Re: IO scheduler, queue depth, nr_requests

[Nick Piggin <piggin@cyberone.com.au>]

Bill Davidsen wrote:

> linux.kernelNick Piggin wrote:
>
>> But the whole reason it is getting blocked in the first place
>> is because your controller is sucking up all your requests.
>> The whole problem is not a problem if you use properly sized
>> queues.
>>
>> I'm a bit surprised that it wasn't working well with a controller
>> queue depth of 64 and 128 nr_requests. I'll give you a per process
>> request limit patch to try in a minute.
>
>
> And there's the rub... he did try what you are calling correctly sized 
> queues, and his patch works better. I'm all in favor of having the 
> theory and then writing the code, but when something works I would 
> rather understand why and modify the theory.
>
> In other words, given a patch which does help performance in this 
> case, it would be good to understand why, instead of favoring a 
> solution which is better in theory, but which has been tried and found 
> inferior in performance.
>
> I am NOT saying we should just block, effective as Miquel's patch 
> seems, just that we should understand why it works well instead of 
> saying it is in theory bad. I agree, but it works! Hopefully 
> per-process limits solve this, but they "in theory" could result in 
> blocking a process in an otherwise idle system. Unless I midread what 
> you mean of course. Processes which calculate for a while and write 
> results are not uncomon, and letting such a process write a whole 
> bunch of data and then go calculate while it is written is certainly 
> the way it should work. I'm unconvinced that per-process limits are 
> the whole answer without considering the entire io load on the system.
>
> Feel free to tell me I'm misreading your intent (and why).


No, I know Miquel's patch works and his analysis of what is happening
is correct (he proved it). The patch is a bit of a hack to get around
the specific problem he is seeing which should never happen with an
appropriately sized queue, and it might actually hurt in some cases.