Re: Which I/O scheduler is Fedora switching to in 4.21? mq-deadline or BFQ?

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> Il giorno 12 dic 2018, alle ore 22:41, stan <stanl-fedorauser@xxxxxxxxxxx> ha scritto:
> 
> On Wed, 12 Dec 2018 16:07:49 -0500
> Jeff Moyer <jmoyer@xxxxxxxxxx> wrote:
> 
> Thanks for your insight.  Doesn't look good for my use of BFQ.
> 
>> Note that you can change the current I/O scheduler for any block
>> device by echo-ing into /sys/block/<dev>/queue/scheduler.  Cat-ing
>> that file will give you the list of available schedulers.
> 
> That's part of the problem.  BFQ doesn't appear in the list of
> available schedulers.  When I cat that location for my disks, I see
> [noop].  Since CFQ does appear there if it is compiled into the kernel,
> I'll have to look into what is done for CFQ and see how hard it would
> be to patch the kernel to repeat that behavior for BFQ.
> 
> My use case in not mq, so after reading one of the links in this
> thread about performance, I saw that BFQ gave ~20 to 30 % boost in
> disk io performance, and enhanced low latency performance (desktop
> responsiveness) for single queue. That's what I want to capture by using
> BFQ.  I wonder if that is my problem.  From what Chris said, an mq
> scheduler is required in order to use BFQ, whether it is for mq or
> single queue use.  I'll try that.  I normally use deadline and CFQ for
> scheduling.  Back to the compiler.
> 
> I'm surprised this is so difficult.  It's been in the kernel since the
> 2.x series, and usually the configuration options are excellent for
> allowing variation in how the kernel is configured.
> 
> On the plus side, I notice only slight degradation in behavior using
> noop scheduling.  :-)  Maybe I should just skip scheduling. :-D

To test the behavior of your system, why don't you check, e.g., how
long it takes to start an application while there is some background I/O?

A super quick way to do this is

git clone https://github.com/Algodev-github/S
cd S/comm_startup_lat
sudo ./comm_startup_lat.sh <scheduler-you-want-to-test> 5 5 seq 3 "replay-startup-io gnometerm"

The last command line
- starts the reading of 5 files plus the writing of 5 other files
- replays, for three times, the I/O that gnome terminal does while;
  starting up (if you want I can tell you how to change the last command
  line so as to execute the original application, but you would get the
  same results);
- for each attempt, measures how long this start-up I/O takes to
  complete.

Paolo

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