linux-kernel - Re: High latency while CPU is under full load

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Date:	Wed, 08 Oct 2014 07:44:40 -0400
From:	Austin S Hemmelgarn <ahferroin7@...il.com>
To:	Grozdan <neutrino8@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: High latency while CPU is under full load

On 2014-10-07 15:28, Grozdan wrote:
> Hi,
>
> Basically, my problem is this:
>
> I'm doing a lot of audio/video encoding on an AMD FX8350. The encoder
> process always runs at nice 10. Even so, my whole system feels very
> sluggish. Switching between different app windows and/or virtual
> desktops takes up usually 3-5 seconds giving the impression that there
> is not enough processing power. Browsing the web is also severely
> impacted.
>
> I had to tune CFS in order to be (much) more responsive during an
> encoding session. This has worked out pretty well thus far, but it is
> my opinion that the user should *not* need to fiddle with buttons to
> make his system respond fluently even under high load. The below is
> what I had to do in order to get a snappy system during such load
>
> kernel.sched_nr_migrate = 64
> kernel.sched_latency_ns = 65000000
> kernel.sched_wakeup_granularity_ns = 100000
> kernel.sched_min_granularity_ns = 100000
> kernel.sched_migration_cost_ns = 7000000
>
> I have tried 3 different kernels, including one compiled myself, but
> the results are the same
> Kernels I tried were: 3.11.10, 3.12 and 3.16.4 (self-compiled)
>
> My system specs are as follows
>
> CPU: AMD FX-8350 @ 4GHz
> RAM: 16GB DDR1333
> GPU: NVIDIA GTX 560 with NV blob driver
> HDD: Seagate Constellation ES.3 128MB cache
> Desktop: KDE 4.11
>
Are you using a kernel with CONFIG_PREEMPT=y?  I've personally found 
that that can help hugely with UI sluggishness (and also tends to get 
better results when doing stuff like live video capture), although to 
get this you may need to build the kernel yourself.  Also, I would 
suggest trying the deadline I/O scheduler; I've found it provides much 
better latency than CFQ for most workloads, and you almost certainly 
don't need I/O priorities.  I've actually got a similar setup (FX-8320 @ 
4GHz, 16GB DDR3-1600, High-end storage, and a Radeon R7-240 GPU), and 
had similar issues when doing processing of big (>500MB) images in GIMP, 
and using a CONFIG_PREEMPT enabled kernel and the deadline I/O scheduler 
has pretty much resolved all of these issues.

Additionally, try with KDE's 'semantic desktop' functionality turned 
off, this eats a huge amount of disk and memory bandwidth, and can 
easily bring a system to it's knees.  Furthermore, unless you can 
reproduce things using nouevau instead of the NV driver, you're not as 
likely to get a lot of help.



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