WARNING - OLD ARCHIVES

This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
   
 
 
Xen 
 
Home Products Support Community News
 
   
 

xen-devel

Re: [Xen-devel] [RFC] Correct/fast timestamping in apps under Xen [1 of

To: Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Subject: Re: [Xen-devel] [RFC] Correct/fast timestamping in apps under Xen [1 of 4]: Reliable TSC
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Fri, 09 Oct 2009 13:28:05 -0700
Cc: "kurt.hackel@xxxxxxxxxx" <kurt.hackel@xxxxxxxxxx>, Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, "Xen-Devel \(E-mail\)" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>, Keir Fraser <Keir.Fraser@xxxxxxxxxxxxx>
Delivery-date: Fri, 09 Oct 2009 13:28:36 -0700
Envelope-to: www-data@xxxxxxxxxxxxxxxxxxx
In-reply-to: <20091009093406.GD20579@xxxxxxxxxxxxxxxxxxxxxxx>
List-help: <mailto:xen-devel-request@lists.xensource.com?subject=help>
List-id: Xen developer discussion <xen-devel.lists.xensource.com>
List-post: <mailto:xen-devel@lists.xensource.com>
List-subscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=unsubscribe>
References: <C6F36DE8.16DCB%keir.fraser@xxxxxxxxxxxxx> <a301fbde-f47f-4120-a4e0-e85fe5dc589e@default> <20091009093406.GD20579@xxxxxxxxxxxxxxxxxxxxxxx>
Sender: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.4pre) Gecko/20090922 Fedora/3.0-2.7.b4.fc11 Lightning/1.0pre Thunderbird/3.0b4
On 10/09/09 02:34, Tim Deegan wrote:
> Your plans for usermode<-->hypervisor direct TSC integration seem to me
> to be an unpleasant hack.  I understand that you have good business
> reasons for wanting it (even if you're not allowed to tell us explicitly
> what they are) and we've seen the justifications enough times that we
> don't need to cover them again here, but it's still a hack.
>
> I'm unhappy with the idea of kicking around the Xen timekeeping code
> (and introducing the usual bug-tail) to support introducing a usermode
> TSC.  If there is to be a new mode for this, it should default to the
> current (works for everyone except the engineering team of a
> not-to-be-named enterprise application) behaviour.
>   

I'm seeing an approx 12x performance improvement with gettimeofday() and
clock_gettime() on systems with my vsyscall support patches
(~1200ns/call -> ~100ns[*]).  I think that should go a long way towards
mitigating the performance concerns using standard APIs.  

There's probably some scope for improving those numbers on systems with
better-than-baseline tsc support (ie rdtscp and/or guaranteed synced
tscs), but I think its enough to get started with, especially given the
broad applicability and relatively simple engineering.

[*] With native tsc; emulated tsc makes that 1700 -> 500, or only ~3.3x
improvement.

    J

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel

<Prev in Thread] Current Thread [Next in Thread>