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

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Message-ID: <5339AB59.4050906@mit.edu>
Date:	Mon, 31 Mar 2014 10:52:25 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Zhanghailiang <zhang.zhanghailiang@...wei.com>,
	"mtosatti@...hat.com" <mtosatti@...hat.com>,
	"johnstul@...ibm.com" <johnstul@...ibm.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
CC:	"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 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.

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

--Andy

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