lkml.org 
[lkml]   [2014]   [Mar]   [31]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: 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



\
 
 \ /
  Last update: 2014-03-31 20:41    [W:0.095 / U:0.524 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site