From mboxrd@z Thu Jan 1 00:00:00 1970 From: Anthony Liguori Subject: Re: [PATCH 0/6] In kernel PIT patch Date: Tue, 04 Mar 2008 18:50:24 -0600 Message-ID: <47CDEE50.3060206@codemonkey.ws> References: <200803041822.40285.sheng.yang@intel.com> <47CD7038.6000200@codemonkey.ws> <1204672734.25172.24.camel@localhost.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: kvm-devel@lists.sourceforge.net To: dor.laor@qumranet.com Return-path: In-Reply-To: <1204672734.25172.24.camel@localhost.localdomain> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: kvm-devel-bounces@lists.sourceforge.net Errors-To: kvm-devel-bounces@lists.sourceforge.net List-Id: kvm.vger.kernel.org Dor Laor wrote: > On Tue, 2008-03-04 at 09:52 -0600, Anthony Liguori wrote: > >> Yang, Sheng wrote: >> >>> Hi >>> >>> Here is the last in-kernel PIT patch for KVM. The mainly change from last >>> version is the supporting to save/restore. I also tested live migration. >>> >>> The other modifies including some date structure changed to be better for >>> supporting the save/restore. I moved the PIT timer to outside of channel >>> structure, which explicitly means only one channel (channel 0) would trigger >>> it. >>> >>> After fix TSC problem on SMP PAE RHEL5/5.1 guest, now the patch works well >>> without any modify of kernel parameter. >>> >>> >> How are you measuring the improvements from an in-kernel PIT? From your >> mails, you're claiming it increases the timer accuracy. How are you >> measuring it and how much does it improve it? >> >> > > It's also a functionality addition: userspace pit & pic combination > needed to use -tdf option (time drift fix). The tdf took care of pending > pit irqs and tried to make the guest ack the right number of irqs the > pit was configured. > I thought there was some discussion about whether -tdf was every useful in practice? > Once we switched to the default in-kernel pic, the userspace pit > couldn't get the acks from the pit. > One can see the effect when running multiple guests (windows, standard > HAL) playing video, the time slows down. > Okay, that makes sense. So have you done any tests to confirm this? We suffered through a fair number of regressions when we moved to an in-kernel APIC. Before moving another big chunk of code in the kernel and going through possible regressions, I want to make sure we have a measurable argument that it's the right thing to do. So how do we measure the benefits of an in-kernel PIT? Regards, Anthony Liguori > This patch set has a pending counter and takes care for it too. > > >> Do you expect an overall performance improvement from this or is it >> simply about improving timer accuracy? >> >> > > It will probably help older kernels with slow HZ run faster HZ guests. > Without CONFIG_DYNTICK the guests behaved jumpy because of that. > > >> Regards, >> >> Anthony Liguori >> >> >> >> ------------------------------------------------------------------------- >> This SF.net email is sponsored by: Microsoft >> Defy all challenges. Microsoft(R) Visual Studio 2008. >> http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ >> _______________________________________________ >> kvm-devel mailing list >> kvm-devel@lists.sourceforge.net >> https://lists.sourceforge.net/lists/listinfo/kvm-devel >> > > ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2008. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/