linux-kernel - question about IO-sched

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Message-ID: <op.whbsn0qvn27o5l@gaoqiang-d1.corp.qihoo.net>
Date:	Thu, 12 Jul 2012 17:01:02 +0800
From:	gaoqiang <gaoqiangscut@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: question about IO-sched

Hi,all

	I have long known that deadline is read-prefered. but a simple test gives  
the opposite result.
	
	with two processes running at the same time,one for read and one for  
write.actually,they did nothing bug IO operation.
	while(true)
	{
		read();
	}
	the other:
	while(true)
	{
		write();
	}
	
	with deadline IO-sched  and ext4 filesystem.as a result, read ratio was  
about below 3M/s.and write about 100M/s. I have tested both kernel-2.6.18  
and kernel-2.6.32,getting the same result.
	
	I add some debug information in the kernel and recompile,found that,it  
has little to do with IO-sched layer because read request dropped into  
deadline was 5% of write request .from /proc/<pid>/stack,the read process  
hands on sync_page most of the time.
	what is the matter ? anyone help me ?
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