[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] xen-unstable, winxp32 very poor performance on AMD FX-8150, I bisected and changeset is 24770:7f79475d3de7
Hi, At 14:59 +0100 on 22 Oct (1350917960), Tim Deegan wrote: > At 19:21 +0200 on 20 Oct (1350760876), Peter Maloney wrote: > > The change was 8 months ago > > > > changeset: 24770:7f79475d3de7 > > user: Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx> > > date: Fri Feb 10 16:07:07 2012 +0000 > > summary: x86/mm: Make p2m lookups fully synchronized wrt modifications > > This change was bad for performnace across the board and most of it has > since been either reverted or amended, but clearly we missed something > here. > > It's interesting that Win8 isn't slowed down. I wonder whether that's to > do with the way it drives the VGA card -- IIRC it uses a generic VESA > driver rather than a Cirrus one. In fact this is to do with the APIC. On my test system, a busy 2-vcpu VM is making about 300k/s accesses to the APIC TPR. These accesses are all trapped and emulated by Xen, and that emulation has got more expensive as part of this change. Later Windows OSes have a feature called 'lazy IRQL' which makes those accesses go away, but sadly that's not been done for WinXP. On modern Intel CPUs, the hardware acceleration for TPR accesses works for XP; on AMD it requires the OS to use 'MOV reg32, CR8' to access the TPR instead of MMIO, which XP is clearly not doing. :( Peter: if you have the option, you might find that installing the PV drivers that ship with Citrix XenServer 6.0 makes things work better. Andres: even though this load of APIC emulations is pretty extreme, it's surprising that the VM runs faster on shadow pagetables! Any ideas for where this slowdown is coming from? Cheers, Tim. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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