[LSF/MM TOPIC] KPTI effect on IO performance

[Ming Lei]

On Wed, Jan 31, 2018 at 11:43:33AM -0700, Scotty Bauer wrote:
> On 2018-01-31 01:23, Ming Lei wrote:
> > Hi All,
> > 
> > After KPTI is merged, there is extra load introduced to context switch
> > between user space and kernel space. It is observed on my laptop that
> > one
> > syscall takes extra ~0.15us[1] compared with 'nopti'.
> > 
> > IO performance is affected too, it is observed that IOPS drops by 32% in
> > my test[2] on null_blk compared with 'nopti':
> > 
> > randread IOPS on latest linus tree:
> > -------------------------------------------------
> > | randread IOPS     | randread IOPS with 'nopti'|
> > ------------------------------------------------
> > | 928K              | 1372K                     |
> > ------------------------------------------------
> > 
> > 
> 
> Do you know if your CPU has PCID? It would be interesting to see these tests
> on older CPUs or older kernels without PCID support.

BTW, I also saw test data in case of vCPU without PCID, and it is said the syscall
time can be close to ~30X compared with nopti, and the test should be setup easily
by adjust CPU model of Qemu.

So in case of no PCID, KPTI effect on IO performance should be much bigger than the
above data.

Thanks,
Ming