Re: [Qemu-devel] [RFC]Two ideas to optimize updating irq routing table

[Gonglei (Arei)]

> On my system I have HZ=100 and lots of CPUs. So RCUs "every cpu has
> scheduled"
> is certainly slower than SRCUs algorithm
> (/*
>  * We use an adaptive strategy for synchronize_srcu() and especially for
>  * synchronize_srcu_expedited().  We spin for a fixed time period
>  * (defined below) to allow SRCU readers to exit their read-side critical
>  * sections.  If there are still some readers after 10 microseconds,
>  * we repeatedly block for 1-millisecond time periods.  This approach
>  * has done well in testing, so there is no need for a config parameter.
>  */
> )
> 
> With HZ==1000 and a NO. CPUs small SRCUs spinning might be in the same
> delay
> range than classic RCU depending on how long the read side critical
> section is (if we move from spinning to blocking)
> So using synchronize_srcu_expedited is certainly something to test as it
> increased the spinning time.
> 
> Christian
Yes, after we changed to synchronize_srcu_expedited, grace period latency 
improves much 
and overall this is good. However as I mentioned in another mail, in our 
setting-IRQ-affinity 
and ping test, we can still see some impact of KVM_SET_GSI_ROUTING ioctl. I 
wrote another 
patch in that mail and want to be examined to see if it is acceptable or has 
any problem, thank you.


Best regards,
-Gonglei