[LSF/MM TOPIC] KPTI effect on IO performance

[Ming Lei]

Hi Scotty,

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.

My CPU has PCID, which can be retrieved via /proc/cpuinfo.

And the above test is run on same kernel binary, and the result is just done
between 'nopti' and no 'nopti' in kernel command line.

Thanks,
Ming