Question about request queues in I/O scheduling

Question about request queues in I/O scheduling

[Max Kanushin]

Hello,

I was searching for a way to find out the length of a request_queue 
(that is defined as a structure in linux/include/linux/blkdev.h). 
However I am new to the kernel development and can't figure out where is 
the actual list of requests to be processed by an elevator.
Is there a way to iterate requests in a queue or at least find a number 
of them?

Please make a personal CC for me. I will really appreciate your help.
Best regards

   Max Kanushin.

Re: Question about request queues in I/O scheduling

[Jeff Moyer]

Max Kanushin <rextuz@gmail.com> writes:

> Hello,
>
> I was searching for a way to find out the length of a request_queue
> (that is defined as a structure in
> linux/include/linux/blkdev.h). However I am new to the kernel
> development and can't figure out where is the actual list of requests
> to be processed by an elevator.
> Is there a way to iterate requests in a queue or at least find a
> number of them?

Hi, Max,

What exactly are you trying to accomplish?

Cheers,
Jeff

Re: Question about request queues in I/O scheduling

[Jeff Moyer]

Max Kanushin <rextuz@gmail.com> writes:

> Thank you very much for the reply. My general idea is take control of all
> request queues to block and unblock them manually depending on their load.
> One of my steps is to find a length of a queue to decide if to block it.
> Actually I think I've found the way. If I got it right, I can move from one
> request to another within the list:
> struct list_head queue_head;
> So that I can calculate how many requests are there in the queue.

Hi, Max,

The queue_head is the dispatch list for I/O, so it does not represent
all I/O queued for a request_queue.  The number of requests in the
scheduler would be reflected by q->nr_sorted.  The number of dispatched
requests is in an array, in_flight.

Cheers,
Jeff



>
> Best regards,
>   Max Kanushin.
> On Apr 29, 2016 6:32 PM, "Jeff Moyer" <jmoyer@redhat.com> wrote:
>
>> Max Kanushin <rextuz@gmail.com> writes:
>>
>> > Hello,
>> >
>> > I was searching for a way to find out the length of a request_queue
>> > (that is defined as a structure in
>> > linux/include/linux/blkdev.h). However I am new to the kernel
>> > development and can't figure out where is the actual list of requests
>> > to be processed by an elevator.
>> > Is there a way to iterate requests in a queue or at least find a
>> > number of them?
>>
>> Hi, Max,
>>
>> What exactly are you trying to accomplish?
>>
>> Cheers,
>> Jeff
>>

Re: Question about request queues in I/O scheduling

[Max Kanushin]

Hello,

I was trying to use spin_trylock(q->queue_lock) on several queues when 
q->nr_sorted exceeds the per-defined number to block them and unlock the 
them later with spin_unlock(q->queue_lock), but I have faced the 
following problem: my system freezes when I am trying to test it by 
moving files around in my system. I thought it might be caused by 
blocking a queue of a system partition or swap, but I do not know how to 
check this as well.
Is it the right way to stop the queue/elevator? What are freezes might 
be caused by? Is there a simple way to find out which block device is a 
queue belong to?
I would really appreciate your reply.

Best regards,
   Max Kanushin.

On 04/29/2016 07:22 PM, Jeff Moyer wrote:
> Max Kanushin <rextuz@gmail.com> writes:
>
>> Thank you very much for the reply. My general idea is take control of all
>> request queues to block and unblock them manually depending on their load.
>> One of my steps is to find a length of a queue to decide if to block it.
>> Actually I think I've found the way. If I got it right, I can move from one
>> request to another within the list:
>> struct list_head queue_head;
>> So that I can calculate how many requests are there in the queue.
>
> Hi, Max,
>
> The queue_head is the dispatch list for I/O, so it does not represent
> all I/O queued for a request_queue.  The number of requests in the
> scheduler would be reflected by q->nr_sorted.  The number of dispatched
> requests is in an array, in_flight.
>
> Cheers,
> Jeff
>
>
>
>>
>> Best regards,
>>    Max Kanushin.
>> On Apr 29, 2016 6:32 PM, "Jeff Moyer" <jmoyer@redhat.com> wrote:
>>
>>> Max Kanushin <rextuz@gmail.com> writes:
>>>
>>>> Hello,
>>>>
>>>> I was searching for a way to find out the length of a request_queue
>>>> (that is defined as a structure in
>>>> linux/include/linux/blkdev.h). However I am new to the kernel
>>>> development and can't figure out where is the actual list of requests
>>>> to be processed by an elevator.
>>>> Is there a way to iterate requests in a queue or at least find a
>>>> number of them?
>>>
>>> Hi, Max,
>>>
>>> What exactly are you trying to accomplish?
>>>
>>> Cheers,
>>> Jeff
>>>