Commit 14c985cffa "target-i386: present virtual L3 cache info for vcpus" introduced and set by default exposing l3 to the guest.
The motivation behind it was that in the Linux scheduler, when waking up a task on a sibling CPU, the task was put onto the target CPU's runqueue directly, without sending a reschedule IPI. Reduction in the IPI count led to performance gain. However, this isn't the whole story. Once the task is on the target CPU's runqueue, it may have to preempt the current task on that CPU, be it the idle task putting the CPU to sleep or just another running task. For that a reschedule IPI will have to be issued, too. Only when that other CPU is running a normal task for too little time, the fairness constraints will prevent the preemption and thus the IPI. This boils down to the improvement being only achievable in workloads with many actively switching tasks. We had no access to the (proprietary?) SAP HANA benchmark the commit referred to, but the pattern is also reproduced with "perf bench sched messaging -g 1" on 1 socket, 8 cores vCPU topology, we see indeed: l3-cache #res IPI /s #time / 10000 loops off 560K 1.8 sec on 40K 0.9 sec Now there's a downside: with L3 cache the Linux scheduler is more eager to wake up tasks on sibling CPUs, resulting in unnecessary cross-vCPU interactions and therefore exessive halts and IPIs. E.g. "perf bench sched pipe -i 100000" gives l3-cache #res IPI /s #HLT /s #time /100000 loops off 200 (no K) 230 0.2 sec on 400K 330K 0.5 sec In a more realistic test, we observe 15% degradation in VM density (measured as the number of VMs, each running Drupal CMS serving 2 http requests per second to its main page, with 95%-percentile response latency under 100 ms) with l3-cache=on. We think that mostly-idle scenario is more common in cloud and personal usage, and should be optimized for by default; users of highly loaded VMs should be able to tune them up themselves. So switch l3-cache off by default, and add a compat clause for the range of machine types where it was on. Signed-off-by: Denis Plotnikov <dplotni...@virtuozzo.com> Reviewed-by: Roman Kagan <rka...@virtuozzo.com> --- include/hw/i386/pc.h | 7 ++++++- target/i386/cpu.c | 2 +- 2 files changed, 7 insertions(+), 2 deletions(-) diff --git a/include/hw/i386/pc.h b/include/hw/i386/pc.h index 087d184..1d2dcae 100644 --- a/include/hw/i386/pc.h +++ b/include/hw/i386/pc.h @@ -375,7 +375,12 @@ bool e820_get_entry(int, uint32_t, uint64_t *, uint64_t *); .driver = TYPE_X86_CPU,\ .property = "x-hv-max-vps",\ .value = "0x40",\ - }, + },\ + {\ + .driver = TYPE_X86_CPU,\ + .property = "l3-cache",\ + .value = "on",\ + },\ #define PC_COMPAT_2_9 \ HW_COMPAT_2_9 \ diff --git a/target/i386/cpu.c b/target/i386/cpu.c index 1edcf29..95a51bd 100644 --- a/target/i386/cpu.c +++ b/target/i386/cpu.c @@ -4154,7 +4154,7 @@ static Property x86_cpu_properties[] = { DEFINE_PROP_STRING("hv-vendor-id", X86CPU, hyperv_vendor_id), DEFINE_PROP_BOOL("cpuid-0xb", X86CPU, enable_cpuid_0xb, true), DEFINE_PROP_BOOL("lmce", X86CPU, enable_lmce, false), - DEFINE_PROP_BOOL("l3-cache", X86CPU, enable_l3_cache, true), + DEFINE_PROP_BOOL("l3-cache", X86CPU, enable_l3_cache, false), DEFINE_PROP_BOOL("kvm-no-smi-migration", X86CPU, kvm_no_smi_migration, false), DEFINE_PROP_BOOL("vmware-cpuid-freq", X86CPU, vmware_cpuid_freq, true), -- 2.7.4