Hi, Specific example of SMP slowdowns; Two hosts (A), (B) running Ubuntu 9.10. One guest on (B) running Ubuntu 9.10 on two cores. VPN connection (OpenVPN) between host (A) and Guest. Copy file over VPN from host (A) to Guest over VPN. Copy seems slow. Guest CPU usage is around 25% using "top" inside guest. Host A usage is minimal (5-10%). Host B CPU usage is around 160%. I'm expecting Host (B) usage to be ~ 30% ish - this is a huge discrepancy. I'm also noticing during lots of disk/network IO (virtio driver) whereas client / VM load averages tend to be up around the 1.0 mark, looking at the host the load average is showing 10+. (not a specific problem, but curious and concerning) In real terms, if I do a raw copy (not via the VPN, so it's all IO and no CPU) the copy (scp) runs through at around 40Mb/sec. Via the VPN, I'm getting ~ 6Mb/sec. If I reduce the VM to 1 core, I get 12Mb/sec .. so doubling the cores on the VM halves the speed! ??? Gareth. ----- Original Message ----- From: "Avi Kivity" <avi@xxxxxxxxxx> To: "Gareth Bult" <gareth@xxxxxxxxxx> Cc: kvm@xxxxxxxxxxxxxxx Sent: Sunday, 6 December, 2009 10:04:10 AM Subject: Re: SMP/DRBVD issues ... On 12/06/2009 04:43 AM, Gareth Bult wrote: > Hi, > > I'm new to the list so apologies if this is known / fixed, but I've not been able to find satisfactory answers in the archives. > > I'm running a number of boxes with KVM on the stock Ubuntu 9.10 kernel. > Generally it works very well and I have live migration working on DRBD volumes - very impressed - good job! > > However I have a number of issues that I note others have also reported, for which I've not seen fixes; > > a. SMP, it appears on Ubuntu 9.10 at the very least - does not work. Whereas setting -smp 2 does actually > start two kvm threads, the overall performance of the VM is slower than if you use -smp 1, AND the combined > kvm threads use way more CPU on the host than they should. > > [note; this is using virt-manager to setup and maintain, CPU's are AMD Phenom II X4 @ 3.2G ] > > I think that some of the performance hit comes down to processes not being tagged to specific CPU's - > I've noticed on Zen that if you run a 4 thread guess on a 4 core CPU with nothing else running, so it doesn't > need to move threads between cores, you get quite a large performance boost. > But, this doesn't really cover the huge impact on the host. The guest can be showing 15% CPU util when > configured with 4 cores, while the host is showing 280%. > > I can supply more information if needed, but the problem seems to blatant I'm hoping people already know > about it and that can someone can supply some details re; a way forward. > > We haven't observed this; what kind of guest is it? > b. DRBD and migration, in order to make this work both hosts it appears must be configured for with the > parameter 'allow-two-primaries'. This makes me a little nervous, but it does seem to work. There is > however one massive flaw, KVM does not seem to be DRBD aware and with two volumes on two machines, > it's possible to start two instances of a given virtual machine. i.e. neither instance successfully locks the > volume to prevent another instance also starting on it. As you will guess, this has a detrimental effect on > the underlying volume. > > Is there some way to make KVM apply a lock to a DRBD device such that another instance of the VM cannot > be started on another host? (incidentally, XEN does this 'out of the box' for drbd volumes, so I'm guessing > it is possible somehow...? ) > > Note; it's nice to configure VM's to auto start on a given machine so in the event of a power failure the VM will > boot with the host. However, if this machine fails and you need to start the VM on an alternative machine, > when the original machine recovers / reboots, it will attempt (and succeed) in auto booting the same VM > leaving you with two copies of the same VM and a screwed guest image. > > This needs support at the management layer. qemu has no way of knowing whether you want to share the disk between two guests or not. -- error compiling committee.c: too many arguments to function -- Gareth Bult (Gareth@xxxxxxxxxx) -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
