linux-kernel - Re: Performance overhead of get_cycles

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Message-ID: <20071211133738.GA8150@elte.hu>
Date:	Tue, 11 Dec 2007 14:37:38 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	dor.laor@...ranet.com
Cc:	tglx@...utronix.de,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	kvm-devel <kvm-devel@...ts.sourceforge.net>
Subject: Re: Performance overhead of get_cycles_sync


* Dor Laor <dor.laor@...il.com> wrote:

> Hi Ingo, Thomas,
>
> In the latest kernel (2.6.24-rc3) I noticed a drastic performance 
> decrease for KVM networking. The reason is many vmexit (exit reason is 
> cpuid instruction) caused by calls to gettimeofday that uses tsc 
> sourceclock. read_tsc calls get_cycles_sync which might call cpuid in 
> order to serialize the cpu.
>
> Can you explain why the cpu needs to be serialized for every gettime 
> call? Do we need to be that accurate? (It will also slightly improve 
> physical hosts). I believe you have a reason and the answer is yes. In 
> that case can you replace the serializing instruction with an 
> instruction that does not trigger vmexit? Maybe use 'ltr' for example?

hm, where exactly does it call CPUID?

	Ingo
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