Re: performance drop after using blkcg

[joeytao]

Hello, 

I also do these tests and find the same results. IMO, on faster storage with
deep queue depth, if device is asking for more requests,but our workload
can't send enough requests, we have to idle to provide service
differentiation. We'll see performance drop if  applications can't drive
enough  IO  to keep disk busy.Especially for writes, with the effect of disk
cache and deep queue depth, we'll often see performance drop .

So I come up with an approach called Self-adaption blkcg that if the 
average total service time for a request is much less,we don' choose to
idle. Otherwise, we choose to idle to wait for the request. The patch is
below. After large tests,the new scheduler can provide service 
differentiation in most cases. When the application can't drive enough
requests and the mean total service time is very small, we don't choose to
idle. In most cases, the performance doesn't drop after using blkcg and the
service  differentiation is good.