linux-kernel - Re: [PATCH 0 of 9] x86/smp function calls: convert x86 tlb flushes to use function calls [POST 2]

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Message-ID: <48AA79C4.8060309@goop.org>
Date:	Tue, 19 Aug 2008 00:44:04 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	x86@...nel.org, Nick Piggin <nickpiggin@...oo.com.au>,
	Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [PATCH 0 of 9] x86/smp function calls: convert x86 tlb flushes
 to use function calls [POST 2]

Andi Kleen wrote:
>> nice stuff!
>>     
>
> If only those 35% wouldn't be there ...
>   

Don't pay attention to that number.  It's only the extra latency of a HT
context->HT context function call + tlb flush.  Which means 1) the tlb
is shared anyway, so the extra flush is redundant, 2) they're not really
concurrent, 3) it's going down the single-cpu call, rather than the
multi-cpu one, 4) it's only measuring the latency for a particular tlb
flush, and doesn't take into account any throughput improvements the
extra queueing may add.

I don't really know what the nett consequence of all that is, but it
means the number has almost no bearing on multicore results, esp with >2
cores.

    J
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