linux-kernel - Re: Question about request queues in I/O scheduling

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Message-ID: <x494makcrl3.fsf@segfault.boston.devel.redhat.com>
Date:	Fri, 29 Apr 2016 12:22:48 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Max Kanushin <rextuz@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Question about request queues in I/O scheduling

Max Kanushin <rextuz@...il.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@...hat.com> wrote:
>
>> Max Kanushin <rextuz@...il.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
>>