linux-kernel - Re: Scheduler(?) regression from 2.6.22 to 2.6.24 for short-lived threads

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Message-ID: <47B0C1E8.2050809@tmr.com>
Date:	Mon, 11 Feb 2008 16:45:12 -0500
From:	Bill Davidsen <davidsen@....com>
To:	Olof Johansson <olof@...om.net>
CC:	Willy Tarreau <w@....eu>, Mike Galbraith <efault@....de>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: Scheduler(?) regression from 2.6.22 to 2.6.24 for short-lived
   threads

Olof Johansson wrote:

>> However, I fail to understand the goal of the reproducer. Granted it shows
>> irregularities in the scheduler under such conditions, but what *real*
>> workload would spend its time sequentially creating then immediately killing
>> threads, never using more than 2 at a time ?
>>
>> If this could be turned into a DoS, I could understand, but here it looks
>> a bit pointless :-/
> 
> It seems generally unfortunate that it takes longer for a new thread to
> move over to the second cpu even when the first is busy with the original
> thread. I can certainly see cases where this causes suboptimal overall
> system behaviour.
> 
I think the moving to another CPU gets really dependent on the CPU type. 
On a P4+HT the caches are shared, and moving costs almost nothing for 
cache hits, while on CPUs which have other cache layouts the migration 
cost is higher. Obviously multi-core should be cheaper than 
multi-socket, by avoiding using the system memory bus, but it still can 
get ugly.

I have an IPC test around which showed that, it ran like hell on HT, and 
progressively worse as cache because less shared. I wonder why the 
latest git works so much better?

-- 
Bill Davidsen <davidsen@....com>
   "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot
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