On Thu, Jan 12, 2017 at 04:41:00PM +0200, Sagi Grimberg wrote: > > >>**Note: when I ran multiple threads on more cpus the performance > >>degradation phenomenon disappeared, but I tested on a VM with > >>qemu emulation backed by null_blk so I figured I had some other > >>bottleneck somewhere (that's why I asked for some more testing). > > > >That could be because of the vmexits as every MMIO access in the guest > >triggers a vmexit and if you poll with a low budget you do more MMIOs hence > >you have more vmexits. > > > >Did you do testing only in qemu or with real H/W as well? > > I tried once. IIRC, I saw the same phenomenons... JFTR I tried my AHCI irq_poll patch on the Qemu emulation and the read throughput dropped from ~1GB/s to ~350MB/s. But this can be related to Qemu's I/O wiredness as well I think. I'll try on real hardware tomorrow. -- Johannes Thumshirn Storage jthumshirn@xxxxxxx +49 911 74053 689 SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: Felix Imendörffer, Jane Smithard, Graham Norton HRB 21284 (AG Nürnberg) Key fingerprint = EC38 9CAB C2C4 F25D 8600 D0D0 0393 969D 2D76 0850 -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
