Re: [Lsf-pc] [LSF/MM TOPIC] KPTI effect on IO performance

On 01/31/18 00:23, Ming Lei wrote:

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                     |	
------------------------------------------------


Two paths are affected, one is IO submission(read, write,... syscall),
another is the IO completion path in which interrupt may be triggered
from user space, and context switch is needed.

So is there something we can do for decreasing the effect on IO performance?

This effect may make Hannes's issue[3] worse, and maybe 'irq poll' should be
used more widely for all high performance IO device, even some optimization
should be considered for KPTI's effect.

For what kind of workload would you like to improve I/O performance? 
Desktop-style workloads where the only third party code is the code that 
runs in the webbrowser and in the e-mail client or datacenter workloads 
where code from multiple customers runs on the same server? I'm asking 
this because the per-task KPTI work seems very useful to me for 
improving I/O performance for desktop-style workloads. I'm not sure 
however whether that work will be as useful for datacenter workloads. 
See also Willy Tarreau, [PATCH RFC 0/4] Per-task PTI activation 
(https://lkml.org/lkml/2018/1/8/568).

Bart.
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