On Thu, Jan 12, 2017 at 04:41:00PM +0200, Sagi Grimberg wrote:

quoted

quoted

**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.

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Johannes Thumshirn                                          Storage
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