linux-kernel - Real-time scheduling policies and hyper-threading

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Date:	Thu, 24 Apr 2014 22:16:12 +0400
From:	Roman Gushchin <klamm@...dex-team.ru>
To:	LKML <linux-kernel@...r.kernel.org>, mingo@...hat.com,
	peterz@...radead.org, tkhai@...dex.ru
Subject: Real-time scheduling policies and hyper-threading

Hello!

I spend some time investigating why switching runtime* tasks to real-time scheduling policies increases
response time dispersion, while the opposite is expected.

The main reason is hyper-threading. rt-scheduler tries only to load all logical CPUs, selecting topologically
closest when the current is busy. If hyper-threading is enabled, this strategy is counter-productive:
tasks are suffering on busy HT-threads when there is a plenty of idle physical cores.

Also, rt-scheduler doesn't try to balance rt load between physical CPUs. It's significant because of
turbo-boost and frequency scaling technologies: per-core performance depends on the number of
idle cores in the same physical cpu.

Are there any known solutions of this problem except disabling hyper-threading and frequency scaling at all?

Are there any common plans to enhance the load balancing algorithm in the rt-scheduler?

Does anyone use rt-scheduler for runtime-like cpu-bound tasks?

Why just don't use CFS? :-)
Rt-scheduler with modified load balancing shows much better results.
I have a prototype (still incomplete and with many dirty hacks), that shows 10-15% 
performance increase in our production.

(*) A simplified model can be described as following:
there is one process per machine, with one thread, that receives request from network and puts them into queue;
n (n ~ NCPU + 1) worker threads, that get requests from the queue and handle them.
Load is cpu-bound, tens of milliseconds per request. Typical CPU load is between 40% and 70%.
A typical system has two physical x86-64 cpus with 8-16 physical cores each (x2 with hyper-threading).

Thanks,
Roman
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