On 28/11/2017 19:54, Michael S. Tsirkin wrote: >> 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.
Hi Denis, thanks for the report. I think there are two cases: 1) The dedicated pCPU case: do you still get the performance degradation with dedicated pCPUs? 2) The non-dedicated pCPU case: do you still get the performance degradation with threads=1? If not, why do you have sibling vCPUs at all, if you don't have a dedicated physical CPU for each vCPU? Thanks, Paolo