 entries the 0x40 entry would be taken and APM has to move further
> up, and has to save/restore the 0x40 entry across BIOS calls.

As each supported TLS entry has its context-switch time cost, I think we
should stay at 2 supported TLS entries.
My understanding was that the GDT patches were written to optimize the
common case (all threaded apps using LDT and with the advent of __thread
support causing every single application to use LDT), with 2 TLS entries
where one is for libc/libpthread and the other one is for application
usage I think it is enough for 99.9% of apps. In the rare
case someone needs more, there is still LDT which offers 8192 entries.

	Jakub
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