From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Dan Magenheimer" Subject: RE: Xen system skew MUCH worse than tsc skew (was RE: RE: [PATCH] record max stime skew (was RE: [PATCH] strictly increasing hvm guest time)) Date: Sat, 19 Jul 2008 11:51:30 -0600 Message-ID: <20080719115130390.00000001344@djm-pc> References: Reply-To: "dan.magenheimer@oracle.com" Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: quoted-printable Return-path: In-Reply-To: List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Keir Fraser , "Xen-Devel (E-mail)" Cc: Dave Winchell List-Id: xen-devel@lists.xenproject.org > > SO XEN SYSTEM TIME MAX SKEW IS >30X WORSE THAN TSC MAX SKEW! > > > > Looks to me like there's still something algorithmically wrong > > and its not just natural skew and jitter. Maybe some corner > > case in the scale-delta code? Also, should interrupts be turned > > off during the calibration part of init_pit_and_calibrate_tsc() > > (which might cause different scaling factors for each CPU)? > = > I didn't measure skew across CPUs. I measured jitter between = > one local TSC > and the chosen platform timer for calibration (in my case I = > think this was > the HPET). I did this because getting a consistent tick rate from the > platform timer, and from each local TSC, is the basis for the = > calibration > algorithm. The more jitter there is between them, the less = > well it will > work. > = > I implemented a user-space program to collect the required = > stats. It used > CLI/STI to prevent getting interrupted when reading the timer pair. Hi Keir - I'm still looking at whether all of the intra-processor stime skew I'm seeing is due to jitter vs algorithmic. Would you expect system load to impact stime skew between processors (using hpet as a system timer)? I can repeatably watch skew get worse when I am launching an hvm domain. It is MUCH worse when the new domain is in its early stages of booting. CPU load on domain0 has little or no impact but I/O load on dom0 seems to make skew get worse. Thanks, Dan