[LSF/MM TOPIC] KPTI effect on IO performance

[Bart Van Assche]

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.