linux-kernel - Re: VDSO pvclock may increase host cpu consumption, is this a problem?

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Message-ID: <20140331213032.GA5866@amt.cnet>
Date:	Mon, 31 Mar 2014 18:30:32 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Zhanghailiang <zhang.zhanghailiang@...wei.com>,
	"johnstul@...ibm.com" <johnstul@...ibm.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Zhouxiangjiu <zhouxiangjiu@...wei.com>,
	zhang yanying <zhuangyanying@...wei.com>
Subject: Re: VDSO pvclock may increase host cpu consumption, is this a
 problem?

On Mon, Mar 31, 2014 at 10:52:25AM -0700, Andy Lutomirski wrote:
> On 03/29/2014 01:47 AM, Zhanghailiang wrote:
> > Hi,
> > I found when Guest is idle, VDSO pvclock may increase host consumption.
> > We can calcutate as follow, Correct me if I am wrong.
> >       (Host)250 * update_pvclock_gtod = 1500 * gettimeofday(Guest)
> > In Host, VDSO pvclock introduce a notifier chain, pvclock_gtod_chain in timekeeping.c. It consume nearly 900 cycles per call. So in consideration of 250 Hz, it may consume 225,000 cycles per second, even no VM is created.
> > In Guest, gettimeofday consumes 220 cycles per call with VDSO pvclock. If the no-kvmclock-vsyscall is configured, gettimeofday consumes 370 cycles per call. The feature decrease 150 cycles consumption per call. 
> > When call gettimeofday 1500 times,it decrease 225,000 cycles,equal to the host consumption.
> > Both Host and Guest is linux-3.13.6.
> > So, whether the host cpu consumption is a problem?
> 
> Does pvclock serve any real purpose on systems with fully-functional
> TSCs?  The x86 guest implementation is awful, so it's about 2x slower
> than TSC.  It could be improved a lot, but I'm not sure I understand why
> it exists in the first place.

VM migration.

Can you explain why you consider it so bad ? How you think it could be
improved ?

> I certainly understand the goal of keeping the guest CLOCK_REALTIME is
> sync with the host, but pvclock seems like overkill for that.

VM migration.

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