From: Ingo Molnar <mingo@elte.hu>
To: dor.laor@qumranet.com
Cc: tglx@linutronix.de,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
kvm-devel <kvm-devel@lists.sourceforge.net>
Subject: Re: Performance overhead of get_cycles_sync
Date: Tue, 11 Dec 2007 14:37:38 +0100 [thread overview]
Message-ID: <20071211133738.GA8150@elte.hu> (raw)
In-Reply-To: <475E8C8B.7070308@qumranet.com>
* Dor Laor <dor.laor@gmail.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
next prev parent reply other threads:[~2007-12-11 13:37 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-12-11 13:11 Performance overhead of get_cycles_sync Dor Laor
2007-12-11 13:37 ` Ingo Molnar [this message]
[not found] ` <20071211133738.GA8150-X9Un+BFzKDI@public.gmane.org>
2007-12-11 14:11 ` Dor Laor
[not found] ` <475E9A92.4030001-atKUWr5tajBWk0Htik3J/w@public.gmane.org>
2007-12-11 14:27 ` Ingo Molnar
[not found] ` <20071211142717.GA15903-X9Un+BFzKDI@public.gmane.org>
2007-12-11 15:03 ` Dor Laor
2007-12-11 21:26 ` Joerg Roedel
[not found] ` <20071211212628.GB6537-5C7GfCeVMHo@public.gmane.org>
2007-12-12 0:19 ` Dor Laor
2007-12-11 16:35 ` Arjan van de Ven
[not found] ` <20071211083513.56c2a385-NIQFrBLA1CpScpXdPBN83iCwEArCW2h5@public.gmane.org>
2007-12-11 17:03 ` Ingo Molnar
[not found] ` <p73abohno0u.fsf@bingen.suse.de>
[not found] ` <20071211201930.GB22397@elte.hu>
[not found] ` <20071211201930.GB22397-X9Un+BFzKDI@public.gmane.org>
2007-12-11 20:29 ` Ingo Molnar
2007-12-11 14:14 ` Dor Laor
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