On 08/21/2012 06:21 PM, Richard Davies wrote: > Avi Kivity wrote: >> Richard Davies wrote: >> > We're running host kernel 3.5.1 and qemu-kvm 1.1.1. >> > >> > I hadn't though about it, but I agree this is related to cpu overcommit. The >> > slow boots are intermittent (and infrequent) with cpu overcommit whereas I >> > don't think it occurs without cpu overcommit. >> > >> > In addition, if there is a slow boot ongoing, and you kill some other VMs to >> > reduce cpu overcommit then this will sometimes speed it up. >> > >> > I guess the question is why even with overcommit most boots are fine, but >> > some small fraction then go slow? >> >> Could be a bug. The scheduler and the spin-loop handling code fight >> each other instead of working well. >> >> Please provide snapshots of 'perf top' while a slow boot is in progress. > > Below are two 'perf top' snapshots during a slow boot, which appear to me to > support your idea of a spin-lock problem. > > There are a lot more "unprocessable samples recorded" messages at the end of > each snapshot which I haven't included. I think these may be from the guest > OS - the kernel is listed, and qemu-kvm itself is listed on some other > traces which I did, although not these. > > Richard. > > > > PerfTop: 62249 irqs/sec kernel:96.9% exact: 0.0% [4000Hz cycles], (all, 16 CPUs) > -------------------------------------------------------------------------------------------------------------------------------- > > 35.80% [kernel] [k] _raw_spin_lock_irqsave > 21.64% [kernel] [k] isolate_freepages_block Please disable ksm, and if this function persists in the profile, reduce some memory from the guests. > 5.91% [kernel] [k] yield_to > 4.95% [kernel] [k] _raw_spin_lock > 3.37% [kernel] [k] kvm_vcpu_on_spin Except for isolate_freepages_block, all functions up to here have to do with dealing with cpu overcommit. But let's deal with them after we see a profile with isolate_freepages_block removed. -- error compiling committee.c: too many arguments to function -- 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
