On Fri, Dec 15, 2017 at 02:36:56PM -0500, Joe Buehler wrote: > I am debugging large latency issues in a realtime KVM instance, trying to make an existing VM (debugged on another type of box) run on an HP proliant now. > > cyclictest came up with the following snippet: > > kworker/-71 0.....11 13483223us : ata_sff_exec_command <-ata_bmdma_setup > kworker/-71 0.....11 13483228us : ata_sff_sync <-ata_sff_exec_command > kworker/-71 0.....11 13483231us : delay_tsc <-__const_udelay > kworker/-71 0.....11 13483231us*: ata_bmdma_start <-ata_bmdma_qc_issue > kworker/-71 0d....11 13496181us : smp_apic_timer_interrupt <-apic_timer_interrupt > kworker/-71 0d....11 13496181us : irq_enter <-smp_apic_timer_interrupt > kworker/-71 0d....11 13496182us : rcu_irq_enter <-irq_enter > > I don't have a lot of experience with this; is this a DMA start for disk I/O, presumably causing an exit from the VM? For almost 13 milliseconds???? > > I haven't been able to find much obvious about eliminating this -- any suggestions? I set caching to writeback without effect. > > <disk type='file' device='disk'> > <driver name='qemu' type='qcow2' cache='writeback'/> > <source file='/boot.qcow2'/> > <backingStore type='file' index='1'> > <format type='raw'/> > <source file='/root.qcow2'/> > <backingStore/> > </backingStore> > <target dev='hdb' bus='ide'/> > <alias name='ide0-0-1'/> > <address type='drive' controller='0' bus='0' target='0' unit='1'/> > </disk> Um. Don't use IDE? Use VirtIO. > > Joe Buehler
